Skip to main content

Full text of "Memoirs of the Museum of Comparative Zoӧlogy, at Harvard College, Cambridge, Mass"

See other formats 


HARVARD  UNIVERSITY 


LIBRARY 


OF  THE 

Museum  of  Comparative  Zoology 


/Demotrs  of  tbe  /iDuseum  ot  (lomparattvc  ZoSloQp 

AT  HARVARD   COLLEGE 
Vol.  LIV.    No.  4 


STUDIES   OF   THE  WATERS   OF   THE 

CONTINENTAL  SHELF,  CAFE  COD 

TO   CHESAPEAKE  BAY 

III 

A  VOLUMETRIC  STUDY  OF  THE  ZOOPLANKTON 


BY 


Henry  B.  Bigelow  and  Mary  Sears 


MU8.  COMP.  ZOOL 
LIBRARY 

Nuv  n    idti4 

HAHVARD 
UNIVERSiTC 


CAMBRIDGE,  U.S.A. 

printeD  tor  tbe  Museum 

1939 


/Demoirs  of  tbe  /IDuseum  of  Comparative  Zoolocis 

AT  HARVARD   COLLEGE 

Vol.  LIV.    No.  4 


STUDIES   OF   TPIE  WATERS   OF   THE 

CONTINENTAL  SHELF,  CAPE  COD 

TO   CHESAPEAKE  BAY 

III 

A  VOLUMETRIC  STUDY  OF  THE  ZOOPLANKTON 


BY 


Henry  B.  Bigelow  and  Mary  Sears 


MUS.  COMP.  ZOOL 
LIBRARY 

NOV  2    1964 

HARVARD 
UNIYERSCDC 


CAMBRIDGE,  U.S.A. 

IprtnteO  for  tbe  /IDuseum 

1939 


>> 


STUDIES  OF  THE  WATERS  OF  THE  CONTINENTAL  SHELF,  CAPE  COD 

TO  CHESAPEAKE  BAY.  III.'  A  VOLUMETRIC  STUDY 

OF  THE  ZOOPLANKTON 

By  Henry  B.  Bigelow  and  Mary  Sears 

Contribution  No.  194 
From  the  Woods  Hole  Oceanographic  Institution 


1  Parts  I  and  II  appeared  in  Papers  in  Physical  Oceanography  and  Meteorology, 
Vol.  II,  No.  4,  1933  and  Vol.  IV.  No.  1,  1935. 


CONTENTS 

Page 

Introduction 189 

Acknowledgments 190 

Geographic  limits  and  subdivisions  of  the  area 190 

History  and  sources  of  information 190 

Methods 192 

Collection    192 

Measurements  and  calculations 193 

Expression  of  results 196 

Validitj^  of  calculated  results 197 

Part  I.     The  volume  of  zooplankton 200 

The  water  column  as  a  whole 200 

Seasonal  cycle 200 

February 200 

April 202 

May 203 

June 206 

July 209 

Autumn 210 

Annual  differences 212 

Vertical  distribution 214 

Diurnal  stratification 214 

^'ertical  stratification  other  than  diurnal 217 

Comparison  with  other  areas 221 

Relative  abundance  of  different  species 228 

Monthly  succession 230 

February 230 

April 231 

-  May 233 

June 237 

July 241 

Autumn 243 

Annual  differences 245 

Sources  of  the  local  plankton 246 

The  area  as  a  feeding  ground  for  plankton-eating  fish 253 


184  memoir:  museum  of  comparative  zoology 

Page 

Conclusions  261 

The  plankton  as  a  whole 261 

Horizontal  distribution 261 

Vertical  distribution 264 

Annual  differences 265 

Comparison  with  other  areas 265 

Relative  importance  of  different  species 265 

Feeding  conditions  for  plankton-eating  fishes 268 

Part  II.    Volumetric  distribution  of  individual  species 270 

Chordates 270 

Doliolum  sp 270 

Salps 272 

Fritillaria  sp • 273 

Oikopleura  dioica 273 

Oikopleura  labradoriensis 274 

Molluscs 275 

Clione  limacina 275 

Frequency 275 

Abundance 277 

Source  of  the  local  stock 278 

Limacina  reiroversa 279 

Frequency 279 

Abundance 280 

Vertical  distribution 284 

Relation  to  temperature 284 

Annual  variations 284 

Source  of  the  local  stock 284 

Other  molluscs 286 

Decapods 286 

Crab  and  hermit  crab  larvae 286 

Crago  sp 289 

Palinurid  larvae 290 

Lucifer  typus   290 

Stomatopods 290 

Euphausiids 291 

Meganyctiphanes  norvegica 291 


BIGELOW   AND    SEARS!   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  185 

Page 

Nematoscelis  megalops 293 

Thysanoessa  inermis 294 

Thysanoessa  gregaria 295 

Thysanoessa  longicaudata 296 

Euphausiid  larvae 297 

Other  euphausiids 298 

Mysids 298 

Amphipods 299 

Euthemisto  compressa 299 

Frequency 299 

Abundance 300 

Annual  variations 302 

Other  amphipods 302 

Copepods 303 

Acartia 303 

Anomalocera  pattersoni 304 

Calanus  finmarchicus 304 

Frequency 304 

Abundance 305 

Breeding  periods 310 

Vertical  distribution 310 

Diurnal  migration 312 

Distribution  in  relation  to  temperature 314 

Annual  fluctuations 316 

Calanus  hyperboreus 317 

Candacia  armata 318 

Centropages  hamatus 320 

Centropages  typicus 321 

Frequency 321 

,  Abundance 321 

Annual  variations 324 

Vertical  distribution 324 

Centropages  violaceus  326 

Corycaeus  sp 326 

Eucalanus  sp 326 

Euchirella  rostrata 328 


186  memoir:  museum  of  comparative  zoology 

Page 

Mecynoccra  clausi 328 

Metridia  lucens 329 

Frequency 329 

Abundance 331 

Annual  variations 333 

Oithona  sp 333 

Oncaea  sp 334 

Pleuromamma  sp 334 

Paracalanus  parvus 336 

Paraeuchaeta  norvegica 337 

Pseudocalanus  minutus 338 

Frequency 338 

Abundance 339 

Annual  variations 340 

Bhincalanus  nasutus 340 

Frequency 340 

Abundance 342 

ScolecUhrix  danae 343 

Temora  longicornis 343 

Temora  stylifera 345 

Other  copepods 345 

Cladocerans 346 

Podon  and  Evadne 346 

Penilia 346 

Chaetognaths 347 

Eukrohnia  hamaia 347 

Sagitta  elegans 347 

Frequency 347 

Abundance 349 

Annual  differences 351 

Sources  of  the  local  stock 351 

Vertical  distribution 352 

Sagitta  enflata 356 

Sagitta  serratodentata 356 

Frequency 356 

Abundance 358 


niGELOW    ANO    sears:    north    ATLANTIC    ZOOPLANKTON    STUDIES  187 

Page 

Annual  variations 360 

Other  chaetognaths 360 

Annelids 361 

Tomopteris  sp 361 

Other  annelids 361 

Medusae 362 

Aglantha  digitale 362 

Regional  distribution 362 

Seasonal  and  annual  variations 363 

Leptomedusae 365 

Other  medusae 366 

Siphonophores 366 

Agalmidae 366 

Muggiaea  kochii 367 

Other  siphonophores 367 

Ctenophores 368 

Beroe  sp 368 

Pleurobrachia  pileus 369 

Other  ctenophores 370 

Protozoans 371 

Bibliography 372 


INTRODUCTION 

111  April,  1!)29,  the  U.  S.  Bureau  of  Fisheries  commenced  an  investigation  of 
the  distribution  of  the  eggs  and  larvae  of  the  mackerel  in  the  waters  of  the  con- 
tinental shelf  between  Cape  Cod  and  Latitude  about  36°,  a  work  continued  dur- 
ing the  vernal  half  year,  in  the  three  subsequent  years.  And  while  the  collection 
of  plankton  was  only  a  secondary  goal,  samples  were  systematically  obtained  on 
all  cruises,  and  turned  over  to  us  for  study. 

Plankton  investigations,  we  conceive,  fall  into  two  chief  groups:  (a)  popu- 
lation and  distribution  studies  of  particular  species  or  groups  of  species,  or  studies 
of  the  relationship  of  one  species  to  another  (e.g.,  feeding  habits  of  fishes) ;  and  (b) 
attempts  to  assay  the  richness  of  one  part  of  the  sea  or  another,  either  in  organic 
production,  or  as  a  feeding  ground  for  larger  animals.  Studies  of  the  first  of 
these  categories  depend  on  enumerations  of  the  specimens  present,  whether  of 
different  species  or  of  different  growth  stages  of  given  species.  And  the  data  that 
have  been  presented  in  the  great  majority  of  recent  publications  on  zooplankton 
have  been  of  this  sort.  If,  however,  we  attempt  to  approach  the  matter  from  the 
other  angle,  information  as  to  the  mass  of  organic  matter  that  is  present  in  the  sea 
from  time  to  time  and  from  place  to  place  becomes  of  prime  import.  And  very 
seldom  can  this  be  deduced  from  a  knowledge  of  the  numbers  of  units  of  which 
this  mass  is  composed,  because  different  planktonic  animals,  whether  different 
species  or  different  stages  in  the  growth  of  one  species,  vary  so  widely  in  size  that 
counts  of  total  numbers  are  apt  to  prove  very  deceptive,  if  interpreted  as  indices 
to  total  mass.  Part  I  of  this  report  attempts  to  give  at  least  a  rough  picture  of 
the  latter  for  the  part  of  the  sea  in  question,  together  with  the  proportions  in 
which  the  leading  species  enter  into  it.  Part  II  includes  such  information  as  to 
the  distribution  of  individual  species  as  our  own  analyses  have  yielded. 

In  studies  of  this  sort,  the  vegetable  and  animal  fractions  of  the  plankton 
can  be  considered  either  as  a  unit,  or  separately:  the  latter  course  has  seemed  to 
us  preferable,  because  of  the  basic  difference  between  the  nutritional  require- 
ments of  the  two  categories.  We  must  realize,  however,  whether  for  the  zooplank- 
ton or  for  the  phytoplankton,  that  measurements  of  mass — however  precise  these 
may  be — are  only  one  step  in  our  path  toward  a  knowledge  of  the  productivity 
of  the  sea  in  organic  substance.  A  second,  coincident,  and  equally  vital  require- 
ment is  a  knowledge  of  the  rate  of  overturn,  which  can  come  only  from  popula- 
tion studies  based  on  enumerations  of  individuals  combined  with  classification 
of  age-frequencies.  A  third  essential  step  is  the  chemical  analysis  of  the  groups  of 


190  memoir:  museum  of  comparative  zoology 

organisms  concerned,  for  these  differ  one  from  another,  in  the  proportional  com- 
ponents of  proteins,  carbohydrates,  fats,  and  sundry  other  compounds. 

The  present  report,  confined  to  a  survey  of  the  mass  distribution  of  the  larger 
animal  plankton,  is  offered  in  the  hope  that  it  may  serve  as  a  preliminary  ap- 
proach to  the  broader  field  outlined  above. 

The  detailed  data  (which  cannot  be  reproduced  here  for  lack  of  space)  are 
on  file  at  the  Woods  Hole  Oceanographic  Institution. 

ACKNOWLEDGMENTS 

Mr.  O.  E.  Sette,  Dr.  Roderick  Macdonald,  and  Dr.  George  L.  Clarke  col- 
laborated in  identifying  the  plankton  caught  during  1929,  Miss  Alice  Beale  in 
identifying  that  caught  during  1930  and  part  of  1931.  And  Dr.  Th.  von  Brand 
has  assisted  us  by  making  the  ether  extracts  and  the  weighings  used  in  our  dis- 
cussions of  the  area  as  a  feeding  ground  for  plankton-feeding  fish. 

GEOGRAPHIC  LIMITS  AND  SUBDIVISIONS 
OF  THE  AREA 

The  account  is  confined  to  the  continental  shelf,  out  to  the  200-meter  con- 
tour, from  the  ofl[ing  of  Martha's  Vineyard,  westward  and  southward  to  about 
Latitude  36°.  One  cruise  only  (February  1931)  extended  south  of  Cape  Hat- 
teras.  Neither  was  the  number  of  hauls  seaward  from  the  200-meter  contour 
large  enough,  for  any  general  estimate  of  plankton  volumes  in  the  slope  water. 

For  convenience  in  regional  comparisons,  we  have  arbitrarily  divided  the 
area  into  a  northern  sector  north  from  (and  including)  the  Atlantic  City  profile, 
a  southern  sector,  south  of  the  latter,  an  inshore  belt  extending  30  miles  out  from 
the  coast,  and  an  offshore  belt,  thence  seaward  to  the  200-meter  contour  (Fig.  1). 

HISTORY  AND  SOURCES  OF  INFORMATION 

So  far  as  we  have  been  able  to  learn,  the  first  published  account  of  the 
zooplanktonic  communities  of  the  region  in  question  was  Rathbun's  (1889)  re- 
port on  tow-net  catches  made  in  April-May  1887,  in  connection  with  the  mackerel 
investigations  of  that  year.  During  subsequent  years,  several  notices  appeared 
of  the  occurrences  of  individual  planktonic  groups  for  restricted  localities,  e.g., 
of  the  copepods  of  the  New  Jersey  coast  (Fowler,  1912),  of  medusae,  ctenophores, 
copepods,  and  amphipods,  etc.,  of  the  Woods  Hole  region  (Fish,  1925;  Hargitt, 
1905;  Hohnes,  1905;  Mayer,  1912;  ^Vlieeler,  1901;  Wilson,  1932),  of  copepods 


BIGELOW   AND    SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  191 


Fig.    1.  Charts  of  the  area:  A,  Locations  of  profiles;  B,  Sub-divisions  of  the  area  as  employed  in  the 
present  report. 


192  memoir:  museum  of  comparative  zoology 

and  other  groups  for  Chesapeake  Bay  (Cowles,  1930;  Wilson,  1932a).  But  it  was 
not  until  1913  that  any  systematic  survey  of  the  plankton  of  the  continental  shelf 
was  undertaken.  In  that  summer,  the  U.  S.  Fisheries  Schooner  "Grampus"  col- 
lected samples  at  a  grid  of  stations  between  the  offings  of  Cape  Cod  and  of 
Chesapeake  Bay,  as  well  as  in  the  Gulf  of  Maine  (Bigelow,  1915).  And  she  sur- 
veyed these  same  waters  again  in  1916,  both  in  August  and  in  November  (Bige- 
low, 1922).  No  data  had,  however,  been  obtained  for  any  other  year  until  1929, 
when  "Albatross  11"  carried  out  cruises  in  April,  May,  June,  and  July.  And  these 
were  repeated  in  the  following  months  and  years: 

1930 — February,  April  (2  cruises).  May,  June  (2  cruises),  July; 

1931 — February,  May,  June,  July; 

1932 — February,  May  (4  cruises),  June  (3  cruises). 
"Atlantis"  of  the  Woods  Hole  Oceanographic  Institution,  also  made  a  cruise  in 
October  1931.    And  periodic  collecting  was  carried  out  from  the  Institution, 
both  with  a  pump-filter  and  with  tow  nets,  at  a  station  a  few  miles  off  Martha's 
Vineyard,  from  late  June  1935  to  September  1936  (Clarke  and  Zinn,  1937). 

We  have  drawn  freely  from  all  of  the  foregoing.  But  the  collections  made 
by  the  U.  S.  Bureau  of  Fisheries  during  the  years  1929-1932,  as  outlined  above 
have  been  the  chief  basis  for  the  present  report,  which  forms  a  sequel  to  earlier 
accounts  of  the  temperature  and  of  the  salinity  of  the  same  region  (Bigelow, 
1933;  Bigelow  and  Sears,  1935). 

METHODS 
Collection 

The  observing  stations  (with  few  exceptions)  were  located  along  the  profiles 
indicated  on  Figure  1,  so  spaced  that  on  each  profile  the  inshore  belt,  the  mid- 
belt,  and  the  outer  edge  of  the  shelf  were  sampled  at  points  seldom  more  than 
15-20  miles  apart,  and  often  much  closer  (for  position  of  stations,  etc.,  see  Bige- 
low, 1933,  p.  104).  The  plan  was  for  observations  to  be  made  at  approximately 
the  same  locaUties,  on  all  the  cruises,  and  this  was  adhered  to  as  far  as  practicable, > 
six  hundred  and  four  plankton  stations  on  the  continental  shelf  being  occupied 
in  all,  377  of  them  in  the  northern  sector,  227  in  the  southern,  330  in  the  inshore 
belt,  and  274  offshore. 

Most  of  the  hauls  made  by  "Atlantis"  in  October  1931  were  vertical,  those 
by  "Albatross  H",  in  1929,  horizontal,  at  the  surface,  and  (except  when  in  very 

'  Bad  weather  sometimes  interfered,  and  some  of  the  cruises  were  abbreviated. 


BIGELOW   AND    SEARS:   NORTH   ATLANTIC    ZOOPLANKTON    STUDIES  193 

shallow  water,  or  when  the  sea  was  very  rough)  at  one  or  more  subsurface  levels 
at  each  station.  In  subsequent  years,  on  "Albatross  11",  oblique  hauls  were  em- 
ployed to  obviate  the  chief  shortcoming  of  horizontal  towing,  namely,  that  it 
may  be  a  matter  of  chance  whether  the  net  hits  or  misses  the  strata  where  the 
planktonic  population  is  richest  or  poorest." 

Ideally,  hauls  of  this  type  should  be  made  by  lowering  the  net  close  to  the 
bottom  and  by  towing  in  a  diagonal  direction  up  to  the  surface.  This,  however, 
is  seldom  practical,  the  best  substitute  being  to  tow  in  a  series  of  short  horizontal 
steps  at  frequent  intervals  from  bottom  to  surface. 

The  standard  procedure  in  the  present  case  (seldom,  however,  attained  in 
Mo),  was  to  tow  for  two  minutes  at  each  five  meter  level,  the  hauls  often  being 
abbreviated  to  one  meter  of  towing  at  every  ten  meter  level,  at  the  deep  stations, 
in  order  to  shorten  the  time  required.  The  column  strained  in  this  way  diverged, 
on  the  average,  by  about  15°  from  the  horizontal.  At  a  towing  speed  averaging 
1.2  knots  or  37  meters  per  minute  (as  observed  repeatedly),  the  obUque  column 
fished  through  was  thus  about  370-518  meters  long,  at  stations  where  the  vertical 
depth  was  20-30  meters,  or  about  777  meters  where  the  depth  was  200  meters. 
The  chief  drawback  to  hauls  of  this  sort,  from  near  bottom  to  surface,  is  that  they 
give  no  information  as  to  the  degree  of  stratification  of  the  communities  at  differ- 
ent levels.  To  meet  this  difficulty,  two  or  more  nets  were  attached  on  the  wire 
at  a  time,  20-35  meters  apart,  on  the  cruises  subsequent  to  February  1931.  Un- 
fortunately, a  considerable  proportion  of  the  hauls  left,  unsampled,  a  stratum 
next  the  bottom,  equalling  as  much  as  50%  of  the  whole  vertical  distance  in  ex- 
treme cases. 

The  nets  were  either  1-meter,  or  J^-meter  in  diameter,  of  silk,  the  forepart 
with  29-38  meshes  per  linear  inch,  the  rear  part  with  48-54  meshes  per  inch. 
Nets  of  these  meshes  and  diameters  may  be  expected  adequately  to  sample 
planktonic  animals  of  the  various  groups  from  the  size  of  the  copepod,  Centro- 
pages,  up  to  fish  fry.  Many  or  most  of  the  smaller  adult  copepods  (Oithona), 
smaller  larval  copepods  of  all  species,  and  other  minute  animals  pass  through. 
And  failure  to  sample  these  is  mentioned  repeatedly  in  the  following  pages. 

Measurements  and  Calculations 

Dry  weight  would  be  the  most  reliable  index,  easily  obtainable,  to  the  mass 
of  the  plankton,  if  the  desiccation  and  weighing  could  be  done  soon  after  the  col- 

'  For  a  recent  discussion  of  the  advantages  of  oblique  towing,  see  Walford,  1938. 


194  memoir:  museum  of  comparative  zoology 

lections  were  made.  But,  for  so  large  a  number  of  samples,  this  would  have  re- 
quired much  more  assistance  than  was  available.  And  so  much  of  the  oil  from 
copepods — and  other  substances  as  well,  both  from  these  and  from  other  groups — 
dissolves  out  into  the  preservative  that  long  preservation  of  the  samples  robs 
dry  weighing  of  much  of  its  initial  advantage  over  volumetric  measurement, 
which  is  a  much  simpler  procedure. 

We  may  also  point  out  that  "volume"  is  usually  translatable  into  "wet 
(preserved)  weight"  within  a  reasonable  limit  of  error,  for  while  the  specific 
gravities  of  different  groups  of  planktonic  animals  differ  considerably — both  in 
life  and  after  preservation  (e.g.,  as  between  shelled  pteropods  and  ctenophores), 
this  is  comparativel}^  constant  within  each  of  the  major  groups. 

Selected  samples,  from  the  collections  of  1929,  showed  the  following  weight- 
volume  relationship,  when  weighed  in  water: 

Calanus,  20  c.c.  by  displacement;  "wet  weight,"  18.35  gms; 

Sagitta  elegans,  20  c.c.  by  displacement;  "wet  weight,"  21.34  gms; 

Limacina,  20  c.c.  by  displacement,  "wet  weight,"  20.18  gms. 

The  method  of  volumetric  measurement  that  has  most  often  been  used  in 
the  past  consists  simply  of  allowing  the  catch  to  settle  for  a  given  length  of  time, 
in  a  graduated  cylinder  of  convenient  size,  and  of  then  measuring  its  bulk.  It 
has,  however,  been  universally  appreciated  that  the  resultant  measurements 
have  only  a  comparative  value,  because  they  include  the  interspaces  between  the 
animals,  as  well  as  the  latter  themselves.  Savage  (1931,  p.  5)  has,  in  fact,  shown 
that  such  measurements  may  average  more  than  twice  as  large  as  those  obtained, 
for  the  same  samples  by  the  so-called  displacement  method.'  And  comparative 
tests,  that  we  have  made  for  the  entire  series  of  "Albatross  II"  catches  for  the 
year  1929,  have  similarly  shown  a  wide  disparity,  with  volumes  averaging  2-4 
times  as  large  by  "settlement"  as  by  "displacement,"  for  all  types  of  plankton 
combined,  the  difference  being  greatest  for  plankton  dominated  by  sagittae 
(extreme  case,  105  c.c.  by  displacement;  935  c.c.  by  settlement),  least  for  plank- 
ton consisting  chiefly  of  Calanus  (210  c.c.  by  displacement ;  260  c.c.  by  settlement). 
All  measurements  in  the  present  report  have,  therefore,  been  made  by  displace- 
ment as  follows: 

The  sample  of  plankton  is  first  drained,  through  a  bolting  silk  strainer  with 
meshes  as  fine  as  that  of  the  nets  in  which  the  catch  was  taken.  The  semi-dried 
mass  is  then  added  to  a  known  volume  of  water,  when  the  resultant  increase  in 

1  See  Johnstone,  1908,  p.  130  for  a  general  discussion  of  these  methods. 


BIGELOW  AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON    STUDIES  195 

volume  is  equal  to  that  of  the  sample,  plus  the  few  drops  of  liquid  that  may  still 
adhere  to  the  latter  after  draining. 

In  the  preliminary  catches  of  1929,  the  respective  percentages  of  the  several 
constituent  species  or  groups  were  determined  either  after  these  had  been  sorted 
out,  or  in  some  cases,  roughly  estimated.  In  the  subsequent  catches,  the  volumes 
of  the  species  represented  were  arrived  at  as  follows: 

The  total  catch  was  first  sorted  into  such  of  its  constituent  parts  as  were 
most  readily  separable,  e.g.,  into  the  chaetognaths,  euphausiids,  larger  medusae, 
adult  amphipods,  pteropods,  and  "residue,"  this  last  consisting  chiefly  of  cope- 
pods,  with  other  forms  of  similarly  small  size.  Each  of  these  groupings  was  then 
measured  by  the  same  displacement  method  as  used  in  obtaining  the  total  volume. 
A  random  and  well  mixed  sample  of  at  least  100  specimens'  was  then  taken  from 
each  grouping,  and  the  number  of  specimens  counted,  for  each  species  repre- 
sented. In  order  next  to  calculate  the  proportional  volumes  of  the  several  species 
included  in  each  grouping,  and  so  to  arrive  at  the  total  volume  of  each  species  in 
the  total  catch,  it  was  necessary  to  weight  each  according  to  the  average  size  of 
its  members.  In  the  cases  of  Calanus  finmarchicus,  Centropages  typicus,  and 
Thysanoessa  inermis,  the  weightings  were  based  on  actual  measurements  of  the 
volume  per  100  specimens  of  large  series  of  adults,  covering  the  average  size 
range.  Volumetric  comparison  with  these,  by  eye,  then  gave  a  rough  ratio  for 
weighting  the  younger  stages  of  these  same  species,  as  well  as  for  the  other  species. 

The  following  actual  example  may  serve  more  graphically  to  illustrate  the 
procedure : 

Station,  Montauk  V,  June  12,  1930,  total  volume  of  catch,  60  c.c,  or  480  c.c. 
per  20  minutes  towing  with  a  1-meter  net;  volume  of  "residue,"  59  c.c,  or  472 
c.c,  per  standard  tow;  number  of  individuals  of  each  species  represented  among 
119  counted  specimens  in  sample  of  "residue",  61  Calanus  finmarchicus,  1  Cen- 
tropages typicus,  52  Metridia  lucens,  5  Pseudocalanus  minutus.  Weights  derived 
as  above,  Calanus,  25;  Centropages,  4;  Metridia,  25;  Pseudocalanus,  2.  Calcu- 
lated percentages  (volumetric),  in  "residue",  Calanus,  53.7%;  Centropages, 
0.1%;  Metridia,  45.7%;  Pseudocalanus,  0.3%.  The  volume  of  "residue"  being 
472  c.c  for  the  standard  haul,  the  calculated  volumes  for  the  several  species 
work  out  at  253  c.c.  of  Calanus;  <  1  c.c  of  Centropages;  216  c.c.  of  Metridia,  and 
1  c.c.  of  Pseudocalanus. 

Comparison  of  catches  made  in  horizontal  hauls  at  the  surface  and  deeper 

'  Fewer,  if  there  were  not  that  many. 


196  memoir:  museum  of  comparative  zoology 

in  1929,  and  in  oblique  hauls  through  shoaler  and  deeper  sections  of  the  water 
column  in  1931  and  1932,  afford  some  information  as  to  the  degree  of  stratifica- 
tion of  the  plankton  in  different  months.  Since  open  nets  were  used,  it  is  obvious 
that  the  volumes  taken  in  the  deeper  tows  represent  not  only  the  abundance  pre- 
vailing at  the  towing  level,  but  also  whatever  was  caught  while  the  nets  were 
being  lowered  and  hauled  in  again.  And  this  contamination  might  well  be  great 
enough  entirely  to  obscure  the  picture  in  individual  case.s — if,  for  example,  it 
chanced  that  the  net  was  hauled  up  through  a  dense  swarm  of  one  organism  or 
another.  But  at  most  of  the  stations,  inside  the  200-meter  contour,  the  vertical 
sectors  averaged  less  than  1/7  as  long  as  the  horizontal  sectors  in  the  cases  of  the 
horizontal  hauls,  and  only  about  1/15  as  long  as  the  horizontal  sectors  in  the 
cases  of  obliques.  It  may,  therefore,  be  assumed  that  this  contamination  is  not 
an  important  factor  for  our  purposes  in  these  shoal  waters,  when  considerable 
numbers  of  catches  are  averaged.  Consequently,  we  have  not  attempted  to 
correct  for  it.' 

In  order  to  render  the  catches  made  in  horizontal  tows  comparable  (from  the 
standpoint  of  depth)  with  those  made  in  obliques,  we  have  further  assumed  that 
the  catches  of  the  latter  may  be  accepted  as  representative  of  the  mid-depths  of 
the  water  columns  sampled.  And  the  depths  stated  in  the  following  discussion 
are  so  derived. 

We  have  credited  a  value  of  1  to  catches  <  1  c.c,  in  the  calculations  of  average 
volumes  and  ratios,  while  in  the  case  of  the  latter,  we  have  also  treated  values  of 
0  as  equivalent  to  1  c.c,  for  the  sake  of  simplicity.  Likewise,  in  the  tables  giving 
percentages  and  abundance,  a  dash  signifies  that  the  particular  subdivision  was 
not  visited  on  a  particular  cruise ;  a  0  that  the  species  was  not  detected  in  that 
particular  subdivision,  though  taken  in  another;  a  blank  that  it  was  not  detected 
in  any  subdivision  on  a  particular  cruise. 

Expression  of  Results 

The  fact  that  the  catches  were  taken  in  nets  of  different  sizes,  as  well  as  in 
hauls  of  different  lengths,  some  obUque,  some  vertical,  and  some  horizontal, 
makes  it  necessary  to  reduce  all  the  measurements  to  one  common  basis  in  order 
to  render  the  results  comparable,  one  with  another. 

The  elements  on  which  such  a  reduction  must  be  based  for  any  particular 
haul  are:—  (a)  the  diameter  of  the  net  used,  (b)  the  duration  of  the  haul  in  time, 
(c)  the  average  speed  of  the  vessel  (or  else  the  linear  extent  of  the  haul),  and  (d) 

'  For  a  case  of  such  correction,  see  Bigelow  and  Sears,  1937,  p.  69. 


BIGELOVV   AND    SEARS :    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES  197 

the  efficiency  of  the  purticuhir  net  used.  In  tlie  cases  of  horizontal  and  oblique 
hauls,  the  first  two  elements  are  precisely  known,  hence  the  first  reduction  is  to 
"catch  per  unit  time  per  unit  net-opening";  the  standards  here  adopted  being  20 
minutes  of  hauling  with  a  net  1  meter  in  diameter.  Up  to  this  point,  the  reduction 
is  precise  mathematically.  But  it  does  not  yet  provide  a  standard  of  comparison, 
unless  "time"  can  be  translated  directly  into  "length  of  water  column  fished". 
Close  attention  was,  therefore,  paid  to  the  speed  of  the  ship,  on  all  the  cruises  of 
"Albatross  11"  and  of  "Atlantis",  and  records  by  R.P.M.  of  the  propeller,  and 
by  ship's  log,  show  that  this  was  close  to  1.2  knots  for  the  series  as  a  whole.  But 
whoever  has  had  experience  with  the  differences  in  the  velocities  and  directions 
of  currents  at  different  levels  in  coastwise  waters  where  the  circulation  is  governed 
by  the  tide,  will  appreciate  that  the  rate  at  which  a  ship  is  moving,  relative  to  the 
surface  water  may  differ  considerably  from  the  rate  of  a  net  relative  to  the  water 
at  some  deeper  level.  Rather  than  attempt  the  calculation  of  the  linear  extent 
of  each  individual  tow,  we  have,  therefore,  thought  it  preferable  to  assume  an 
average  speed  of  1.2  knots  in  all  the  calculations,  and  the  catches  of  the  horizontal 
and  oblique  hauls  were  reduced  to  the  common  standard  accordingly. 

On  this  basis,  the  linear  extent  of  the  standard  tow  of  20  minutes  would 
average  741  meters.  And  there  would  be  as  good  justification  for  expressing  the 
volumes  caught  "per  unit  volume  of  water"  as  "per  unit  time,"  for  the  one  of 
these  expressions  includes  the  same  probable  errors  as  the  other.  But  we  must 
impress  upon  tlie  reader  that  neither  of  these  expressions  is  susceptible  of  direct 
translation  into  "volume  of  plankton  present,"  whether  per  unit  time  or  per  unit 
volume  of  water,  because  of  uncertainty  as  to  the  efficiency  of  the  nets  (discussed 
on  p.  198).  Hence,  to  avoid  any  possible  misconception  as  to  the  rehabihty  of  the 
observations,  we  have  endeavored  to  draw  a  sharp  distinction,  in  this  respect 
throughout  the  descriptive  sections  of  the  present  report.  It  has  also  been  sug- 
gested to  us  that  "catch  per  unit  volume  of  water"  might  suggest  a  higher  degree 
of  precision  than  "catch  per  unit  tow",  whereas  actually  the  two  are  interchange- 
able.   We  have,  therefore,  adopted  the  latter. 

All  the  results,  then,  are  expressed  as  catch,  in  c.c.  per  20  minutes  towing 
with  a  1-meter  net  at  an  assumed  speed  of  1.2  knots  (2222  meters  per  hour), 
unless  otherwise  stated. 

VALIDITY  OF  CALCULATED  RESULTS 
No  modern  student  of  plankton,  we  fancy,  would  claim  that  quantitative  cal- 
culations, based  on  tow  nettings,  can  be  any  more  than  approximations  to  the  truth. 


198  memoir:  museum  of  comparative  zoology 

To  begin  with,  we  must  face  what  may  be  termed  the  "catching  error"  in- 
herent in  the  tow  net  method.  We  have  no  intention  of  reviving  the  controversies 
as  to  the  rehabiUty  of  the  latter  that  gave  planktonologists  so  much  concern  dur- 
ing the  last  quarter  of  the  nineteenth  century.  It  seems  pertinent,  however,  to 
remind  the  reader  that  a  tow  net,  of  the  usual  conical  form,  and  of  mesh  appropri- 
ate for  the  capture  of  planktonic  organisms,  filters  somewhat  less  than  the 
amount  of  water  that  would  pass  through  a  simple  hoop  of  equal  diameter,  the 
loss  (i.e.,  the  amount  regurgitated)  depending  on  the  shape  of  the  net,  on  the 
proportionate  areas  occupied  by  the  threads  and  by  the  spaces  between  the 
latter,  and  on  the  pressure,  i.e.,  on  the  velocity  with  which  the  net  is  drawn 
through  the  water.  The  slower  the  towing,  the  more  complete  filtration,  as  every 
planktonologist  knows  from  observation.  In  the  case  of  fine  meshed  silk  (e.g., 
#20) ,  the  loss  may  be  so  serious  at  ordinary  towing  speeds  as  to  necessitate  special 
types  of  net,  to  increase  the  filtering  surface  relative  to  the  mouth  opening.  And 
even  with  silk  as  coarse  meshed  as  #0  (38  meshes  per  linear  inch),  such  as  used  in 
the  fore  parts  of  our  nets,  only  about  30%  as  much  water  would  pass  through  a 
given  area,  drawn  transversely  at  2  knots,  as  through  an  open  cyhnder  of  the 
same  area,  according  to  the  relationship  between  pressure  and  filtration  in 
Hensen's  (1895)  experiments^ 

This  loss  is  so  minimized  by  the  increase  in  straining  area  relative  to  mouth 
opening,  resulting  from  the  conical  form,  that  it  probably  averaged  less  than  10%, 
at  the  usual  towing  speed  for  our  nets,  though  experiments  at  various  hands  have 
proved  that  the  catches  made  in  parallel  hauls  with  similar  nets  may  vary  as 
much  as  10-30%  or  even  more  (see  Winsor  and  Walford,  1936,  p.  190,  for  a 
recent  discussion).  Neither  is  clogging  likely  to  be  serious,  for  nets  such  as  those 
used,  except  on  rare  occasions  when  diatoms  or  Phaeocystis  may  swarm,  or  when 
the  gelatinous  bodies  of  ctenophores,  appendicularians,  etc.,  may  block  the 
meshes.  Much  more  serious  is  the  uncertainty  as  to  what  relation  the  fraction 
of  the  plankton  that  is  adequately  sampled  bears  to  the  remainder  that  the  net 
fails  to  capture,  it  being  common  knowledge  that  no  one  type  of  net  will  equally 
well  sample  the  various  size  categories  of  planktonic  animaLs.  The  nets  used  in 
the  present  studies  (meshes,  38-54  per  linear  inch)  being  comparatively  coarse, 
the  failure  of  small  copepods  such  as  Oithona,  or  the  young  of  others,  to  figure 
more  largely  in  the  following  volumetric  lists,  does  not  necessarily  mean  that 


'  Pressure  at  the  given  velocity  calculated  from  the  formula,  . 

"      •'  2g 


/v« 

ila,  p  =  \/  —     . 
V  2k 


BIGELOW  AND    SEARS :   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  199 

they  may  not  actually  have  been  present  in  considerably  greater  abundance 
(numerically,  at  least)  than  the  catches  would  suggest. 

In  extended  surveys,  errors  also  creep  in  through  the  fact  that  it  is  necessary 
to  classify  the  hauls  by  the  time — i.e.,  by  the  duration  of  the  tow — ,  for  in  most 
cases,  no  precise  measure  of  the  speed  of  the  net  through  the  water,  i.e.,  the  dis- 
tance, is  available,  as  already  remarked  (p.  196). 

WTiile  errors  of  the  sorts  just  mentioned  may  reach  extreme  proportions  in 
individual  cases,  they  can  usually  be  minimized  by  combining  a  sufficiently  ex- 
tensive series  of  data.  More  vital  is  the  question  whether  the  grid  formed  by  the 
stations,  in  any  particular  plankton  survey,  has  been  close  enough  to  warrant 
generalization  for  the  included  area  as  a  whole.  In  this  respect,  the  consistency 
of  results  seems  to  us  sufficient  warrant,  as  already  argued  by  Walford  (1938)  for 
a  similar  survey  carried  out  on  George's  Bank. 

If  these  shortcomings  of  various  kinds  should  chance  to  be  cumulative  for  a 
given  haul,  the  calculated  volume  for  the  latter  may  very  likely  be  as  much  as 
100%  in  error — perhaps  more.  But,  when  so  many  observations  are  in  hand,  the 
plus  or  minus  error  no  doubt  averages  much  less  than  this — perhaps  not  more 
than  20-30%,  which  is  far  smaller  than  the  variations  observed  among  the  values 
that  form  the  basis  of  study.  And  since  the  latter  also  show  very  clear  consistency, 
both  regional  and  secular,  not  only  for  the  volumes  of  plankton  as  a  whole,  but 
also  for  the  more  abundant  of  the  constituent  species,  we  think  no  further  argu- 
ment is  needed  in  justification  of  our  conclusion  that  the  picture  they  have  yielded 
may  be  accepted  as  representative  (within  reasonable  limits)  of  the  larger  zoo- 
plankton  from  place  to  place  within  the  area,  from  season  to  season,  and  from 
year  to  year,  for  the  period  1930-1932. 

The  basis  for  comparison  between  this  group  of  years  and  the  year  1929  is 
not  so  soUd,  because  of  the  use  in  that  year  of  horizontal  hauls.  The  best  we  can 
do,  in  this  case,  is  to  assume  that  the  average  catch,  of  the  two  or  more  of  these 
that  were  made  at  different  levels  at  each  station,  at  least  approximates  the  cor- 
rect average  for  the  water  column  as  a  whole,  bottom  to  surface.  It  is  probable 
that  this  is  close  to  the  truth  through  the  period  February- April,  i.e.,  before  any 
marked  stratification  has  developed.  But  from  June  on,  as  the  plankton  like  the 
temperature  tends  to  become  increasingly  stratified,  it  becomes  increasingly 
doubtful  how  thick  the  stratum  is,  for  which  the  yield  of  a  net  working  horizon- 
tally, at  any  particular  level,  can  be  accepted  as  representative. 


200 


memoir:  museum  of  comparative  zoology 


PART  I.    THE  VOLUME  OF  THE  ZOOPLANKTON  AS  A  UNIT 
The  Water  Column  as  a  Whole 

The  volumes  in  c.c.  per  standard  haul  for  each  month  of  the  series  are  shown 
on  Figures  2-6,  8-9,  and  average  and  maximum  volumes  for  the  several 
subdivisions  in  the  following  table : 


Month 

Year 

Inshore 

Offshore 

North 

South 

Total  area 
Surveyed 

Av. 

Max. 

Av. 

Max. 

Av. 

Max. 

Av. 

Max. 

Av. 

Max. 

February 

1930 
1931 
1932 

249 
218 
550 

1000 

432 

2400 

49 

109 

83 

128 
140 
306 

Ill 
57 
57 

497 

108 

91 

175 
200 

472 

1000 

432 

2400 

144 
153 
368 

1000 

432 

2400 

April 

1929 
1930 

203 
302 

520 
1124 

349 
322 

1701 
1026 

312 
396 

520 
1124 

224 
244 

1701 
700 

263 
199 

1701 
1124 

May 

1929 
1930 
1931 
1932 

243 

467 
385 
274 

1448 
1294 

821 
771 

326 

1026 

622 

329 

805 
3288 
1880 
1026 

334 

689 
507 

285 

805 
1847 
1880 
1026 

198 
746 
436 
320 

1448 

3288 

821 

746 

273 
714 
487 
299 

1448 
3288 
1880 
1026 

June 

1929 
1930 
1931 
1932 

174 
430 
535 
232 

541 

1712 

978 

478 

294 
311 

777 
302 

447 

789 

1756 

790 

249 
420 
654 

228 

541 
1712 
1756 

467 

168 
242 
578 
333 

514 

900 

1373 

790 

219 
381 
626 
261 

541 
1712 
1756 

790 

July 

1929 
1930 
1931 

296 
600 
702 

550 
1681 
1410 

211 
341 
912 

474 

800 

2341 

285 
448 
782 

550 

1681 
2341 

203 

512 

257 

448 
782 

5501 
1681 
2341 

October 

1931 

— 

— 

234 

560 

236 

331 

231 

560 

234 

560 

'  One  haul  of  67109  c.c.  omitted. 

The  most  striking  feature  of  volumetric  distribution  throughout  has  been 
the  irregularity  from  station  to  station,  often  with  volumes  differing  up  to  a 
hundred  fold,  between  localities  only  a  few  miles  apart.  As  a  concrete  illustration, 
we  may  instance  the  fact  that  the  maximum  volume  was  3  times  as  large  as  the 
minimum,  even  on  the  cruise  (May  24-28,  1932)  when  the  difference  between  the 
two  was  smallest.  And  on  the  occasion,  when  widest  (July  10-18,  1930),  the 
range  was  373  to  one. 

Comparatively  regular  gradients  nevertheless  appear,  both  secular  and 
regional,  when  averages  are  compared,  for  subdivisions  large  enough  to  include 
several  stations  each,  as  follows: 


Seasonal  Cycle 

February.     February  may  be  chosen  as  the  starting  point  in  our  survey,  it 
being  highly  probable  (though  not  yet  actually  proven)  that  the  zooplanktonic 


BIGELOW   AND    SEARS:    NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


201 


community  in  the  waters  in  question,  as  in  boreal  seas  in  general,  is  at  its  lowest 
ebb,  at  the  end  of  winter,  or  in  early  spring. 

The  three  surveys  for  this  month  (1930,  1931,  1932)  agree  in  showing  the 


Fig.    2.  Volumes  of  plankton,  per  standard  haul:  A,  February  5-13,  1930  and  February  13-March  5, 
1931  (underlined);  B,  February  10-March  1,  1932,  contour  lines  100,  500,  1000,  1500,  2000  c.c. 

volumes  as  largest  over  the  inner  half  of  the  shelf,  and  to  the  south,  decreasing 
offshore  and  to  the  north  (Fig.  2) ,  though  with  great  irregularity  from  station  to 
station,  as  is,  in  fact,  the  case  throughout  the  year. 


202  memoir:  museum  of  comparative  zoology 

The  magnitude  of  this  inshore-offshore  gradient  in  winters  of  normal  tem- 
perature may  be  illustrated  by  the  fact  that,  of  the  37  tows  made  in  1930  and 
1931,  the  average  of  the  ten  closest  to  shore  was  294  c.c,  four  yielding  more  than 
250  c.c,  with  fourteen  hauls  over  the  mid-belt  of  the  shelf  averaging  104  c.c, 
whereas  thirteen  along  the  continental  edge  averaged  only  72  c.c,  with  four  alone 
of  the  latter  yielding  as  much  as  100  c.c.  The  plankton  thus  averaged  roughly 
twice  as  voluminous  inshore  as  along  the  mid-belt  of  the  shelf,  four  times  as 
voluminous  as  along  the  outer  edge  of  the  latter.  And  in  most  cases  the  inshore- 
offshore  relationship  was  of  this  same  order  along  individual  profiles,  though 
with  occasional  exceptions  as  in  1930  off  Cape  May,  where  the  volume  was  136 
c.c.  at  the  inshore  station,  but  191  c.c  at  the  station  next  seaward,  and  again 
in  1931,  off  Martha's  Vineyard  where  the  catch  closest  to  land  was  only  35  c.c, 
but  108  c.c  farther  out. 

The  contrast  prevaihng  at  this  season  between  small  volumes  in  the  north- 
eastern sector  (bounded  by  the  New  York  profile)  and  large  in  the  southern  is 
even  more  striking,  February  volumes  having  averaged  only  about  1/3  as  great 
for  the  former  as  for  the  latter  in  1930  and  in  1931  combined,  and  l/lO  as  great 
in  1932.  Furthermore,  no  catch  as  great  as  200  c.c.  was  made  in  the  eastern  sector 
in  any  February,  whereas  fourteen  such  February  catches  were  recorded  to  the 
south  and  west.  But  this  abundance  seems  not  to  extend  south  of  Cape  Hatteras 
"which  may  be  regarded  as  the  southern  boundary  in  winter  to  the  cold  boreal 
water"  (Bigelow,  1933,  p.  11),  tows  made  in  1931  in  the  vicinity  of  the  Cape 
having  yielded  only  15,  80,  92,  and  205  c.c.  respectively. 

April.  The  sequence  of  events  in  the  development  of  the  plankton,  with 
the  advance  of  spring,  is  obscured  in  our  data  by  the  long  period  that  elapsed  be- 
tween the  first  and  second  surveys  of  each  year,  no  coUectioas  having  been  made 
in  March.  In  1930,  which  we  must  perforce  accept  as  representative,  being  the 
only  year  when  collections  were  made  both  in  February  and  in  April,  average 
volumes  had  increased  about  eight  or  nine  fold  from  the  one  month  to  the  other 
over  the  offshore  belt,  northward  and  eastward  from  the  Barnegat  profile,  where 
the  catch  averaged  325  c.c.  in  early  April,  contrasting  with  39  c.c.  in  February. 
In  the  southern  sector,  however,  the  volume  of  plankton  still  averaged  about  the 
same  order  of  magnitude  in  April  (244  c.c)  as  it  had  in  February  (175  c.c). 

This  combination  of  relatively  stationary  conditions  in  the  south,  with 
marked  augmentation  in  the  north  resulted — in  the  year  in  question — in  a  re- 
versal of  the  north-south  relationship  that  existed  in  February,  so  that  by  early 
April,  the  volume  of  plankton  averaged  nearly  three  times  as  great  (419  c.c) 


BIGELOW   AND    SEARS:   NORTH   ATLANTIC   ZOOPLANKTON    STUDIES  203 

eastward  from  the  New  York  profile,  as  south  of  the  hitter  (157  c.c),  instead  of 
only  about  1/3  as  great,  as  it  had  at  the  end  of  the  winter,  this  being  a  case  where 
the  north-south  gradient  would  have  been  hidden,  had  the  comparison  been 
made  between  sectors  separated  by  the  boundary  (Atlantic  City  profile)  that 
has  usually  proved  significant. 

Unfortunately,  it  was  not  possible  to  include  the  immediate  offing  of  Chesa- 
peake Bay  in  this  calculation,  lacking  an  oblique  haul  there  in  that  April.  A  very 
rich  surface  catch  (1200  c.c.)  was  made  there,  it  is  true,  in  that  month,  but  we 
have  no  information  as  to  whether  the  average  volume  had  or  had  not  increased 
at  this  particular  locahty  meantime.  And  volumes  for  April  similarly  averaged 
larger  in  the  northern  sector  than  in  the  southern  in  1929,  as  well. 

Vernal  augmentation  spread  southward  in  1930,  between  the  first  and  third 
weeks  of  April  as  far  as  the  ofRng  of  Chesapeake  Bay,  causing  a  five-fold  increase 
since  February,  along  the  mid-belt  of  the  shelf,  as  illustrated  by  the  expanding 
outlines  of  the  area  where  the  catches  were  greater  than  500  c.c.  (Fig.  3A,  B). 
And  there  is  evidence  of  some  slight  alteration  of  the  same  order  to  the  south  of 
the  Bay  as  well,  where  the  late  April  average  was  181  c.c.  (6  stations,  oblique 
hauls),  contrasting  with  98  c.c.  (3  stations,  1  oblique,  2  horizontal  hauls),  early 
in  the  month.  Consequently,  the  plankton  averaged  much  the  richest  along  the 
mid-belt  of  the  shelf  by  mid-April  of  that  year  (16  stations,  average,  496  c.c.),' 
whereas  the  coastal  waters — which  had  been  richest  in  February — were  now  rela- 
tively barren  (8  stations,  average,  192  c.c),  as  was  also  the  case  along  the  con- 
tinental edge  (8  stations,  average,  133  c.c). 

The  facts,  (a)  that  the  richest  aggregations  recorded  for  February  and  April 
(Fig.  2,  3)  have  not  been  at  the  most  easterly  stations,  and  (b)  that  the  plankton 
of  the  Gulf  of  Maine  is  on  the  whole  sparse  in  late  winter  and  early  spring  (Bige- 
low,  1926;  Fish  and  Johnson,  1937),  combined  with  the  vernal  histories  of  the 
dominant  species,  individually,  is  suflficient  evidence  that  in  years  when  volumes 
increase  significantly  in  early  spring,  this  results  chiefly  from  local  reproduction, 
not  from  immigration  from  waters  farther  to  the  east.  Neither  have  we  any  evi- 
dence of  mass  immigration  from  oiTshore,  or  from  the  south,  at  this  season. 

May.  In  one  of  the  years  (1930)  when  April  can  be  compared  with  May, 
the  area  that  had  been  well  populated  (  >  500  c.c.)  during  the  earlier  month,  had 
expanded  seaward  to  the  continental  edge,  by  the  latter,  all  along  from  the  oflfing 
of  Chesapeake  Bay  to  that  of  Montauk  (though  apparently  no  farther  eastward), 

'  In  early  April,  only  two  stations  yielded  more  than  500  c.c.  (Fig.  3A),  both  of  them  east  of  New  York. 
In  late  April  this  was  the  case  at  nine  stations  scattered  as  far  south  as  Chesapeake  Bay. 


204 


memoir:  museum  of  comparative  zoology 


Fig.    3.  Volumes  of  plankton,  per  standard  haul,  contour  lines  for  100,  500,  1000  c.c:  A,  April  3-11,  1930, 
with  surface  hauls  underlined:  B,  April  22-May  1,  1930;  C,  April  14-24,  1929. 


BIGELOW   AND    SEARS:   NORTH   ATLANTIC    ZOOPLANKTON    STUDIES  205 


Fig.    4.  Volumes  of  plankton,  per  standard  haul,  contour  lines  100,  500,  1000,  1500,  2000,  2500,  3000:  A, 
May  10-18,  1929;  B,  May  12-23,  1930;  C,  May  16-22,  1931. 


206  memoir:  museum  of  comparative  zoology 

coupled  with  the  development  of  considerable  pools  richer  than  1000  c.c.  (Fig. 
4B).  A  considerable  increase  in  the  volumes  of  plankton  had  also  taken  place 
inshore  between  the  offings  of  Delaware  Bay  and  of  New  York,  where  three  sta- 
tions now  averaged  358  c.c,  contrasting  with  only  113  c.c.  at  the  end  of  April. 
As  a  result  of  these  changes,  the  volume  of  plankton  averaged  approximately 
twice  as  great  in  that  May  as  it  had  in  April,  both  in  the  northern  sector  and  in 
the  southern,  with  an  alteration  of  the  same  order,  inshore  as  well  as  offshore. 
But  it  is  doubtful  whether  any  significant  or  general  alteration  had  taken 
place  in  this  respect  from  the  one  month  to  the  next,  in  1929.  In  fact,  the  volume 
of  plankton  may  not  have  averaged  significantly  higher  in  April  or  in  May  of 
that  year  than  at  the  end  of  the  preceding  winter.^  And  it  is  certain  that  no  sig- 
nificant augmentation  took  place  between  February  and  May  in  1932,  contrasting 
with  the  great  increase  that  certainly  took  place  during  that  interval  both  in  1930 
and  1931  (Fig.  4),  unless  possibly  for  a  brief  period  in  March  or  April,  months 
for  which  no  information  is  available  for  that  particular  year. 

Averages  for  the  different  subdivisions  suggest,  however,  that  differences 
from  year  to  year  in  these  respects  are  not  wide  enough  to  obscure  the  general 
rule  that  in  May  the  offshore  belt  supports  an  appreciably  larger  volume  of 
plankton  than  the  inshore  belt  (in  fact,  this  was  the  case  to  a  greater  or  less  extent 
in  each  year).  But  no  prevailing  difference  for  this  month  is  suggested,  between 
the  northern  sector  and  the  southern. 

June.  In  one  year  of  the  series  (1931)  the  average  volume  continued  to 
increase  somewhat  in  each  subdivision,  from  May  to  June  (Table,  p.  200).  In  each 
of  the  other  three  years,  a  decrease  was,  however,  recorded.  Thus,  in  1929,  when 
the  rich  ( >500  c.c.)  areas  continued  about  as  scattered  in  June  as  in  May  (Fig. 
4A,  6A),  average  volumes  fell  between  mid-May  and  early  June  from  about  334 
c.c.  to  about  249  c.c.  in  the  northern  sector,  from  198  c.c.  to  168  c.c.  in  the  south- 
ern with  corresponding  decreases  in  the  maxima  from  1448  c.c.  in  May  to  541  c.c. 
in  June.  And  it  appears  that  the  vernal  maximum, — whether  for  extent  of  the 
rich  ( >500  c.c.)  area,  or  for  average  and  maximum  volumes, — was  also  reached 
by  May,  in  1930,  for  in  that  year  the  average  volume  for  the  area  as  a  whole^ 
decreased  from  about  714  c.c.  in  that  month  to  about  381  c.c.  for  June,  the  rich 
(  >  500  c.c.)  areas  meantime  contracting  by  the  end  of  the  latter  month  to  a  few 
scattered  centers,  one  close  to  Delaware  Bay  (due  to  a  local  swarm  of  Sagitta 

'  We  have  no  February  data  for  1929. 

'  The  June  cruises  of  1930  extended  southward  only  as  far  as  the  mouth  of  Delaware  Bay. 


BIGELOW   AND   SEARS :   NORTH   ATLANTIC    ZOOPLANKTON    STUDIES  207 

elegans),  one  near  the  edge  of  the  shelf  off  New  York,  and  others  near  shore  off 
Montauk  Point  and  Martha's  Vineyard  (Cf.  Fig.  4B  with  Fig.  5B).  And  seven 
successive  surveys  (Fig.  7),  at  close  intervals  in  1932,  again  showed  the  trend  as 


Fig.  5.  Volumes  of  plankton,  per  standard  haul :  A,  maximum  and  minimum  at  each  station  for  the 
four  May  cruises  of  1932,  hatched  areas  where  catches  were  greater  than  500  c.c;  B,  June  7-18, 
1930;  C,  June  24- July  1,  1930.    Contour  Unes  100  and  500  c.c.  in  B  and  C. 


generally  downward  from  early  May  (299  c.c.)  to  late  June  (261  c.c),  except 
in  the  southernmost  sector,  an  alteration  which  seems  sufficiently  consistent 
to  be  accepted  as  significant,  though  not  large. 


208 


memoir:  museum  of  comparative  zoology 


Fig.  6.  Volumes  of  plankton,  per  standard  haul,  contour  lines  100,  500,  1000  c.c:  A,  May  28-June  5, 
1929;  B,  June  12-19,  1931 ;  C,  maximum  and  minimum  at  each  station  for  the  three  June  cruises 
of  1932. 


BIGELOW   AND    SEARS :    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES 


209 


Thus,  it  appears  that  while  the  alteration — for  the  area  as  a  whole — is  not 
Ukely  to  be  great  in  either  direction,  from  May  through  June,  the  trend  is  more 
Ukely  to  be  downward  than  upward,  at  this  season  in  any  given  year. 


C.C. 
350 

300 

2  50 

200 

350 

300 

250 

200 


NORTH 


ll 


INSHORE 


ilL 


A 


SOUTH 


OFFSHORE 


IL 


T 


MAY  '  JUNE  MAY  '  JUNE 

Fig.    7.  Average  volumes  of  plankton,  in  the  different  subdivisions,  in  May  and  June,  1932. 

July.  In  the  only  year  (1929),  when  a  general  survey  was  carried  out  in 
July,  as  well  as  in  June,  the  average  volume  of  plankton  had  ostensibly  increased 
somewhat  from  the  one  month  (June,  average,  219  c.c.)  to  the  next  (July,  average, 
257  c.c.),'  southward  from  the  New  York  profile,  but  had  decreased  somewhat 
near  the  eastern  boundary  of  the  area  (average,  Montauk-Martha's  Vineyard 
profiles,  275  c.c,  in  June;  250  c.c.  in  July).  And  while  these  changes  were  so  small 
that  they  may  best  be  interpreted  as  evidence  of  a  roughly  stationary  state,  so 
far  as  the  average  abundance  is  concerned,  a  corresponding  shift  in  the  center  of 
population  from  east  of  the  New  York  profile  to  south  of  the  latter,  may  be  regu- 
larly characteristic  of  this  season,  since  a  similar  shift,  westward,  also  took  place 
between  mid-  and  late  June  and  mid- July  of  1930  and  1931,  as  illustrated  by  the 
following  average  volumes  for  the  Martha's  Vineyard  and  Montauk  profiles,  con- 
trasted with  the  New  York  and  Barnegat  profiles : 


Year 

Season 

Barnegat-New  York 

Martha's  Vineyard-Montauk 

1930 

Late  June 

327 

478 

Mid-July 

605 

403 

1931 

Mid-June 

535 

730 

Mid-July 

852 

711 

'One  large  haul  (67109  c.c.)  omitted  from  calculations  of  the  Julj'  value. 


210  memoir:  museum  of  comparative  zoology 

An  interesting  feature  of  the  volumetric  picture  for  July  is  the  striking  irreg- 
ularity of  distribution,  with  centers  of  great  abundance  sporadically  scattered  on 
the  shelf.  In  such  cases,  it  is  not  unusual  to  make  a  poor  catch  close  to  an  ex- 
traordinarily rich  one.  One  haul,  for  example,  off  Montauk,  on  July  10,  1930,  pro- 
duced only  3  c.c,  but  another  not  ten  miles  away  yielded  1120  c.c,  chiefly, 
Calanus  finmarchicus.  And  other  instances  of  this  same  sort  might  be  cited. 
On  the  other  hand,  three  productive  (  >  500  c.c.)  hauls  from  contiguous  localities 
(Fig.  8A)  in  the  ofRng  of  New  York  may  perhaps  have  formed  part  of  a  center  of 
abundance  to  the  south  of  the  area  surveyed  in  that  July. 

Autumn.  No  quantitative  information  is  available  for  the  months  of  August 
and  September.  In  1931  (Fig.  9B),  however,  the  volume  of  plankton  in  the  off- 
shore belt  had  decreased  from  an  average  of  714  c.c.  in  July  to  236  c.c.  in  October 
in  the  northeastern  sector,  and  from  an  average  of  743  c.c.  in  June  to  231  c.c.  in 
October  in  the  southern.  In  fact,  only  one  of  the  stations  yielded  as  much  as 
500  c.c.  in  that  month.  The  contrast  between  the  richer  and  the  poorer  catches 
on  the  offshore  part  of  the  shelf  (the  inshore  belt  was  not  sampled) ,  was,  however, 
no  less  striking  in  October  (richest  haul  about  20  times  the  poorest)  than  it  had 
been  in  the  preceding  summer  (richest  about  14  times  the  poorest).  Hence,  great 
irregularity  of  quantitative  distribution  appears  to  be  as  characteristic  of  a  popu- 
lation that  is  (or  recently  has  been)  declining,  as  of  one  that  is  increasing,  or  near 
its  maximum. 

The  only  year  (1916)  in  which  towing  was  done  in  late  autumn,  was  one 
when  the  summer  plankton  seems  to  have  been  more  than  usually  abundant, 
following  tardy  vernal  warming  and  consequent  low  summer  temperatures.  It 
appears  from  the  original  notes  on  the  horizontal  catches  made  by  the  late  Will- 
iam Welch  (no  vertical  or  obUque  tows  were  made)  that  the  inshore  waters  still 
supported  considerable  amounts  of  plankton  of  one  sort  or  another  in  that  No- 
vember, for  "rich"  catches  were  recorded  locally  on  the  inner  half  of  the  shelf 
south  of  Martha's  Vineyard,  close  to  Delaware  Bay,  and  close  in  to  Chesapeake 
Bay,  contrasting,  however,  with  only  "fair"  to  "scanty"  catches  near  the  outer 
edge  of  the  shelf  all  along  from  the  ofHng  of  Martha's  Vineyard  to  that  of  Chesa- 
peake Bay.  So  far  as  these  data  go,  in  combination  with  those  for  1931,  they 
point  toward  a  comparatively  stable  state  for  the  last  half  of  autumn. 

There  are  no  quantitative  data  for  December  or  January,  but  it  is  likely 
that  these  months,  and  perhaps  early  February,  see  a  progressive  impoverish- 
ment in  most  years,  especially  in  the  northern  half  of  the  area,  leading  to  the 
barrenness  prevailing  at  the  end  of  the  winter.  But  considerable  production  may 


BIGELOW  AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON    STUDIES  211 


Fig.    8.  Volumes  of  plankton,  per  standard  haul,  contour  lines  100,  500,  1000  c.c.:-A,  July  10-18,  1930; 
B,  July  10-16,  1931. 


212 


memoir:  museum  of  comparative  zoology 


take  place  in  the  southernmost  sector  in  a  normal  February,  judging  from  the 
fact  that  the  center  of  abundance  lies  inshore  and  to  the  south  in  that  month,  as 
described  above  (p.  201),  and  the  probability  of  this  is  especially  strong  in  warm 
winters,  to  judge  from  the  volumes  that  were  recorded  in  February  1932  (p.  200). 


•  304 


308 


40* 


ir 


Fig.    9.  Volumes  of  plankton,  per  standard  haul,  contours  100,  500,  1000,  50000  c.c;   A,  July  11-August 
1,  1929;  B,  October  19-28,  1931,  as  calculated  from  vertical  tows. 


Annual  Differences 
In  February,  the  plankton  was  decidedly  more  abundant  in  the  southern 
sector  after  the  warm  winter  of  1932  (average,  472  c.c,  maximum,  2400  c.c, 


BIGELOW   AND    SEARS:    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES 


213 


Fig.  2B),  than  ill  oithcr  1930  or  1931  (average,  175-200  c.c,  maximum,  1000  c.c.) 
which  were  much  alike  in  this  respect.  Whether  this  means  that  more  animals 
survived  the  preceding  months  in  1932,  than  usual,  on  account  of  the  abnormally 
high  temperatures  of  that  winter,  or  whether  vernal  augmentation  had  already 
started  at  the  time  of  that  February  survey  is  an  open  question.  However,  we 
may  take  1932  as  an  example  of  the  extreme  in  the  variation  to  be  expected 
ill  tlic  one  direction,  i.e.,  in  a  February  of  extraordinary  abundance.  We  might 
perhaps  expect  the  other  extreme — a  great  paucity — after  an  unusually  severe 
winter.  Unfortunately,  data  are  lacking  here,  for  while  1929  was  the  coldest 
February  for  which  we  have  a  complete  temperature  survey  (Bigelow,  1933),  no 
contemporaneous  plankton  tows  were  made. 


c.c 
700 

600 

500 

400 

300 

200 

100 


1932  X'.. 


1931 


J. 


J. 


J. 


J. 


J. 


J. 


FEB.       MAR.       APR         MAR        JUNE      JULY        AUG       SEPT       OCT. 
Fig.  10.   Average  volumes  of  plankton,  for  the  area  as  a  whole,  in  different  months,  for  the  .several  years. 

The  most  interesting  aspects  of  the  annual  comparison  for  spring  and  summer 
(Fig.  10)  are  that  vernal  augmentation  was  most  pronounced,  and  summer  volumes 
consequently  greatest,  in  a  year  (1931)  when  the  amount  of  plankton  had  aver- 
aged relatively  small  at  the  end  of  the  winter,  whereas  there  was  no  general  aug- 
mentation of  the  plankton  during  the  spring  in  the  year  (1932)  when  volumes  had 
averaged  largest  in  February:  on  the  contrary,  a  slight  impoverishment.  But  the 
facts  that  vernal  augmentation  was  certainly  of  no  great  magnitude  in  1929  either, 
following  a  cold  winter  (summer  volumes  consequently  low),  and  that  the  positive 
anomaly  of  temperature  existing  in  February  1932  had  been  entirely  obliterated 


214  memoir:  musexjm  of  comparative  zoology 

by  that  May,  forbids  our  invoking  temperature  to  explain  year  to  year  differences 
of  this  sort,  unless  the  causes  lie  in  conditions  prevailing  some  time  earlier  in  the 
season. 

According  to  present  information,  the  plankton  as  a  whole  may  be  expected 
to  average  about  twice  as  voluminous,  in  our  area,  in  a  rich  year  as  in  a  poor,  at 
the  end  of  the  winter,  a  difference  increasing  to  about  3  to  1  by  midsummer. 

Quantitative  data,  for  the  summers  of  1913  and  1916,  are  not  sufficient  for 
inclusion  in  the  preceding  comparison,  though  the  qualitative  pictures  for  these 
two  years  are  of  much  interest  as  representative  of  a  warm  and  of  a  cold  summer 
respectively. 

Vertical  Distribution 

Studies  of  the  vertical  distribution  of  a  mixed  population  are  complicated 
by  the  fact  that  the  existing  state  may  depend  on  the  precise  proportions  in 
which  different  species  may  enter  into  the  general  stock  at  any  particular  time 
and  place.  A  case  in  point  is  the  antithetical  influence  that  would  be  exerted  by  a 
preponderance  of  Centropages,  which  usually  has  its  center  of  abundance  near 
the  surface  (p.  324)  as  contrasted  with  a  preponderance  of  Calanus,  which, 
through  much  of  the  year,  is  most  abundant  some  distance  below  the  surface 
(p.  310).  We  have  also  to  distinguish  between  two  categories  of  stratification 
that  may  be  largely  independent,  first,  that  which  results  from  diurnal  vertical 
migrations,  and  second,  what  we  name  the  "residual"  stratification  upon  which 
the  diurnal  is  superimposed.  For  the  sake  of  clarity,  it  seems  desirable  to  con- 
sider these  two  types  separately. 

Diurnal  Stratification 

The  volumetric  ratio  of  deep  catch  to  shoal  has  varied  widely  from  place  to 
place  on  every  cruise,  by  day  as  well  as  by  night,  depending  largely  on  the  pre- 
cise qualitative  composition  of  the  community.  In  February,  for  example,  of 
1932 — the  only  year,  when  hauls  were  made  at  two  levels  in  that  month, — the 
ratio  of  deep  catch  (22-24  meters)  to  shoal  (7  meters)  was  about  0.3  to  1  at  3 
A.M.  on  the  12th,  midway  out  on  the  shelf  off  Cape  May,  where  Euthemisto  and 
Paracalanus  jointly  constituted  43%  of  the  catch,  but  about  1  to  1  at  midnight 
on  the  26th,  at  about  the  same  relative  position  off  Bodie  Island,  where  Metridia 
lucens  was  the  leading  species  (38%).  But  the  ratio,  deep  volume  to  shoal,  aver- 
aged nearly  the  same  between  8  A.M.  and  4  P.M.  (0.9  to  1,  5  cases)  as  between 
8  P.M.  and  4  A.]\I.    (0.7  to  1,  8  cases),  for  that  cruise  as  a  whole.   Evidently 


BIGELOW   AND   SEARS:   NORTH    ATLANTIC    ZOOPLANKTON   STUDIES  215 

diurnal  migration  docs  not  assume  mass  proportions  at  this  time  of  year,  either 
because  the  particular  species  concerned  do  not  show  this  type  of  phototropic 
response  on  a  broad  scale,  under  the  conditions  of  illumination  then  existing,  or 
because  the  effects  are  then  nuUified  by  turbulent  movements  of  the  water. 

But  this  ratio  averaged  significantly  larger,  however,  during  the  hours  of 
high  illumination  than  during  those  of  low,  on  each  of  the  pertinent  cruises  for 
April,  May,  and  June,  as  follows: 

Average  ratio  of  subsurface  volume  to  surface  volume, 
day  and  night,  1929 

April;  8  A.M.-4  P.M.,  12-30  M.,  28.8  to  1,  5  cases;  31-70  M.,  1.4  to  1,  6  cases. 

8  P.M.-4  A.M.,  12-30  M.,  1.2  to  1,  9  ca.ses;  31-70  M.,  0.6  to  1,  5  cases. 
May;  8  A.M.-4  P.M.,  12-30  M.,  9.2  to  1,  4  cases;  39-64  M.,  3.2  to  1,  3  cases. 

8  P.M.-4  A.M.,  12-30  M.,  1.2  to  1,  4  cases;  39-64  M.,  1.0  to  1,  2  cases. 
June;  8  A.M.-4  P.M.,  12-30  M.,  5.5  to  1,  11  cases;  34-71  M.,  2.4  to  1,  7  cases. 

8  P.M.-4  A.M.,  12-30  M.,  1.9  to  1,  15  cases;  34-71  M.,  3.9  to  1,  5  cases. 

Average  ratio  of  deep  (  >  20  meters)  volumes  to  shoal  (5-10  meters), 
day  and  night,  1931  and  1932 

1931,  May;  8  A.M.-4  P.M.,  19-32  M.,  10.9  to  1,  7  cases;  39-64  M.,  3.2  to  1,  3  cases. 

8  P.M.-4  A.M.,  19-32  M.,  2.1  to  1,  5  cases;  39-64  M.,  1  to  1,  2  cases. 
June;  8  A.M.-4  P.M.,  22  M.,  6.8  to  1,  12  cases;  39  M.,  7.4  to  1,  6  cases. 
8  P.M.-4  A.M.,  22  M.,  2.0  to  1,  11  ca.ses;  39  M.,  1.6  to  1,  2  cases. 

1932,  May;  8  A.M.-4  P.M.,  17-36  M.,  1.4  to  1,  17  cases. 

8  P.M.-4  A.M.,  17-36  M.,  0.9  to  1,  30  cases. 
June;  8  A.M.-4  P.M.,  20-28  M.,  20.5  to  1,  14  cases. 
8  P.M.-4  A.M.,  20-28  M.,  1.1  to  1,  22  cases. 

Combination  of  the  foregoing  shows  the  ratio  of  volumes  at,  say,  18-36  meters, 
relative  to  that  at  10-0  meters,  to  have  averaged  about  10.4  to  1  by  day  (37 
cases)  contrasted  with  1.0  to  1  by  night,'  for  April  and  May  as  a  whole.  Thus  it  is 
evidently  characteristic  of  our  area  and  of  the  particular  assemblage  of  plank- 
tonic  animals  there  existing,  that  diurnal  migrations  do  attain  sufficient  magni- 
tude during  spring  and  early  summer,  to  cause  considerable  enrichment  of  the 
mid-depths  by  day,  at  the  expense  of  the  surface  stratum,  the  stratification  that 
is  temporarily  built  up  in  that  way,  being  largely  obUterated  by  night. 

But  the  day-night  contrast  was  usually  much  narrower  at  depths  greater 
than  35  meters  than  in  the  mid-strata.  In  fact,  no  sign  of  diurnal  alteration  in 
this  respect  was  recorded  at  35  meters  at  the  one  station,  where  successive  hauls 

>  Meaning  by  "day,"  8  A.M.-4  P.M.,  and  by  "night,"  8  P.M.-4  A.M. 


216  memoir:  museum  of  comparative  zoology 

were  made  several  hours  apart,  though  it  was  well  marked  at  20  meters  and  at 
10  meters.  Hence,  we  hazard  the  guess  that  diurnal  migrations  do  not  signifi- 
cantly affect  the  mass  distribution  of  the  plankton  much  below  40  meters  depth. 
Diurnal  stratification,  though  still  widespread,  is  much  less  pronounced 
in  June,  to  judge  from  an  average  ratio,  deep  catch  to  shoal,  of  about  3.8 
to  1  by  day  and  of  2.2  to  1  by  night  (47  cases)  for  that  month  in  1929,  1931, 
and  1932  combined,  while  on  one  of  the  cruises  (June  1929)  a  contrast  of  the 
opposite  order  was  recorded  for  the  39-64  meter  level.  And  the  difference  in  this 
respect,  between  the  hours  of  strong  illumination  and  weak,  averaged  still 
smaller  in  July,  as  follows: 

Ratio  of  deep  to  surface  volumes,'  July  1929 

8  A.M.-4  P.M.,    10-30  M.,  8.3  to  1,  7  ca.scs;  30-60  M.,  4.0  to  1,  6  cases;  >  60  M.,  3.3  to 

1,  3  cases. 
8  P.M.-4  A.M.,    10-30  M.,  7.8  to  1,  5  cases;  30-60  M.,  3.2  to  1,  3  ca.ses;  >  60  M.,  2.0  to 

1,  4  cases. 

Ratio  of  deep  volumes  to  those  at  7-10  meters,  July  1931 

8  A.M.-4  P.M.,   20-30  M.,  5.8  to  1,  4  cases. 
8  P.M.-4  A.M.,   20-30  M.,  4.8  to  1,  5  cases. 

But  these  July  ratios,  being  consistently  larger  by  day  than  by  night,  show  that 
diurnal  migration  is  a  factor  to  be  taken  into  account,  even  at  this  time  of  year, 
though  its  effects  appear  then  to  be  strongly  opposed  by  the  vertical  distribution  of 
temperature  (p.  219),  at  least  for  the  particular  assemblage  of  species  with  which 
we  are  concerned. 

We  have  no  information  as  to  diurnal  migrations,  for  autumn,  or  early  winter. 

The  physical  factors  most  ob\aously  to  be  considered,  in  connection  with 
vernal  intensification  of  diurnal  migration  as  a  mass  phenomenon,  followed  by 
weakening,  in  midsummer,  are  (1)  increasing  vertical  stability  of  the  water 
column,  (2)  the  intensity  of  illumination,  and  (3)  vernal  warming  of  the  surface. 
During  the  winter,  and  first  two  months  of  spring,  when  temperature  and  salinity 
are  close  to  uniform  from  surface  to  bottom,  o^'er  our  area  as  a  whole,  the  water 
has  no  significant  vertical  stability.  And  the  vertical  turbulence  is  evidently  ac- 
tive enough  to  counteract  any  general  tendency  that  may  exist  for  the  plank- 
tonic  community  to  carry  out  diurnal  migrations  through  February,  when 
illumination  is  still  relatively  weak.  In  short,  the  plankton,  like  the  other  charac- 
teristics of  the  water,  is  kept  thoroughly  stirred,  down  to  a  depth  of  about  30 

'  The  tabulation  is  confined  to  the  groups  in  which  there  were  at  least  three  cases. 


BIGELOW   AND    SEARS:   NORTH    ATLANTIC    ZOOPLANKTON    STUDIES  217 

meters.  In  this  connection,  we  should  perhaps  remind  the  reader  that  vertical 
stability  influences  the  vertical  distribution  of  the  planktonic  community  chiefly 
by  its  control  of  vertical  circulation,  for  the  vertical  gradient  of  specific  gravity 
is  not  great  enough,  even  when  the  water  is  most  stable,  seriously  to  aff'ect  the 
rate  of  vertical  swimming  for  active  organisms,  or  the  rate  of  sinking,  for  passive. 
It  seems  probable  that  the  development  of  diurnal  migrations  on  a  mass 
scale  by  April,  in  spite  of  the  fact  that  vertical  stability  still  continues  negligible, 
and  vertical  stirring  consequently  active,  results  from  the  increasing  stimulus 
to  phototropic  response  provided  by  the  increasing  intensity  of  illumination. 
Then,  as  the  season  advances,  the  vernal  warming  from  above,  combined  with 
some  decrease  in  the  sahnity  of  the  surface  waters  (Bigelow,  1933;  Bigelow  and 
Sears,  1935)  progressively  renders  the  water  column  so  much  more  stable  that  a 
corresponding  frictional  force  (wind  or  current)  would  produce  only  about  1/3 
as  active  stirring  in  the  upper  40  meters  of  our  area,  as  a  whole,  in  June,  and  1/8 
as  active  in  July  as  it  would  in  mid-May'.  The  mechanical  barrier  to  effective 
vertical  swimming,  thus  breaks  down  in  the  stratum  within  which  it  was  previ- 
ously strongest.  The  positive  phototropic  stimulus  to  diurnal  migration  also 
continues  to  strengthen,  during  the  late  spring  and  early  summer,  as  the  intensity 
of  illumination  and  the  length  of  day  increase,  with  increasing  decUnation  of  the 
sun.  These  factors  in  combination,  ofTer  a  reasonable  explanation  for  the  fact 
that  diurnal  migrations  have  proved  to  have  much  more  effect  on  the  vertica 
distribution  of  the  planktonic  community  in  April,  May,  and  June,  than  in 
February.  And  the  evidence  discussed  on  page  219,  indicates  that  its  decline,  as 
a  mass  phenomenon  from  June  to  July  is  due  to  the  barrier  to  upward  swimming 
imposed  by  the  high  temperature  to  which  the  surface  warms  in  midsummer. 

Vertical  stratification  other  than  diurnal 

In  February,  the  oblique  tows  made  in  1932,  at  the  groups  of  stations  where 
two  or  more  levels  were  sampled,  yielded  volumes  averaging  about  as  large  at 
20-27  meters  (average,  267  c.c.)  as  at  3-8  meters  (average,  251  c.c),  but  only 
13%  as  large  at  39-49  meters  (35  c.c).  And  the  vertical  gradient  is  still  stronger 
if  the  one  richest  catch  for  each  depth-category,  be  omitted  from  the  calculation. 
Nor  was  there  any  apparent  evidence  of  any  mass  effects,  by  diurnal  migration, 
in  that  month  (p.  214).  It  thus  appears  that  while  some  of  the  February  stations 
showed  one  stratum  considerably  the  more  productive,  and  other  stations  the 

'  Calculations  by  C.  O'D.  IseUn. 


218  memoir:  museum  of  comparative  zoology 

other,  the  plankton,  on  the  whole,  was  distributed  nearly  uniformly,  down  to 
about  25-30  meters,  but  that  it  averaged  less  than  one  fourth  as  abundant  in 
deeper  water,  a  state  probably  reflecting  the  dominance  of  the  community  by 
Centropages  and  by  Limacina  at  this  season,  and  the  prevailing  scarcity  of 
Calanus  (p.  305). 

But  the  center  of  population  evidently  tends  to  sink  some  20-30  meters 
below  the  surface  by  early  spring,  there  to  continue  until  early  summer,  for  the 
catches  averaged  3  to  15  times  as  large  from  hauls  centering  at  12-30  meters  as 
from  those  centering  shoaler  than  10  meters,  in  six  of  the  seven  cruises  of  record, 
April-June, — the  one  exception  being  May  1932,  when  the  volume  of  plankton 
averaged  about  the  same  at  the  one  level  as  at  the  other.  And  the  fact  that  the 
ratio  of  average  volume  at  20-30  meters  to  volume  at  10  meters  or  shoaler  aver- 
aged 10.5  to  1  in  July  1929,  and  25.7  to  1  in  that  month  of  1931,  contrasting  with 
about  3.8  to  1  for  May  and  about  3.1  to  1  for  June  (all  cruises  combined)  is  evi- 
dence that  concentration  of  plankton  in  the  mid-depths  tends  to  become  still 
more  pronounced  as  summer  advances.  It,  also,  appears  that  this  enrichment, 
relative  to  the  superficial  stratum,  likewise  tends  to  extend  to  the  deepest  water  on 
the  shelf  from  June  to  July — though  in  lesser  degree — for  the  ratio  of  volume  at 
depths  greater  than  60  meters  to  those  at  the  surface  rose  from  1.3  to  1  in  May, 
and  1.9  to  1  in  June,  to  3.6  to  1  in  July,  in  the  one  year  (1929)  when  pertinent 
data  were  obtained. 

It  seems  certain  that  the  type  of  stratification,  described  above,  is  not  the 
result  of  diurnal  migration — general  though  that  phenomenon  be  from  April 
on — ,  because  the  observational  series  for  April,  May,  and  June  includes  a  smaller 
number  (70)  of  day  time  hauls,  that  would  tend  to  magnify  the  effects  of  such 
migrations,  than  of  night  hauls  (104)  that  would  tend  to  obscure  them,  while  the 
picture  for  July  includes  about  as  many  hauls  of  the  one  category  as  of  the  other 
(11  day,  10  night).  Neither  can  the  initiation  of  vertical  stratification  of  plankton 
be  credited  to  temperature,  for  while  this  factor  seems  of  great  importance  later 
in  the  season  (p.  219),  enrichment  of  the  mid-level  relative  to  the  surface  is  estab- 
lished early  in  the  spring,  i.e.,  long  before  the  upper  strata  have  warmed  to  a  value 
that  could  be  considered  unfavorable  for  the  species  concerned.  An  alteration  in 
the  qualitative  composition  of  the  plankton,  does,  however,  offer  a  reasonable  ex- 
planation, for  while  the  community  was  dominated  in  the  one  February  of  record 
by  Limacina  and  Centropages  (combined,  67%),  species  that  are  usually  most 
abundant  near  the  surface,  they  have,  on  the  average,  been  greatly  surpassed, 
from  April  through  July,  by  Calanus  finmarchicus  and  Sagitta  elegans,  both  of 


BIGELOW   AND    SEARS:    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES 


219 


which  average  most  abundant  at  mid-depths,  as  illustrated  by  the  following  tab- 
ulation, for  the  cruises  on  which  hauls  were  made  at  two  or  more  levels : 

Average  Percentage  in  Total  Catch 


Feb. 

April 

May 

June 

July 

Centropages  and  Limacina 

67% 

13% 

19% 

11% 

17% 

Calanus  finmarchicus  and  Sagilla  elegans 

2 

68 

43 

55 

64 

Comparison  between  catches  and  temperatures  at  the  mid-towing  levels, 
argues  strongly,  however,  that  the  progressive  warming  of  the  surface  is  the  factor 
chiefly  responsible  for  the  rather  abrupt  intensification  of  the  vertical  plankton 
gradient,  with  corresponding  impoverishment  of  the  surface  locally,  that  takes 
place  from  June  to  July,  as  was,  indeed,  forecast  long  ago,  by  the  relation.ship 
between  the  stratum  of  chief  abundance  of  Calanus  and  the  temperature 
(Bigelow,  1922). 

The  fact  that  the  majority  of  the  dominant  species  are  boreal,  not  arctic, 
would  make  it  unlikely  that  any  thermal  control  of  vertical  distribution  would 
become  operative  until  the  surface  had  warmed  above,  say,  14°,  nor  do  the  data 
yield  any  suggestion  of  it  in  April  or  May,  except  perhaps  at  two  stations  close  to 
the  southern  boundary  of  the  area,  where  (in  1929)  volumes  were  twice  and  eleven 
times  as  great  at  15-30  meters,  as  at  the  surface,  in  temperatures  of  17.8°-18.6°. 
The  catches  continued,  in  fact,  to  average  nearly  as  large  from  one  temperature 
as  from  another,  through  June,  in  one  of  the  years  (1932)  when  subsurface  read- 
ings were  obtained  in  that  month  —  perhaps  because  of  the  scarcity  of  Calanus  — 
although  the  surface  had  already  warmed  above  16°  over  considerable  areas.  A 
negative  correlation  had,  however,  developed  by  June  between  average  catch  and 
temperature,  in  the  other  pertinent  year  (1931),  though  with  wide  irregularity 
from  station  to  station,  as  appears  from  the  following  tabulation. 


<12° 

12.1°— 16° 

>16° 

June  1931 

903  c.  c. 
38  cases 

322  c.  c. 

11  cases 

313  c.  c. 

4  cases 

June  1932 

302  c.  c. 
46  cases 

225  c.  c. 
28  cases 

251  c.  c. 
14  cases 

And  the  limitation  of  plankton  by  high  temperature  (again  independent  of 
depth)  proved  more  obvious  by  July  in  each  of  the  two  years  (1929,  1931)  when 


220  memoir:  museum  of  comparative  zoology 

tows  were  made  at  two  levels  in  that  month  as  follows: 


<12° 

12.1°-16° 

16°- 18° 

18°-20° 

>20° 

July  1929 

383  c.  c. 
23  cases 

485  c.  c. 
8  cases 

— 

149  c.  c. 
16  cases 

77  c.  c. 
24  cases 

July  1931 

1025  c.  c. 
21  cases 

664  c.  c. 
3  cases 

280  c.  c. 
4  cases 

127  c.  c. 
7  cases 

197  c.  c. 
3  cases 

Similar  evidence  lies  in  the  fact  that  7  out  of  8  catches  of  1000  c.c.  for  July 
1931  were  in  hauls  where  the  temperature  at  the  mid-towing  level  was  lower  than 
11°  and  none  where  it  was  warmer  than  14°.  It  seems  definitely  established, 
therefore,  that  temperature  and  not  depth  per  se,  nor  illumination  (which  is 
about  the  same  in  July  as  in  June)  is  the  factor  responsible  for  the  fact  that  the 
colder  levels  average  several  times  as  productive  of  zooplankton  in  midsummer 
as  the  warmer. 

The  thermal  contrast  between  July,  on  the  one  hand,  and  May-June,  on  the 
other,  consists  not  only  in  higher  surface  temperature,  but  also  in  the  develop- 
ment of  a  more  pronounced  thermocline,  raising  the  question  whether  the  latter 
be  significant  in  the  present  connection,  or  whether  we  have  simply  to  do  with  the 
relative  suitability  of  different  temperatures  for  the  particular  species  concerned. 

Unfortunately,  a  definite  answer  cannot  be  expected  from  available  data,  be- 
cause it  seldom  happened  that  the  one  haul  was  made  just  above  the  thermocline, 
and  the  other  just  below  it,  the  picture  being  especially  obscure  for  1929,  when 
the  vertical  intervals  between  hauls  were,  as  a  rule,  broad  enough  to  include  both 
the  thermocline  and  a  considerable  column  of  water  within  which  the  vertical  gradi- 
ent of  temperature  was  small.  The  following  comparison  between  the  vertical  gra- 
dient for  temperature  per  unit  depth  and  the  ratio  of  shoal  catch  to  deep,  at 
stations  (1929  and  1931  combined)  where  the  depth  intervals  between  the  mid- 
points of  successive  hauls  were  not  greater  than  30  meters,  might,  it  is  true,  sug- 
gest a  very  strong  positive  correlation  between  the  two,  if  taken  by  itself. 


Temperature 

difference 

per  10 

meter  depth 

Average  ratio 

of  shoaler 

catch  to  next 

deeper 

Extreme 
ratios 

Cases 

0-2° 

1  to  3.4 

1  to  0.5;    1  to  22.6 

9 

2.1-4° 

1  to  2.6 

1  to  0.5;    1  to  9.0 

10 

4.1-6° 

1  to  18.6 

1  to  0.4;    1  to  142.1 

12 

6.1-8° 

1  to  41.4 

1  to  4.8;    1  to  140.1 

4 

BIGELOW   AND    SEARS :    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES  221 

But  it  happened  tliat  the  deeper  haul  was  either  made  in  water  colder  than  17°, 
or  the  shoalcr  in  water  warmer  than  12°,  wherever  the  thermal  gradient  per  10 
meters  was  grt^ater  than  4°.  In  other  words,  either  the  one  catch  was  made  within 
the  thermal  range  which  appears  most  favorable  for  boreal  plankton,  or  the  other 
was  made  at  a  temperature  that  is  much  less  so.  In  short,  it  is  doubtful  whether 
the  steepness  of  the  thermal  gradient,  per  se,  is  of  any  importance  in  the  distribu- 
tional picture  now  under  discussion. 

The  fact  that  the  boreal  plankton  remains  on  the  whole  so  definitely  con- 
centrated in  the  cooler  part  of  the  water  column  at  the  season  when  the  surface 
is  warmest,  in  spite  of  the  widespread  tendency  toward  diurnal  migration,  intro- 
duces interesting  speculations  as  to  the  physiological  mechanisms  and  responses 
involved — phototropism,  geotropism  (both  of  these  perhaps  being  reversible  by 
temperature),  and  also  the  possibility  that  such  animals  as  Calanus,  etc.,  may 
simply  become  immobiUzed  at  a  temperature  unfavorably  high,  and  thus  passively 
sink  down  again,  to  a  more  favorable  environment. 

We  have  no  information  as  to  vertical  distribution  for  the  community  as  a 
whole,  for  the  autumn  or  early  winter. 

Comparison  With  Other  Areas 

The  great  majority  of  zooplankton  studies  that  have  appeared  during  the 
last  quarter  century  have  been  based  on  the  enumeration  of  individuals,  and 
when  measurements  have  been  made  of  the  volumes  of  plankton  present,  these 
have  in  most  cases  been  only  incidental  to  the  main  thesis  of  the  particular  inves- 
tigation in  hand.  For  example,  the  only  systematic  records  of  volumes  of  zoo- 
plankton  as  distinct  from  phytoplankton  that  are  included  in  the  extensive  lists 
of  collections  made  by  the  Fisheries  Services  of  the  various  North  European 
countries  in  different  years  between  1902-1912,  published  by  the  International 
Council  for  the  Exploration  of  the  Sea  in  their  several  series  of  bulletins,  are  those 
contributed  by  the  Swedish  Fisheries  Service  for  the  Skagerak  and  for  the  Baltic 
(Conseil  Permanent  International  pour  I'Exploration  de  la  Mer,  1904-1907). 
And  it  is  obvious  that  there  can  be  no  basis  for  comparison  between  volumetric 
studies  and  numerical,  unless  one  or  the  other  includes  some  indication  as  to  the 
relationship  between  the  number  of  individuals  and  their  volumes. 

It  is  perhaps  somewhat  astonishing  that  such  should  be  the  case,  because  a 
majority  of  the  campaigns  of  towing  have  received  their  impetus  from  fisheries 
investigations,  from  which  standpoint,  it  would  seem  perhaps  not  less  important 


222  memoir:  museum  of  comparative  zoology 

to  know  the  mass  of  animal  plankton  present  than  to  know  the  number  of  in- 
dividual animals  making  up  that  mass — at  least  if  the  point  at  issue  be  the  food- 
stuffs available  for  fishes. 

The  result  has  been  that  while  a  vast  body  of  information  has  been  accumu- 
lated about  life  histories  of  the  species  of  planktonic  animals  that  are  the  most 
numerous  in  northern  seas  and  the  most  important  there  as  food  for  herring  or 
other  fishes,  about  the  variations  in  their  populations  in  space  and  time,  and  about 
their  community  associations,  we  still  have  only  the  haziest  ideas  as  to  the  mass 
of  animal  substance  that  is  characteristic  of  any  part  of  the  sea,  at  any  depth,  at 
any  time  of  year — and  even  less  idea  as  to  how  much  may  be  produced  annually. 
In  this  respect,  the  study  of  the  zooplankton  has  lagged  far  behind  that  of  the 
phytoplankton. 

We  also  face  the  further  difficulty  that  only  a  rough  comparison  is  possible 
in  most  cases,  even  between  different  studies  primarily  volumetric,  because  of 
differences  in  the  methods  of  collection  and  in  the  precision  with  which  necessary 
information  as  to  towing  speeds,  etc.,  has  been  presented.  For  example,  the 
coarse  nets  used  on  the  "Thor"  and  "Dana"  expeditions  (Jespersen,  1917,  1923, 
1935),  adequately  sample  only  the  larger  planktonic  animals  such  as  are  more 
abundant  on  the  high  seas,  but  fail  for  everything  smaller  than  the  largest  cope- 
pods,  i.e.,  for  many  of  the  species  that  are  often  predominant  inshore,  as  illus- 
trated by  the  fact  that  catches  with  a  2-meter  stramin  net,  at  eight  of  our  own 
stations  in  June  1932,  averaged  only  about  1/6  as  voluminous  as  those  made  at 
the  same  stations  with  the  silk  net,  when  reduced  to  a  common  standard.  On 
the  other  hand,  fine  meshed  nets  (#20  silk,  173  meshes  per  linear  inch),  such  as 
have  often  been  employed  in  quantitative  studies,  fail  for  the  larger  animals,  but 
yield  a  combined  catch  of  smaller  zooplankton  and  of  phytoplankton.  Hence, 
catches  made  in  them  very  seldom  give  a  measure  of  the  total  animal  fraction  of 
the  planktonic  community,  as  distinct  from  the  vegetable  fraction. 

Consequently,  the  allowances  that  must  be  made  for  divergences  in  pro- 
cedure Umit  to  the  roughest  such  comparisons  as  we  are  able  to  make  between 
our  area  and  others  where  the  volume  of  animal  plankton  has  been  recorded. 

The  following  tables,  summarizing  the  more  extensive  of  the  volumetric 
measurements  of  the  zooplanktonic  community  as  a  whole,  that  have  been  made 
off  the  east  coast  of  North  America  and  off  the  northwestern  coasts  of  Europe, 
are  here  expressed  arbitrarily  as  cubic  centimeters  per  cubic  meter  of  water, 
rather  than  per  unit  tow,  in  the  hope  of  giving  the  reader  a  more  graphic  picture 
(for  a  discussion  of  the  methods  of  expression,  see  p.  197). 


BIGELOW   AND    SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


223 


Volume  of  Plankton  along  the  east  coast  of  North  America 
and  West  Greenland 


6 

6 

t-  .■— ^ 

0^ 

fcS 

°P 

03 

CO  -^ 

>    m 

'"& 

.& 

^  c 

6.2 

>> 

c 
m 

1 

Settlem 
Displac 

Calcula 
per  cub 

Cape  Cod  to 

Feb.,  1930-1932 

D 

0.4 

0.4 

Chesapeake 

Apr.,  1929-1930 

0.5 

0.5 

Bay 

May,  1929-1932 

0.8 

0.8 

June,  1929-1932 

0.7 

0.7 

July,  1929-1931 

0.8 

0.8 

Oct.,  1931 

0.4 

0.4 

Martha's 

Sept.-Oct.,  1935 

Clarke  and 

S 

0.08 

0.04 

Vineyard 

Nov.-Dec,  1935 

Zinn  (1937) 

1.4 

0.7 

Jan.-Mar.,  1936 

1.9 

0.9 

Apr.-June,  1936 

0.24 

0.12 

July- Aug.,  1936 

0.19 

0.09 

Gulf  of  Maine 

July-Aug.,  1913, 
1916 

Bigelow 
(1926) 

S 

0.7 

0.35 

March,  1920 

0.4 

0.2 

April,  1932 

Fish  and  Johnson  ' 

D 

0.7 

0.7 

May,  1932 

(1937) 

0.34 

0.34 

June,  1932 

0.21 

0.21 

Aug.-Sept.,  1932 

0.16 

0.16 

September,  1933 

Redfield 

D 

0.21 

0.21 

October,  1933 

(unpublished  data) 

0.13 

0.13 

December,  1933 

0.17 

0.17 

January,  1934 

0.12 

0.12 

March,  1934 

0.05 

0.05 

April-May,  1934 

0.16 

0.16 

May-June,  1934 

0.26 

0.26 

June-July,  1934 

0.32 

0.32 

September,  1934 

0.5 

0.5 

Bay  of  Fundy 

April,  1932 

Fish  and  Johnson 

D 

0.04 

0.04 

May,  1932 

(1937) 

0.19 

0.19 

June,  1932 

0.2 

0.2 

Aug.-Sept.,  1932 

0.09 

0.09 

Nova  Scotia, 

May,  1915 

Huntsman 

S 

0.9 

0.45 

Newfoundland 

June,  1915 

(1919) 

0.5 

0.25 

Shelf 

July,  1915 

0.6 

0.3 

Gulf  of  St. 

May-June,  1915 

Huntsman 

S 

0.5 

0.25 

Lawrence 

August,  1915 

(1919) 

0.5 

0.25 

West 

August,  1924 

St0rmer " 

0.35 

0.17 

Greenland 

(1929) 

•  Dr.  Fish  informs  us  that  the  towing  speed  was  very  nearly  1.5  miles  per  hour. 
2  Stas.  32-63,  250-0  meters. 


224  memoir:  museum  of  comparative  zoology 

These  series  are  all  comparable  with  our  own  data,  in  that  phytoplankton 
does  not  enter  into  the  resultant  volumes  in  more  than  minimal  proportions.  And 
the  number  of  observations  on  which  the  average  is  based  is  large  enough  in  each 
case  for  results  to  appear  significant.  But,  in  some  cases,  the  measurement  was 
by  "settlement,"  which  yields  results  averaging  more  than  twice  as  large  as  does 
the  "displacement"  method.  If  allowance  be  made  for  this  difference  in  method, 
the  tabulation  suggests  that  there  is  but  little  latitudinal  difference,  in  the  rich- 
ness of  plankton  from  the  offing  of  Chesapeake  Bay  northward  to  the  Gulf  of 
Maine,  in  late  winter  or  in  early  spring,  when  tlie  zooplankton  (near  its  yearly 
minimum  for  the  continental  shelf  as  a  whole)  averages  only  about  0.2-0.3  c.c. 
per  cubic  meter.  But  from  May  through  July,  the  catches  have  averaged  at 
least  twice  as  large  west  and  south  from  Cape  Cod  (about  0.6-0.7  c.c.  per  cubic 
meter)  as  for  the  Gulf  of  Maine  (including  the  Bay  of  Fundy),  for  eastern  Cana- 
dian waters,  or  for  West  Greenland,  with  the  sector  between  Cape  Cod  and  Dela- 
ware Bay  averaging  somewhat  the  richest  of  all,  at  the  season  of  maximum 
abundance;  the  Bay  of  Fundy,  and  the  Gulf  of  St.  Lawrence  somewhat  the 
poorest. 

The  largest  volume  per  cubic  meter,  as  yet  actually  recorded  from  American 
waters  is  116.1  c.c,  at  the  edge  of  the  continental  shelf  off  Delaware  Bay  on 
July  18,  1929.  But  this — being  the  product  of  a  local  swarm  of  salpae — cannot  be 
regarded  as  representative  of  more  than  the  actual  column  of  water  fished.  No 
doubt,  a  net  drawn  through  one  of  the  windrows  of  Aurellia,  for  example, — that 
are  so  commonly  encountered  in  northern  seas  in  summer — would  yield  a  catch 
as  large  or  larger.  Aside  from  this  (which  may  well  have  been  equalled  on  many 
unrecorded  occasions  elsewhere),  the  maximum  is  6.9  c.c.  per  cubic  meter,  in  the 
offing  of  New  Jersey,  May  20,  1930.  The  Gulf  of  St.  Lawrence  ranks  second  in 
this  respect,  with  9.4  c.c.  measured  by  the  ".settlement"  method,  equivalent  to 
perhaps  4-5  c.c.  by  displacement  (Huntsman,  1919),  the  Ciulf  of  Maine  third 
(4.3  c.c.  per  cubic  meter  by  "settlement,"  or  perhaps  about  2  c.c.  by  displace- 
ment, Bigelow,  1926,  Table,  p.  94),  while  the  maxima  so  far  recorded  for  West 
Greenland  and  for  the  Scotian  shelf  are  only  2.6  c.c.  and  2.1  c.c,  respectively  by 
"settlement,"  corresponding  to  about  1  c.c  by  displacement. 

It  has  long  been  a  matter  of  common  knowledge  that  rich  concentrations  of 
plankton  occur  in  summer  in  the  upper  water  layers  around  Iceland.  Assuming 
a  towing  speed  of  1.5  knots,  Damas'  (1905,  p.  15)  record  of  a  catch  of  more  than 
1000  c.c.  of  young  Calanus  alone  in  a  five  minute  haul  at  the  surface  with  a  one 
meter  net — frequently  quoted  as  an  instance  of  extraordinary  abundance  of 


BIGELOW   AND   SEARS:   NORTH    ATLANTIC    ZOOPLANKTON   STUDIES 


225 


Volume  of  Plankton  in  North  European  Waters 


i  i 

d 

+3 
■  c 

S  «s 

o 

3  S 

^  a 

0^ 

•n 

o  i^ 

o  a 

o 

^1 

o;g 

>. 

.4-3 

1  ^--Q 

5" 

c 

g 

?«^ 

03  'X! 

OJ 

CO 

1 

cc.S  s 

w  a 

Iceland 

July,  1924 

St0rmer  (1929) 

s 

0.4 

0.2 

Iceland,  Faroes, 

Jespersen  (1923) 

Dr.' 

0.84 

0.84 

Ireland 

Southern 

July-August, 

St0rmer  (1929) 

s 

0.6 

0.3 

Norwegian  Sea 

1924 

North  Sea 

June,  1926 

Savage  (1931) 

S 

1.15 

0.57 

Aug.,  1926 

1.0 

0.5 

Nov.,  1926 

1.2 

0.6 

Jesperson  (1923) 

Dr.' 

0.013 

0.013 

English 

April,  1925 

Russell  (1927) ' 

S 

0.03 

0.01 

Channel 

May,  1925 

0.20 

0.10 

June,  1925 

0.04 

0.02 

July,  1925 

0.11 

0.05 

Aug.,  1925 

0.10 

0.05 

Skagerak 

Feb.,  1903 

Conseil  permanent 

S 

0.08 

0.04 

Aug.,  1903 

international  pour 

0.09 

0.05 

Nov.,  1903 

I'exploration  de  la 

0.28 

0.14 

May,  1904 

mer  (1904-1907) 

0.18 

0.09 

Aug.,  1904 

0.09 

0.05 

Nov.,  1904 

0.32 

0.16 

Aug.,  1906 

0.04 

0.02 

Baltic 

Aug.,  1903 

Conseil  permanent 

s 

0.04 

0.02 

Aug.,  1904 

international  pour 

0.45 

0.22 

Aug.,  1906 

I'exploration  de  la 
mer  (1904-1907) 

0.07 

0.04 

May-June, 

Mielk  and  Kiinne 

s 

0.95 

0.5 

1931 

(1935,  p.  69) 

plankton — corresponds  to  about  6  c.c.  per  cubic  meter,  if  the  volumetric  measure- 
ment was  made  by  displacement,  which  ranks  with  the  richer  catches  in  the  Gulf 

'  We  have  found  by  experiment  that  measurement  by  "draining"  agrees  closely  with  that  by  "dis- 
placement." 

'  We  are  informed  by  Dr.  Russell  that  his  towing  speed  was  between  1 .5  and  2  knots. 


226  memoir:  museum  of  comparative  zoology 

of  St.  Lawrence,  and  south  of  New  York.  It  is  more  impressive,  in  the  present 
connection  that  the  average  of  0.84  c.c,  for  82  "Thor"  catches  between  Iceland, 
the  Faroes,  and  Ireland  (Jespersen,  1923,  p.  4,  Fig.  1;  Schmidt,  1912,  p.  8)  may 
well  have  represented  a  community  richer  than  any  that  has  thus  far  been  re- 
ported for  any  considerable  area  in  the  western  Atlantic,  most  of  the  smaller 
copepods,  etc.,  having  very  likely  been  lost  through  the  coarse-meshed  nets  that 
were  used.  And  while  St0rmer  (1929)  reported  much  smaller  volumes  from  Ice- 
landic waters  (average,  0.4  c.c.  per  cubic  meter,  by  settlement,  or  perhaps  0.2  c.c. 
by  displacement)  his  measurements  were  made  at  two  stations  only,  and  after 
removing  some  of  the  more  bulky  forms  (medusae,  etc.). 

The  average  recorded  by  St0rmer  (1929)  at  23  "Michael  Sars"  stations,  in 
the  southern  part  of  the  Norwegian  Sea  of  about  0.6  c.c.  per  cubic  meter,  with  a 
maximum  for  any  one  haul  of  about  3.4  c.c.  per  cubic  meter,  is  about  half  that  for 
our  own  area  (p.  223),  if  allowance  be  made  for  the  difference  between  measure- 
ments by  "settlement"  and  by  "displacement." 

In  the  North  Sea,  many  measurements  of  plankton  volume,  per  cubic  meter, 
were  long  ago  made  by  Apstein  (1906)  and  Kraefft  (1910),  yielding  very  large 
values.  But  their  catches  were  made  in  such  fine  nets  (173  meshes  per  Unearinch) 
that  the  recorded  volumes  include  the  phytoplanktonic  fraction,  nor  do  the  pub- 
lished data  afford  any  way  of  estimating  the  volume  of  the  zooplanktonic  fraction, 
as  distinct  from  the  latter,  and  this  applies  equally  to  the  volumes  given  for  the 
Baltic  and  North  Sea,  byMielck  (1911),  Lucke  (1912),  and  Biise  (1915).  On  the 
other  hand,  the  very  low  average  (corresponding  to  0.013  c.c.  per  cubic  meter) 
reported  for  the  North  Sea  by  Jespersen  (1923,  Fig.  1),  may  be  chargeable — at 
least  in  part — to  the  very  coarse  mesh  of  his  stramin  nets.  Volumes  for  the  North 
Sea  calculated  from  the  lists  more  recently  published  by  Savage,  (1931,  Tables 
13-17),  in  connection  with  his  studies  of  the  food  of  the  herring,  agree  clo.sely 
with  the  summer  average  for  our  own  area,  when  reduced  to  the  "displacement" 
basis  according  to  Savage's  comparison  of  methods,  though  the  maximum  re- 
ported by  him  of  7  c.c.  per  cubic  meter  by  "settlement"  was  somewhat  lower.* 

Zooplankton  would  appear  to  be  considerably  less  abundant  in  the  English 
Channel  than  in  the  North  Sea,  according  to  Russell's  (1925,  1927)  records.  And 
Dr.  Russell  writes  us  that  "the  mixed  oceanic  and  coastal  water  near  the  con- 
tinental shelf  is  probably  richer,  and  the  English  Channel  and  southern  North 
Sea  are  rather  poorer  in  plankton  content"  (quoted  from  a  letter  of  January  28, 

'  For  description  of  the  type  of  net  employed,  see  Cons.  Int.,  Publ.  Cire.,  No.  84. 


BIGELOW  AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  227 

1938),  though  this  regional  contrast  may  be  not  as  wide  as  the  calculated  volumes 
indicate,  because  the  nets  used  in  the  Channel  would  retain  "only  the  more  bulky 
of  the  planktonic  organisms"  (Russell,  1927).  And  the  long  series  of  vertical 
tows  made  by  the  Swedish  Fisheries  Service  show  a  decided  poverty  in  the  Skag- 
erak,  as  contrasted  with  the  North  Sea,  with  the  English  Channel,  or  with  the 
coastal  waters  of  northeastern  America  as  a  whole.  But  the  volumes — chiefly 
copepods — reported  for  the  western  part  of  the  Baltic  by  Mielck  and  Kiinne 
(1935,  p.  69)  correspond  quite  closely  to  the  averages  east  of  Great  Britain,  when 
reduced  to  a  common  standard  (see  Table,  p.  225).  And  this  also  applied  in  two 
surveys  out  of  three,  to  the  region  farther  east  and  north  in  the  Baltic  examined 
by  the  Swedish  cruises  in  August  of  1903,  1904,  and  1906. 

The  foregoing  comparisons  point  to  the  waters  between  Iceland,  the  Faroes, 
and  Scotland  as  probably  somewhat  more  productive  of  zooplankton,  at  the 
season  of  chief  abundance  than  is  any  other  considerable  sector  of  the  boreal  belt 
(including  tributary  seas),  of  the  North  Atlantic.  In  fact,  it  would  not  be  aston- 
ishing if  these  waters  should  finally  prove  to  support  4-6  times  as  large  a  volume 
of  plankton,  as  does  the  most  closely  competing  region,  to  judge  from  the  allow- 
ance to  be  made  for  the  size  of  mesh  used  in  the  nets.  Next  in  rank,  come  the 
American  sector  between  Cape  Cod  and  Chesapeake  Bay  on  the  one  side  of  the 
Atlantic,  and  the  North  Sea  on  the  other,  the  former  with  an  average  of  about 
0.7-0.8  c.c.  per  cubic  meter,  at  the  peak  season,  as  measured  by  displacement^ 
the  latter  with  1.4-1.9  c.c,  as  measured  by  settlement,  which  corresponds  to  per- 
haps 0.5-0.6  c.c.  by  displacement.  And  it  is  further  interesting  that  1932  was  a 
poor  year  for  plankton  in  each  of  these  areas,  on  opposite  sides  of  the  Atlantic 
(Savage,  1937).  The  Gulf  of  Maine  and  Scotian  shelf  in  the  west  and  the  southern 
North  Sea  and  Baltic  in  the  east,  similarly  fall  together  at  the  season  when  the 
plankton  is  richest,  but  with  somewhat  smaller  volumes,  corresponding  to  about 
0.2-0.4  c.c,  or  perhaps  somewhat  less,  by  displacement.  But  no  attempt  has  been 
made  to  estimate  the  total  yearly  production  (volumetric)  of  zooplankton  for  any 
part  of  the  sea — so  far  as  we  are  aware.  And  the  relative  ranking  on  this  basis 
may  differ  widely  from  the  foregoing,  with  Icelandic  waters,  where  active  produc- 
tion is  probably  confined  to  a  short  season,  perhaps  falling  below  our  own  waters 
and  below  the  North  Sea  region. 

The  striking  feature  of  the  comparison  is  not  the  demonstrated  diversity, 
i.e.,  that  one  boreal  locality  may  average  several  times  as  rich  as  another,  whether 
for  the  column  of  water  as  a  whole  on  the  continental  shelf,  or  for  the  upper  strata 
farther  out  to  sea,  but  rather,  that  the  average  volumes  for  different  regions,  at 


228  memoir:  museum  of  comparative  zoology 

the  season  of  maximum  abundance  (Tables,  pp.  223,  225),  should  differ  so  little, 
one  from  another,  relative  to  the  volume  of  water  containing  them.  If  allowance  be 
made  for  the  method  of  measurement  and  for  the  size  of  mesh,  the  extreme  range 
is  only  from  about  1  to  about  8  volume.s  of  animal  plankton,  of  the  size  of 
medium-sized  copepods  and  larger,  per  10  million  volumes  of  water,  though 
individual  catches  may  run  as  high  as  1100  volumes  or  more  (p.  224). 

There  would  be  little  profit  in  extending  the  comparison  farther  afield,  be- 
cause the  published  records  of  volumetric  abundaiice  for  other  seas  are  based 
chiefly  on  catches  made  in  nets  either  of  mesh  so  fine  that  they  retain  the  vegetable 
as  well  as  the  animal  fraction,  e.g.,  Hensen's  (1890)  study  of  volumetric  distribu- 
tion between  Bermuda  and  the  Cape  Verdes,  and  Boschma's  (1936),  for  the  East 
Indies,  or  so  coarse  that  they  may  have  lost  a  considerable  fraction  of  the  stock 
of  animals  in  some  localities,  but  perhaps  very  little  at  others.  We  need  merely 
remind  the  reader  of  Jespersen's  (1923,  1924)  well  known  demonstration,  based 
on  catches  with  very  coar.se  (stramin)  nets,  of  poverty  in  the  Mediterranean  and 
in  the  Sargasso  Sea  region,  contrasted  with  richness  in  the  northeastern  Atlantic, 
and  of  his  more  recent  comparison  (1935,  Fig.  27)  between  the  upper  water  layers 
in  various  parts  of  the  Atlantic  and  Pacific  at  mid-  and  low  latitudes. 

RELATIVE  ABUNDANCE  OF  DIFFERENT  SPECIES 
Dominant  species.  Previous  information  (p.  192)  had  shown  that  the  waters 
on  the  continental  shelf  more  than  a  few  miles  out  from  the  land,  and  from  Cape 
Cod  as  far  west  as  the  offing  of  New  York  are  dominated  in  late  summer  and 
autumn  by  much  the  same  boreal  communities — lead  by  Calanus  finmarchicus — 
as  characterize  the  Gulf  of  Maine  to  the  eastward.  And  similar  conditions  had 
been  found  to  extend  southward  at  least  to  the  latitude  of  Chesapeake  Bay  in  a 
cool  summer  (illustrated  by  1916),  though  in  a  warm  one  (e.g.,  1913),  the  waters  in 
this  southern  sector  may  be  monopolized  locally  by  swarms  of  neritic  species  of 
ctenophores,  on  the  one  hand,  or  of  salpae  (of  oceanic  origin),  on  the  other,  with 
a  considerable  variety  of  other  warm  oceanic  visitors  recorded  here  and  there, 
though  never  in  any  significant  volume. 

The  records  for  the  period,  1929-1932,  now  add  the  information  that  the 
same  boreal  "Calanus"  community  similarly  dominates  our  area  as  a  whole, 
southward  at  least  to  Latitude  36°  N.,  from  the  end  of  the  winter,  through  spring 
and  early  summer,  in  the  relati\'e  proportions  tabulated  below,  and  that  immi- 
grants from  the  warm  oceanic  community  that  constantly  inhabit  the  waters 
along  the  continental  edge,  are  no  more  important,  volumetrically,  on  the  shelf 
during  the  vernal  half  year  than  they  are  in  summer  or  autumn. 


BIGELOW   AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


229 


Average  Percentages  in  the  Total  Zooplankton  of  Species  that  have  Formed 
1%  or  more,  by  Volume,  in  any  One  of  the  Subdivisions 


Diite 

Sector 

o 

a 

a 

d 

3 
'p. 

d 

a 

o 

_3 

1 

1 

a; 

3 
3 

a 

's 
a; 

> 

£ 
1 
►J 

a 

a 

0) 
•T3 
O 

CO 

i 

0. 

a 
8 

w 

3 

a 

3 
H 

t 

0 

■§. 

o 
a 

o 

— 

'5) 
'■B 

< 

0) 

at 
3 

a 

a 

01 
> 

d 

.2 

a 

.i 
o 

1 

s 
6 

a 
H 

a 

_3 
O 

"o 

Q 

a 
'o 

J 

d 

a 

s 

o 

'a 

CU 

o 

Feb., 
1930 

Inshore 
Offshore 
North 
youth 
Total  area 

3 
10 
9 
1 
5 

49 
2 
11 
59 
41 

<1 

12 
4 
1 
4 

6 

7 
6 

7 
7 

<l 

8 

4 

<1 

2 

7 
4 
17 
<1 
6 

21 

20 
31 
14 
20 

<i 

2 
<1 
<1 

1 

1 
20 
2 
4 
3 

<1 

<1 
<1 
<1 

1931 

Inahore 
Offshore 
North 
South 
Total  area 

13 
28 
9 
16 
19 

<1 
11 
2 
5 
5 

<1 

21 
3 

10 
9 

2 
8 
20 
2 
5 

<1 
<1 
<1 
<1 
<1 

55 
15 
51 
37 
38 

25 

T 
18 
15 

3 
1 
2 
2 

<1 
2 
<1 
<1 
<1 

1932 

Inshore 
Offshore 
North 
South 
Total  area 

15 
1 
1 

31 
6 
14 
29 
29 

6 
15 

12 

7 
7 

6 
19 
21 

6 
8 

4 
<1 
1 
1 
3 

41 

9 

1 

41 

38 

<1 
<1 
<1 
<1 
<1 

3 
18 
3 
5 
5 

1 

9 

12 

1 

2 

<i 

3 

7 

2 

<1 

April, 
1929 

Inshore 
Offshore 
North 
South 
Total  area 

56 
73 
73 
58 
66 

11 
6 
6 

12 
9 

0 
5 

4 
1 
3 

<1 
2 
2 

<T 

1 

9 
<1 
4 
5 
4 

5 
<1 
3 
0 
2 

12 

4 

<1 

17 

8 

<1 
<1 
<1 
<1 

<1 

<1 
2 
2 

<1 

1 

1930 

Inshore 
Offshore 
North 
South 
Total  area 

40 
40 
44 
30 
41 

11 
3 

2 

15 

7 

5 
27 
18 
12 
16 

11 

5 

6 
9 

8 

12 
13 
12 
9 
6 

7 
4 
11 
3 

8 

3 
3 
2 
5 
3 

3 

2 
2 
4 
2 

<1 

1 

1 

<1 

<1 

May, 
1929 

Inshore 
Offshore 
North 
South 
Total  area 

21 
47 
41 
18 
33 

6 
5 
4 
8 
5 

<1 

T 
1 

1 

<1 
<1 
<1 
<1 
<1 

18 
6 

15 
9 

12 

9 

10 

14 

1 

9 

1 

3 

<1 

7 
2 

<1 
<1 
<1 
<1 
<1 

8 
2 

5 
1 

4 

25 

0 

0 

46 

13 

1 

<i 
<i 

2 
<1 

2 
<1 
<1 

3 

1 
<i 
<i 
<i 
<l 

0 

1 

1 

<1 

<1 

0 
1 
0 
2 
1 

1930 

Inshore 
Offshore 
North 
South 
Total  area 

35 
25 
38 
18 
29 

5 
1 
1 
3 
2 

2 
6 
8 
1 
5 

<1 
<1 
<1 
<1 
<1 

24 
56 
36 
55 
45 

13 
3 

0 

8 
7 

<1 

1 
<1 
<1 

1 

5 
3 
3 
3 
4 

<1 
<1 
<1 
<1 
<1 

2 

1 
<1 

1 
<1 
<1 
<1 
<1 

1931 

Inshore 
Offshore 
North 
South 
Total  area 

30 
69 
61 
24 
52 

<1 
<I 
<1 
<1 
<1 

<1 
9 
6 
3 
5 

7 
5 
7 
4 
6 

12 
4 
3 

21 

8 

34 
3 
11 
36 
17 

<1 
<1 
<1 

<1 
<1 

<1 
<1 
<1 
<1 
<1 

8 
1 
3 
8 
4 

<1 
<1 
<1 
<1 
<1 

1932 

Inshore 
Offshore 
North 
South 
Total  area 

8 
28 
21 

To 

16 

6 
1 
3 
15 
8 

3 
14 
10 

6 

8 

12 
6 

11 

8 

29 
17 
25 
21 
23 

10 
2 
4 
2 
6 

1 

2 
1 
2 
2 

<1 
<1 
<1 
<1 
<1 

<1 

5 
3 
4 

7 
1 

4 
4 
4 

2 
<1 

2 

<1 

1 

4 

1 
1 
5 
3 

8 
4 
8 
3 
6 

0 
3 
2 
1 

<1 

1 

<1 

1 
1 

<1 

2 
<1 

2 

1 

June, 
1929 

Inshore 
Offshore 
North 
South 
Total  area 

25 
46 
43 
17 
37 

9 
4 
2 
16 
6 

6 
9 
4 
6 
5 

1 
<1 

1 
<1 
<I 

9 
3 
6 

4 
5 

23 
11 

22 

1 

18 

<T 

1 

3 

<1 

<1 

5 

1 

1 
6 
6 
1 
5 

1 
0 
1 
1 

1 

5 
1 
3 
2 

2 

1 

1 
1 

0 
9 
5 
4 
3 

3 

1 
1 
6 
1 

1930 

Inshore 
Offshore 
North 
South 
Total  area 

41 
61 
57 
17 
52 

1 
<1 
<1 
<1 
<1 

<1 

7 

3 

<1 

2 

<1 
<1 
1 
<1 
<1 

20 
6 

18 
1 

15 

19 
4 

8 
48 
13 

<i 
<i 
<i 
<i 
<i 

9 
3 
5 
17 

7 

<1 

2 

1 

<1 

1 

1 
<1 

1 
<1 

1 

<1 
<1 
<1 
<1 
<1 

1 
<1 

1 
<1 

1 

<1 
<1 
<1 
<1 
<1 

0 
<1 
<1 

0 
<1 

<1 
<1 
<1 
<1 
<1 

1931 

Inshore 
Offshore 
North 
South 
Total  area 

59 
74 
76 
41 
65 

2 
1 

1 
2 

1 

1 
3 
2 
1 
2 

6 
2 
5 
3 
4 

1 
<1 
<1 

3 

1 

25 
11 
9 
39 
17 

<i 

2 

1 
2 

<1 
<1 
<1 
<1 
<1 

<1 

<1 
<1 

<1 
<1 

<1 
<1 
<1 
<1 
<1 

1 
<1 
<1 
<1 
<1 

2 
<1 

2 

1 

<1 
<1 
<1 
<1 
<1 

0 

1 
<1 

0 

<1 

<1 

<1 

<1 

1 

<1 

1932 

Inshore 
Offshore 
North 
South 
Total  area 

8 
26 
IS 
14 
16 

14 
7 
6 
17 
11 

2 
12 
6 
4 
6 

4 
3 
4 
3 
3 

6 
12 

7 
12 

9 

14 
4 

17 
8 

12 

<1 

2 

1 
4 
2 

<1 
1 
1 
2 
1 

2 
6 
4 
4 
4 

28 
6 
19 
15 
17 

2 
<1 

2 

<1 

1 

2 
<1 
<1 
<1 

2 

<1 

<1 
<1 
<1 
<1 

0 
9 
2 
8 
4 

2 
8 
1 
9 
5 

July, 
1929 

Inshore 
Offshore 
North 
South 
Total  area 

27 
32 
35 
9 
29 

20 
14 
19 
13 
18 

<1 

4 

2 

<1 

1 

<1 

<1 
<I 
<1 
<1 
<1 

2 
<1 
1 
2 
1 

35 
21 
28 
36 
30 

<1 
3 
1 
2 
1 

1 

1 
1 
2 
1 

1 

6 

3 

<1 

2 

<1 
4 

<1 
5 
2 

2 
<1 

2 
<1 

1 

<1 

<1 

<1 

1 

1 

0 

7 
3 
1 
2 

I 
3 
0 
5 
1 

<1 
3 

<1 
4 
2 

1930 

Inshore 
Offshore 
North 

47 
69 
57 

35 
3 

20 

<1 

7 
3 

<1 
<1 
<1 

3 
4 
3 

7 
5 
6 

<1 
<1 
<I 

6 
<1 

6 

<1 

<1 
<1 

<1 
<1 
<1 

<1 

0 

<1 

<1 
<1 
<1 

0 
<1 
<1 

<1 
<1 
<1 

1931 

Inshore 
Offshore 
North 

57 
64 
61 

20 

1 

12 

<1 

<1 
<1 

3 
I 
1 

3 
0 
1 

6 
2 
4 

<1 
<1 
<1 

<1 
<1 
<1 

<1 
<1 
<1 

<1 
<1 
<1 

3 
<1 

1 

<1 
<1 
<1 

0 
24 
11 

<1 
<1 
<1 

Oct., 
1931 

Offshore 

North 

South 

7 
8 
7 

34 
47 
14 

7 
11 
2 

17 
26 
12 

2 
T 
4 

<1 
<1 

<1 

5 
3 

7 

5 
2 
9 

<1 
<1 
<1 

<1 

<1 
<1 

<1 

0 
1 

5 

1 

12 

230  memoir:  museum  of  comparative  zoology 

Monthly  Succession 

February.  The  only  species  that  have  averaged  more  than  1%  (by  volume) 
of  the  total  catches,  in  any  one  of  the  years,  for  the  region  as  a  whole,  in  February 
are  the  following: — Calanus  finmarchicus,  Centropages  typicus,  Eulhemisto  com- 
pressa,  Limacina  retroversa,  Metridia  lucens,  Paracalanus  parvus,  Pseudocalanus 
minutus,  Sagitta  elegans,  and  Sagitta  serratodentala.  Combined,  these  have  con- 
stituted from  86%  to  94%,  by  volume,  of  the  winter  catches.  In  no  instance,  in 
fact,  was  as  much  as  50  c.c.  of  any  other  species  taken  at  any  of  the  February 
stations. 

The  tabulation  (p.  229)  of  relative  percentages  shows  that  in  a  given  year, 
either  Centropages  typicus,  Sagitta  elegans,  or  Limacina  retroversa  may  be  the 
leading  species  at  this  season.  As  a  rule,  this  applies,  also,  for  individual  localities 
where  rich  (  >  300  c.c.)  catches  were  made,  the  only  notable  exception,  being  for 
one  station,  where  Sagitta  serratodentata  ranked  high.  Calanus  finmarchicus 
(dominant  later  in  the  season)  occupies  a  minor,  or  at  most  an  intermediate  posi- 
tion inshore  at  the  end  of  winter,  ranking  fifth  there,  on  the  average,  while  off- 
shore, it  ranks  second  (next  to  Metridia) .  In  February,  Calanus  has  also  averaged 
more  important  to  the  north  of  the  Cape  May  profile  (ranking  third,  for  the  three 
years  combined),  than  southward  (ranking  fifth);  so,  too,  Euthemisto.  On  the 
other  hand,  Centropages  typicus  was  relatively  much  more  important  in  the  south 
than  in  the  north,  in  the  two  years  when  it  occurred  in  significant  proportion  in 
either  subdivision.  In  the  cases  of  the  other  dominant  species,  the  north-south 
relationships  have  been  so  irregular  that  no  general  rule  can  be  derived  from  so 
short  a  series  of  observations. 

The  February  list  of  dominant  species  includes  no  benthonic  derivatives  of 
any  sort,  no  neritic  larvae,  no  visitors  from  ofTshore;  evidence  that  contributions 
from  the  coast  line,  and  from  the  bottom  beneath,  on  the  one  hand,  or  from  the 
continental  slope,  on  the  other,  are  negligible.^  Neither  does  it  include  any  im- 
migrants, whether  from  colder  coastal  waters  to  the  north,  or  from  warmer  to  the 
south. 

But  the  following  additional  species  have  occasionally  constituted  1%,  or 
more,  at  individual  stations  in  February,  north  of  Latitude  36°  N. : 

Aglantha  digitate,  4  stations,  1-10  c.c,  1-7%;  Candacia  armata,  1  station,  27  c.c, 
5%;  Centropages  hamatus,  1  station,  10  c.c,  1%;  Crago  sp.,  2  stations,  4-22  c.c, 
3-15%;  cumaceans,  2  stations,  2-6  c.c,  1-4%;  Eucalanus  sp.,  1  station,  2  c.c,  1%; 

'  Perhaps  on  account  of  failure  regularly  to  sample  close  to  bottom,  see  page  193. 


BIGELOW  AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  231 

Euphausia  sp.,  2  stations,  9  and  28  c.c,  9  and  32%;  Nematoscelis  megalops,  4  sta- 
tions, 5^0  c.c,  25-81%;  Neo7nysis  americana,  1  station,  4  c.c,  2%;  Oithona  sp.,  3 
stations,  1-30  c.c,  1-2%;  Pleuromamma  gracilis,  5  stations,  4-15  c.c,  10-41%; 
Rhincalanus  nasutus,  1  station,  9  c.c,  2%;  salps,  2  stations,  11-17  c.c,  32-57%; 
Thysanoessa  inermis,  1  station,  16  cc,  3%. 

The  winter  stations  occupied  in  1931,  in  the  immediate  vicinity  of  Cape 
Hatteras,  show  that  the  eight  species  which  dominate  to  the  northward  of  Lati- 
tude 36°  N.,  are  outclassed  farther  south,  by  species  of  oceanic  origin,  for  while 
the  former  category  constituted  only  11-12%  of  the  average  catch  there,  Nocti- 
luca  formed  8%,  Sagitta  enflala,  11%,  and  oceanic  copepods,  42%,  with  the  re- 
maining 34%  consisting  of  a  varied  assemblage  of  oceanic  decapods,  amphipods, 
siphonophores,  and  medusae,  among  which  some  51  species  were  identified. 

April.  The  same  species  have  not  only  dominated  in  April  (Fig.  IIB),  as 
in  February,  but,  in  combination,  they  have  formed  about  the  same  average 
proportion  of  the  total  catches  (89-93%),  for  the  area  as  a  whole.  And  their 
combined  importance  is  not  appreciably  affected  by  the  entrance  by  April  into 
the  "dominant"  category  of  a  new  member — euphausiids — for  these  were  still  in 
low  percentage  (1%)  and  counterbalanced  by  a  decline  on  the  part  of  Paracala- 
nus.  In  fact,  the  seven  leading  species  monopoUzed  the  large  (  >  500  c.c.)  catches, 
even  more  strongly  (80%  in  each  of  the  19  cases)  in  April  than  in  February. 

The  most  instructive  feature  of  the  lists  for  April,  compared  with  February, 
of  the  one  year  (1930)  when  surveys  were  made  in  both,  is  that  Calanus  finmarchi- 
cus  had  increased  so  greatly,  in  relative  importance,  from  the  one  month  to  the 
other  as  to  make  it  by  far  the  most  prominant  species  in  the  northern  sector, 
while  in  the  southern  sector,  it  was  dominant  at  the  time  of  the  first  April  cruise 
of  that  year,  and  about  equalled  its  closest  rivals  (Centropages  and  Pseudocalanus) 
at  the  time  of  the  second.  The  area  where  Calanus  averaged  20%  or  more  of  the 
total  volume  of  plankton  (confined  to  locaUties  in  the  northeast  in  February)  had 
also  expanded  by  early  April  to  cover  the  waters  generally  (though  with  local  ex- 
ceptions) down  to  the  offing  of  Chesapeake  Bay.  And  the  fact  that  Calanus  was 
present  in  even  larger  percentage  in  April  of  1929  (when  no  information  was  ob- 
tained in  February),  than  of  1930,  is  evidence  that  the  seasonal  sequence  re- 
corded in  the  latter  year  may  be  accepted  as  normal.  This  increase  in  Calanus 
was,  in  fact,  chiefly  responsible  for  the  great  augmentation  of  the  plankton  as  a 
whole  that  took  place  in  the  early  spring  of  1930  (p.  203).  And  this  statement 
applies  not  only  to  the  region  as  a  whole,  but  to  individual  localities  as  well,  for 
wherever  the  total  catch  was  much  larger  in  that  April,  than  it  had  been  in  the 


232 


memoir:  museum  of  comparative  zoology 


Fig.  1 1 .  Percentages  (by  volume)  of  dominant  species  in  average  catch  for  the  area  as  a  whole :  A,  Felaru- 
ary  5-13,  1930;  B,  April  3-11,  1930;  C,  May  12-23,  1930;  D,  June  7-18,  1930;  E,  July  11- 
August  1,  1929,  and  F,  October  19-28,  1931. 


BIGELOW   AND    SEARS:    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES  233 

preceding  February,  the  percentage  of  Calanus  was  also  much  larger,  whereas  its 
percentage  was  higher  in  some  cases,  and  lower  in  others  at  stations  where  little 
alteration  had  taken  place  in  total  volume. 

It  also  deserves  emphasis  that  Calanus — so  far  as  can  be  judged  from  the 
rather  unsatisfactory  series — is  the  only  member  of  the  dominant  group  that 
usually  experiences  a  great  and  general  augmentation  in  relative  abundance, 
within  the  confines  of  our  area,  in  early  spring.  At  the  same  time,  it  appears 
to  be  rather  more  common  in  April  than  in  February — perhaps  forecasting 
greater  diversity  in  the  general  averages  in  May — for  species  other  than  the 
dominants,  to  form  as  much  as  1  %  of  the  community,  at  individual  stations,  the 
recorded  instances  being: 

Aglantha  digitale,  4  stations,  2-10  c.c,  1-8%;  amphipods,  4  stations,  3-22  c.c,  1-9%; 
annelid  worms,  1  station,  47  c.c,  24%;  Anomalocera  pattersoni,  1  station,  1  c.c,  12%; 
Candacia  aimata,  1  station,  4  cc,  3%;  Centropages  hamatus,  5  stations,  3-35  cc, 
1-15%;  Corycaeus  sp.,  1  station,  2  c.c,  1%;  decapod  larvae,  6  stations,  2-82  cc, 
1-43%;  euphausiid  larvae,  17  stations,  3-60  c.c,  1-54%;  Eucalanus  attenualus,  6 
stations,  3-18  cc,  1-29%;  Euchaeta  sp.,  1  station,  2  cc,  1%,;  Euchirella  rostrata, 
2  stations,  3-6  cc,  2-3%;  leptomedusae,  2  stations,  12-25  cc,  21-21%;  Lucifer 
typus,  1  station,  8  cc,  7%;  Nematoscelis  megalops,  1  station,  23  c.c,  1%;  Oikopleura 
labradoriensis,  1  station,  9  c.c,  3%;  Pleuromamma  gracilis,  2  stations,  2-9  c.c,  1-5%; 
Rhincalanus  nasutus,  4  stations,  5-40  c.c,  4-30%;  oceanic  sagittae,  2  stations,  2-4 
c.c,  1-2%,;  Sagitia  enflata,  2  stations,  11-12  c.c,  10-10%o;  salps,  4  stations,  4-30 
c.c,  4-27%;  Temora  stylifera,  2  stations,  4-7  cc,  3-6%. 

May.  Calanus  averaged  about  as  important  in  the  total  community,  for 
the  area  as  a  whole  in  May  (30-50%o)  as  in  April  (40-60%),  in  the  years  when  the 
winter  had  been  normally  cool,  i.e.,  1929,  1930,  1931,  but  after  a  warm  winter 
(1932),  it  was  much  less  so  (16%).  In  all  years  (1929-1932),  however,  it  averaged 
much  more  important  relatively  in  the  ofTshore  belt  than  inshore,  though  with  con- 
siderable difference  in  this  respect  from  year  to  year,  and  about  twice  as  im- 
portant in  the  north  as  in  the  south.  On  the  other  hand,  while  Calanus  was  the 
largest  single  item  in  April  in  both  years  of  record,  in  May  this  was  true  in  two 
years  (1929,  1931)  only,  whereas  in  the  other  years  (1930,  1932),  it  was  surpassed 
by  Limacina.  And  the  percentage  of  Calanus  in  the  rich  (  >  500  c.c.)  hauls  also 
averaged  considerably  lower  in  May  (16-27%)  than  it  had  in  April  (33-77%) 
in  the  two  years  (1929,  1930)  for  which  information  is  available  for  both  these 
months.  Conflicting  evidence  of  this  sort  is  perhaps  more  reasonably  explained 
as  due  to  the  brevity  of  the  observational  series,  and  to  the  roughness  of  the 
methods,  than  to  any  definitely  seasonal  trend,  one  way  or  the  other,  from  the 
one  month  to  the  next. 


234  memoir:  museum  of  comparative  zoology 

Centropages,  contrasting  with  Calanus,  declined  greatly  in  relative  abun- 
dance, between  February,  or  April,  and  May  in  three  of  the  four  years  of  record 
(1929,  1930,  1932),  as  appears  from  its  average  volumes  and  percentages  for  the 
area  as  a  whole  (Table,  p.  229),  while  in  the  fourth  year  (1931),  this  species  was  so 
scarce  throughout  the  season  that  the  calculated  monthly  values  are  not  signifi- 
cant for  it  in  this  connection.  Furthermore,  Centropages,  which  formed  more 
than  50%  of  the  catch  at  five  out  of  eleven  winter  stations  where  more  than  300 
c.c.  of  plankton  was  taken,  equalled  30%  at  only  one  of  the  60  "rich"  (  >  500  c.c.) 
hauls  for  April  and  May  combined. 

Corresponding  to  this  vernal  decrease  in  relative  importance,  the  area  where 
Centropages  averaged  as  much  as  10%  of  the  tbtal  volume  contracted  consider- 
ably in  the  north  from  February  to  April  in  1930,  though  expanding  ofTshore  in 
the  south  meantime,  while  in  1932,  a  decided  contraction  of  the  same  sort  had  in 
anj^  case  taken  place  by  May,  showing  that  this  copepod  is  on  the  average,  only 
a  minor  element (<  1-5%)  in  the  northern  sector  by  the  end  of  spring.  And 
it  is  not  much  more  important  then,  in  the  southern  sector(  <l-8%)  in 
normal  years  (e.g.,  1929,  1930),  though  after  a  very  warm  winter  (as  in  1932),  its 
percentage  may  average  but  little  lower  in  May  (15%)  than  in  February  (29%). 
A  vernal  decline  in  relative  importance  seems  equally  characteristic  of  Sagitta 
serratodentata ,  judging  from  the  fact  that  its  percentage  in  the  total  catches 
averaged  only  about  1/20  as  great  in  May  as  in  February  of  1930,  less  than  1/15 
as  great  in  1931,  and  1/5  as  great  in  1932.  In  fact,  only  two  out  of  ten  spring 
cruises  showed  it  as  forming  as  much  as  2%  of  the  general  community  (Table, 
p.  229).  And  it  seems  that  it  usually  dechnes  to  less  than  5%  by  May,  even  in  the 
southern  sector,  where  a  considerable  percentage  of  it  may  persist  until  April  in 
some  years  (e.g.,  1929,  16%). 

The  vernal  histories  of  other  members  of  the  group  that  are  dominant  at  the 
end  of  winter  are  less  regular,  for  they  may  show  increases  in  some  years,  de- 
creases in  others,  though  these  alterations  have  usually  been  of  a  smaller  order  of 
magnitude  than  for  Calanus,  for  Centropages,  or  for  Sagitta  serratodentata.  In 
some  years,  for  example,  Pseudocalanus  shows  an  upward  trend  at  first,  but  then 
a  dechne,  as  in  1930,  when  its  percentage  about  doubled  between  February  and 
April,  but  then  fell  to  an  insignificant  figure  by  May.  And  it  may  have  experi- 
enced a  similar  succession  in  1929,  when  it  formed  less  than  1%  in  May  (no 
information  for  that  February).  But  the  alteration  was  of  the  reverse  order  in 
1932,  when  its  percentage  more  than  doubled  between  February  and  May,  while 
in  the  fourth  year  of  the  series,  its  relative  standing  was  about  the  same  in  the 
one  month  as  in  the  other. 


BIGELOW   AND    SEARS:    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES  235 

The  vernal  cycle  for  Metridia  lucens  seems  to  have  paralleled  that  for  Pseudo- 
calanus  in  1930,  when  its  average  percentage  about  quadrupled  from  February 
(4%)  to  April  (16%),  with  a  corresponding  expansion  of  the  area  where  it  aver- 
aged as  much  as  10%,  but  then  fell  again  by  about  1/3  to  May  (5%).  But  it  is 
doubtful  whether  any  significant  alteration  in  its  relative  abundance  took  place 
during  the  spring  of  1929,  of  1931,  or  of  1932,  because  its  percentage  in  each  of 
these  years  averaged  about  the  same  in  May  as  it  had  in  February.  Neither  is 
there  any  evidence  that  notably  rich  centers  ever  develop  for  Metridia  at  this 
time  of  year,  as  happens  frequently  for  Calanus,  and  at  least  occasionally  for 
Pseudocalanus.  Metridia,  however,  continues  as  definitely  an  "offshore"  species 
through  the  spring,  as  it  is  in  February,  its  percentage  having  averaged  about 
2-17  times  as  high  offshore  as  inshore,  both  in  April,  and  in  May  of  each  year  of 
record,  also  about  twice  as  high  in  the  north  (8%)  as  in  the  south  (4%)  for  the 
April-May  cruises  combined. 

The  year  (1930)  that  saw  the  most  pronounced  fluctuations  for  Metridia 
was,  however,  one  of  comparative  constancy  from  February  through  April  to 
May,  for  Sagitta  elegans.  But  the  area  where  this  chaetognath  was  relatively 
important  (  >  10%), — even  less  extensive  early  in  that  April  than  it  had  been  in 
February — then  greatly  expanded  southward  by  May.  And  in  1929,  its  per- 
centage in  the  catches  about  quadrupled  from  April  (2%;  no  February  data  avail- 
able) to  mid-May  (9%).  Likewise  in  1932  it  averaged  more  important  in  May 
(6%),  than  it  had  in  the  preceding  February  (1%),  while  in  1931  (thanks  to  more 
rapid  multiplication  by  Calanus),  it  was  in  only  about  half  as  great  percentage  in 
the  one  month  (17%)  as  it  had  been  in  the  other  (38%),  although  it  had  mean- 
time increased  in  average  abundance  from  about  58  c.c.  to  about  84  c.c.  Region- 
ally, however,  the  picture  for  S.  elegans  is  more  consistent  than  the  foregoing 
might  suggest,  for  in  all  years  of  record,  it  ranked  considerably  higher  in  per- 
centage iashore  (5-34%)  than  offshore  ( <  1-10%)  in  April  and  May,  as  seems 
in  fact  to  be  the  general  rule  for  it;  also  higher  in  the  northern  sector  (3-14%) 
than  in  the  southern  (  <  1-4%)  on  seven  of  the  ten  spring  cruises. 

In  the  case  of  Limacina,  a  3-fold  increase  in  percentage  was  registered  be- 
tween April  (4%)  and  May  (12%o)  in  1929,  a  15-fold  between  early  April  (3%) 
and  mid-May  (45%)  in  1930,  and  an  8-fold  between  February  (  <  1%)  and  May 
(8%)  in  1931,  with  corresponding  wide  southward  expansion  of  the  areas  where 
it  averaged  as  much  as  10%  of  the  total  community  (Fig.  12).  On  the  other  hand, 
its  percentage  decreased  by  nearly  J^  between  February  (38%)  and  May  (23%) 
in  1932  (all  stations  combined).  Neither  has  the  north-south  or  inshore-offshore 


236 


memoir:  museum  of  comparative  zoology 


relationship  of  Limacina  proved  any  more  consistent  at  this  season,  its  percentage 
having  a\'eraged  at  least  ^  2  greater  in  the  north  than  south  on  four  of  the  ten 
spring  cruises,  but  the  reverse  on  two  others,  and  significantly  greater  inshore 
than  offshore  on  five  cruises,  but  greater  offshore  than  inshore  on  two. 


^HH 

^H 

8^^  4f 

^^^Kl 

i9^^ 

^^^^^H| 

..- 

wm 

^^n 

t  / 

A 

FEBRUARY     1930 

70" 


Fig.  12.  Areas  where  Limacina  relroversa  made  up  more  than  10%  of  the  total  volume  of  plankton:  A, 
February  5-13,  1930;  B,  May  12-23,  1930. 

The  general  result  of  these  mutual  fluctuations  among  the  leading  species  is, 
that  while  the  percentage  of  Calanus,  for  the  area  as  a  whole,  has  averaged  about 
the  same  in  May  as  a  month  earlier,  its  rank,  relative  to  its  nearest  competitors 
has  averaged  lower  in  May  than  in  April,  but  that  of  Limacina  and  of  Sagitta 
elegans  much  higher.  Consequently,  if  we  name  April  the  "Calanus"  month,  we 
might  equally  name  May  the  "Calanus- Limacina-<Sogfi<ta  elegans"  month  for  the 
area  as  a  whole,  and  for  the  offshore  belt.  But  the  average  ranking  for  May  in 
the  inshore  belt,  and  in  the  southern  sector,  is  the  reverse,  i.e.,  "Limacina- 
Calanus."  And  the  picture  is  not  greatly  altered,  even  if  one  very  productive 
catch  of  Limacina  (2666  c.c,  midway  out  on  the  Corson  profile.  May  20,  1930) 
be  omitted  from  the  calculation. 

Apart  from  fluctuations  in  the  relative  strengths  of  the  leading  species  as 
just  outlined,  the  most  interesting  alteration  from  April  to  May  is  increasing 
diversification  in  the  general  community  made  evident  by  the  fact  that  the  list,  for 
the  latter  month,  of  other  species  that  have  averaged  as  much  as  1%  (Table, 
p.  229)  includes  larval  euphausiids,  and  crab  larvae  (these  had  already  appeared  in 
significant   proportions,    locally   in   April),    besides    Oikopleura   labradoriensis, 


BIGELOW   AND    SEARS:    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES  237 

Aglantha  digitale,  leptoline  medusae,  and  salps.  Analysis  of  the  rich  (  >  500  e.c.) 
hauls  points  in  the  same  direction,  for  while  the  eight  leading  species  were  respon- 
sible for  80%,  or  more  of  the  catch  in  every  rich  haul  in  April,  this  was  the  case  in 
38,  only,  out  of  46  such  cases  in  May.  We  may  also  note  that  Calanus  hyperboreus 
(which  did  not  form  as  much  as  1%  of  any  April  catch)  constituted  2-11%  of 
the  14  rich  catches  that  were  made  in  May,  in  1930,  occasionally  also  in  1932. 
However,  the  list  of  species,  which — while  never  averaging  1  %  for  any  cruise  as 
a  whole — have  reached  that  level  at  individual  stations,  is  shorter  for  May  than 
for  April,  as  follows: 

Acartia  longiremis,  4  stations,  5-16  c.c,  1-7%;  amphipods,  2  stations,  4  and  23  c.c, 
2  and  4%;  anthomedusae,  1  station,  85  c.c,  36%;  Arachnactis  larvae,  1  station,  16 
c.c,  9%;  Candacia  armala,  3  stations,  8-38  c.c,  3-22%;  Cenlropages  hamatus,  7 
stations,  5-59  c.c,  2-26%;  Clione  limacina,  10  stations,  3-75  c.c,  1-13%;  Doliolum 
sp.,  2  stations,  53-80  c.c,  8-61%;  Euchirella  rostrata,  1  station,  3  c.c,  1%;  Evadne 
sp.,  1  station,  9  c.c,  2%;Mnemiopsis  leidyi,  1  .station,  8000±  cc,  90%;  mysids,  1  sta- 
tion, 2  cc,  1%;  Oikopleura  dioica,  1  .station,  7  c.c,  7%;  Paracalanus  parvus,  1  sta- 
tion, 14  c.c,  7%;  Paraeuchaeta  norvegica,  I  station,  27  cc,  15%;  Pleuromanima 
gracilis,  5  stations,  6-18  c.c,  1-10%;  Rhim:alanus  nasutus,  9  stations,  4-74  cc^ 
2-24%;  Temora  longicornis,  1  station,  10  cc,  4%;  Temora  slylifera,  1  station,  2  c.c, 
4%;  Tomopteris  sp.,  1  station,  2  c.c,  5%. 

And  the  only  rich  centers  for  any  member  of  this  list  in  May  was  a  swarm  of 
lobate  ctenophores  off  Currituck,  on  May  15,  1929  (p.  371). 

It  is  especially  interesting  that  the  percentage  of  benthonic  derivatives,  of 
all  sorts,  combined,  i.e.,  decapod  larvae,  gammarids,  mysids,  and  leptoline  medu- 
sae, was  so  low  in  May  for  the  area  as  a  whole  (maximum,  8%,  May  1931),  even 
in  the  inshore  belt,  where  the  depth  of  water  is  less  than  50  meters  for  most  of 
the  area  included.  The  relative  paucity  of  oceanic  forms  of  any  sort  anywhere  in 
May  is  also  worth  emphasis,  for  salps,  representing  this  group,  assume  some 
importance  locally  later  in  the  season  (p.  240). 

June.  The  percentages  for  the  eight  leading  species  have  averaged  about 
the  same  for  June  (72-97%,  Table,  p.  229),  as  for  May  (63-92%,),  and  the  Hst  of 
dominant  species  includes  only  one  member  (CUone)  on  any  June  cruise,  that 
does  not  appear  in  the  list  for  May.  Calanus,  on  the  whole  stands  relatively  some- 
what higher  in  June  than  in  May,  ranking  first,  by  far,  in  three  of  the  years  (1929, 
1930,  1931)  and  about  equalling  its  closest  rival  (Aglantha)  in  the  fourth  year 
(1932),  whereas  in  May,  it  ranked  second  in  two  of  the  four  years,  first  in  two 
only.  And  a  still  more  definite  trend  in  relative  importance,  upward  from  May 
to  June,  appears,  for  Calanus,  in  the  relative  frequency  with  which  it  has  formed 
upwards  of  80%  in  the  rich  ( >  500  c.c.)  catches,  for  this  happened  in  3  out  of 


238  memoir:  museum  of  comparative  zoology 

18  such  cases  in  April,  in  4  out  of  40  cases  in  May,  and  in  17  out  of  36  cases  in 
June,  but  not  at  all  in  February.  On  the  other  hand,  it  appears  that  no  signif- 
icant alteration  is  to  be  expected  from  April  through  June,  in  the  inshore-off- 
shore and  north-south  gradients  for  C.  finmarchicus,  for  it  has  averaged  in 
about  twice  as  great  percentage  offshore  and  in  the  north,  as  inshore  and  in 
the  south,  throughout  this  part  of  the  year  (Table,  p.  229).  And  this  last  gener- 
alization is  further  supported  by  the  fact  that  very  strong  dominance  by  Calanus 
(80%)  is  also  much  more  frequent  offshore  in  June  (25%  of  the  stations)  than 
inshore  (7%  of  the  stations),  whereas  relative  scarcity  (less  than  5%)  was  much 
less  frequent  offshore  (12%  of  the  stations)  than  inshore  (26%  of  the  stations). 

Lest  the  reader  gather  the  impression  from  the  foregoing  that  Calanus  (or 
any  other  species  for  that  matter)  is  distributed  with  anything  approaching  rela- 
tive uniformity  over  our  area,  at  anj'  season,  we  must,  however,  emphasize  the 
fact  that  this  is  very  far  from  the  truth.  Actually,  a  chart  for  any  given  cruise, 
whether  for  relative  abundance,  or  for  absolute,  shows  wide  and  irregular  varia- 
tions within  short  distances. 

Sagitta  elegans  also  increased  appreciably  in  relative  importance  from  May 
to  June  in  three  of  the  years  (1929,  1930,  1932),  in  fact,  doubled  in  one  (1932),  its 
trend  being  also  upward,  in  the  rich  (  >  500  c.c.)  hauls,  though  this  increase  was 
not  sufficient  to  bring  it  into  rivalry  with  Calanus.  In  the  fourth  year  (1931),  it 
showed  no  definite  trend  in  percentage,  one  way  or  the  other,  during  this  period. 
It  has  also  averaged  much  more  important  inshore  (about  27%,  for  all  years 
combined)  than  offshore  (about  7%),  in  June,  just  as  in  May  but  with  no  con- 
sistent contrast  between  north  and  south. 

Limacina,  contrasting  with  C.  finmarchicus  and  S.  elegans,  decreased  in  im- 
portance from  May  to  June,  in  each  year,  its  average  percentage  for  all  cruises 
combined  being  only  1/3  as  great  in  the  latter  month  (8%)  as  in  the  former  (20%) 
in  the  north,  only  1/5  as  great  in  June  (5%)  as  in  May  (26%)  in  the  south. 
Similarly,  it  formed  80%  of  the  total  catch  in  only  one  of  39  rich  ( >  500  c.c.) 
hauls  in  June,  contrasting  with  5  out  of  41  such  cases  in  May.  It  is  thus  evident 
that  this  pteropod  has  passed  its  peak  in  importance  by  the  end  of  the  spring, 
with  a  subsequent  decline  by  about  2/3,  in  its  percentage  in  the  plankton,  through 
June,  as  a  reasonable  expectation.  As  a  result  of  this  seasonal  decline,  Limacina, 
ranking  first  or  second  in  May  has  only  once  ranked  second  in  June  (1930),  and 
only  fourth  or  fifth  in  the  other  years. 

It  is  doubtful  whether  any  very  pronounced  seasonal  trend  in  relative  per- 
centage in  either  direction  is  characteristic  for  any  of  the  other  leading  species 


BIGELOW   AND    SEARS :   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  239 

from  May  to  June.  Thus,  the  percentage  of  Aglantha,  which  increased  about 
4-fold  during  this  period  in  one  year  (1932),  did  not  appreciably  alter  in  any 
other  year.  The  average  percentage  of  Metridia,  also,  altered  very  little  from  the 
one  month  to  the  other  in  any  year,  ranging  for  the  whole  series  only  between 
1-2%  and  6%,  though  its  percentage  in  the  rich  catches  markedly  declined  in 
one  year  (1930).  The  period,  May-June,  also,  appears  one  of  relative  constancy 
for  the  average  percentage  of  Centropages,  which  did  not  alter  then  by  more  than 
1-3%  or  so,  in  any  year  of  the  series.  It  is,  in  fact,  during  the  period,  May-June, 
that  this  copepod  is  least  important,  for  it  has  then  averaged  only  about  1-11% 
of  the  total  plankton,  for  the  area  as  a  whole,  or  1-14%  in  the  inshore  belt,  con- 
trasted with  1-49%  in  February.  We  may  also  point  out  that  while  Centropages 
monopohzed  (formed  more  than  80%)  one  out  of  11  rich  catches  in  February, 
this  was  not  recorded  at  all  in  May  and  June.  And  euphausiids,  as  a  group,  have 
also  averaged  about  the  same  in  June  as  in  May,  i.e.,  1-5%,  whether  for  the  area 
as  a  whole,  or  for  rich  ( >  500  c.c.)  catches. 

Species,  however,  that  rank  low  in  average  percentage,  may  rank  high 
locally,  or  even  dominate,  in  June,  as  at  other  seasons.  Euthemisto,  for  example, 
which  has  averaged  only  7%,  in  general,  in  that  month  constituted  19-84%  in 
eight  out  of  48  catches  in  the  year  1930,  which  ranks  it  among  the  leaders,  at 
times.  Clione  limacina  also  formed  7-73%  in  10  June  catches  in  1932  (bringing 
its  average  for  that  month  as  a  whole  up  to  5%  for  the  area  as  a  whole),  though 
it  did  not  form  as  much  as  7%  at  any  station  in  June  of  the  other  years,  and 
agalmids  ranked  high  (about  42%)  in  one  June  haul  of  220  c.c,  Centropages 
hamatus  formed  26%  of  another  of  218  c.c,  (both  in  1932)  while  euphausiids 
made  up  9%  of  one  of  the  three  rich  catches  (  >  500  c.c.)  that  were  recorded  for 
June  1932.  These  rich  spots  may  perhaps  have  been  potential  reservoirs  for 
future  local  swarms  of  these  species.  And  it  is  possible  that  the  following  in- 
stances where  some  particular  species  formed  1%  or  more  of  the  June  catch  (listed 
below)  may  also  be  so  considered: 

Acariia  longiremis,  6  stations,  2-15  c.c,  1-7%;  Aequorea  aequorea,  1  station,  11  c.c, 
11%;  Agalma  okeni,  1  station,  13  c.c,  1%;  agalmids,  4  stations,  4-94  c.c,  3-42%; 
amphipods,  6-35  cc,  1-48%;  Arachnactis  larvae,  3  stations,  8-37  c.c,  1-17%; 
Calanus  hyperboreus,  5  stations,  4-17  cc,  1-6%;  Candacia  armata,  1  station,  6  c.c, 
1%;  Centropages  hamatus,  6  stations,  2-189  c.c,  1-60%;  Crago  sp.,  3  stations,  5-35 
CO.,  3-13%;  cumaceans,  1  station,  2  c.c,  2%;  diphyid  eudo.xids,  1  station,  25  c.c, 
4%;  Doliolum  sp.,  3  stations,  19-82  c.c,  3-50%;  Eucalanus  .sp.,  16  c.c,  4%;  Evadne 
sp.,  7  stations,  2-25  c.c,  1-17%;  Lensia  conoidea,  2  stations,  6-13  c.c,  2-3%;  Mecy- 
nocera  dausi,  1  station,  4  cc,  1%;  Neomysis  americana,  2  stations,  22-100  cc, 
9-83%;  Oikopleura  dioica,  4i stations,  7-17  c.c,  2-17%;  Pleuromamma  gracilis,  1  sta- 


240  memoir:  museum  of  comparative  zoology 

tion,  4  c.c,  3%;  Sagitta  enflata,  2  stations,  2  c.c,  1%;  Temora  longicornis,  6  stations, 
2-12  c.c,  2-11%;  Tomopteris  catharina,  1  station,  4  c.c,  4%. 

It  is  especially  interesting  that  benthonic  animals  of  all  sorts  combined, 
have  averaged  no  more  important  in  June  than  in  May,  for  the  area  as  a  whole 
(Table,  p.  229),  because  they  may  then  be  the  leading  forms  close  in  to  the  land, 
at  localities  where  the  more  generally  dominant  species  happen  to  be  only  in 
small  amount.  Cases  in  point  are  Neomysis  americana  (100  c.c.)  and  crab  larvae 
(68  c.c.)  forming  83%  and  48%  respectively  of  the  total  catches,  at  two  stations 
near  the  mouth  of  Delaware  Bay  in  June  1930;  crab  larvae  (70  c.c.  and  65  c.c.) 
forming  19^^  and  \5%  near  S'eagirt  and  Winterquarter,  on  two  occasions  in 
1931;  and  leptoline  medusae  (120  c.c.)  forming  about  half  the  catch  midway  out 
on  the  shelf  off  Martha's  Vineyard  on  June  19,  1932;  this  last  instance  being  par- 
ticularly interesting  because  the  source  of  supply  in  this  case  was  probably  Nan- 
tucket Shoals,  i.e.  to  the  eastward.  On  the  other  hand,  salps  of  one  species 
or  another  have  appeared  locally  near  the  outer  edge  of  the  shelf  in  considerable 
aggregations  in  June,  foreshadowing  the  much  greater  importance  they  assume  a 
month  later  in  the  summer  (p.  242).  They  may  indeed  constitute  as  much  as  9% 
of  the  general  June  average  for  this  belt  as  a  whole,  in  some  years  (e.g.,  1929, 
1932),  though  in  others,  as  in  1930  and  1931,  they  may  not  average  more  than 
1%  there,  in  that  month.  And  in  1929  Doliolum  (recorded  at  one  May  station, 
p.  270)  had  entered  the  southernmost  sector  in  such  numbers  by  June,  that 
it  was  not  only  omnipresent  then  along  the  Hog  Island  and  Chesapeake  pro- 
files, but  constituted  21  ^^  of  the  average  catch  there,  though  not  detected  at 
all  at  any  of  the  stations  farther  north,  or  anywhere  within  the  confines  of  our 
area  in  any  other  June,  for  that  matter.  But  the  increase  in  the  relative  abundance 
of  salps  and  Doliolum  has  not  been  accompanied  by  any  corresponding  increases 
in  the  number  of  oceanic  species  encountered.  On  the  contrary,  fewer  of  these 
species  were  detected  in  June  than  in  May. 

The  combined  annual  tabulation  for  June  (p.  267) — probably  representing 
the  normal  ranking  for  the  month  better  than  the  percentage  distribution  for  any 
one  individual  cruise — shows  Calanus  so  far  in  the  lead  offshore  and  in  the  north, 
that  it  outranks  its  closest  competitor  {Sagitta  elegans)  more  than  2  to  1  for  the 
area  as  a  whole.  S.  elegans  about  equals  Calanus  inshore,  however,  and  in  the 
south.  The  lower  rankings  for  June  have  shown  wide  variation  from  year  to 
year,  second  for  the  region  as  a  whole  being  either  S.  elegans,  Limacina,  or  Ag- 
lantha,  third,  S.  elegans,  Centropages  typicus,  or  Pseudocalanus,  and  fourth, 
S.  elegans,  Centropages  typicus,  Metridia,  Limacina,  Euthemisto,  or  euphausiids. 


BIGELOW    AND    SEARS:    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES  241 

July.  The  data  for  July  are  confined  to  one  general  cruise  in  1929,  and  two 
in  the  northern  sector  (north  from  Barnegat)  in  1930  and  1931.  But  the  results  of 
these,  added  to  previous  information  for  the  summers  of  1913  and  191G  (Bigelow, 
1915;  1922)  give,  at  least,  an  indication  of  the  seasonal  succession,  and  of  the 
orders  of  magnitude  to  be  expected  in  normal  years. 

If  we  omit,  from  our  calculations,  one  phenomenally  rich  catch  of  salps 
(67109  c.c),  at  the  edge  of  the  continent  ofT  Cape  May,  July  18,  1929,  the  relative 
volumetric  strengths  of  the  leading  species  in  the  July  catches  have  averaged  as 
tabulated  on  page  229.  In  the  year  1929,  which  may  perhaps  be  accepted  as  repre- 
senting the  median  condition,  the  most  striking  alterations  from  June  to  July, 
for  the  region  as  a  whole  (Fig.  11),  were  (A)  that  Sagitta  elegans  had  forged  ahead 
of  Calanus  throughout  the  inshore  belt,  though  still  lagging  behind  it  offshore, 
(B)  that  Centropages  typicus,  which  had  fallen  to  a  low  rank  by  May  and  June 
(p.  229)  had  again  tripled  in  relative  importance  by  July,  i.e.,  was  then  about 
1/2-2/3  as  abundant,  volumetrically,  as  Calanus  in  the  north  (indifferently  off- 
shore-inshore) and  ranked  ahead  of  Calanus  to  the  south. 

Euthemisto  compressa  should  perhaps  be  ranked  higher  in  relative  importance 
for  July  than  might  be  suggested  by  its  low  percentage  (1-6%)  for  the  several 
years  combined,  because  local  centers  of  high  abundance  may  develop  for  it. 
In  1929,  for  example,  it  formed  28%  of  a  rather  small  catch  (67  c.c.)  on  one  occa- 
sion (St.  20556)  near  Cape  May;  in  1930,  again,  it  formed  35%  of  the  total  at  a 
station  near  Shinnecock,  and  it  was  found  swarming  midway  out  on  the  shelf  off 
Martha's  Vineyard  and  off  Montauk  in  July  1913  (Bigelow,  1915,  p.  281).  The 
case  is  parallel  for  Temora  longicornis,  for  while  this  copepod  did  not  average  as 
much  as  1%  on  any  July  cruise,  it  formed  1-2%  of  the  rich  ( >  500  c.c.)  catches 
on  three  occasions  in  1931,  while  in  1916,  it  swarmed  near  Martha's  Vineyard 
(Bigelow,  1922,  p.  146).  Clione  limacina,  may,  likewise,  be  relatively  prominent 
locally  in  July,  for  it  formed  9%  of  a  540  c.c.  catch  close  in  to  New  York  on  one 
occasion  in  1930,  though  it  did  not  average  as  much  as  1%  on  any  July  cruise 
as  a  whole.  It  is  also  suggestive,  on  the  negative  side,  that  while  euphausiid 
larvae  formed  35%  of  one  rich  (  >  500  c.c.)  catch,  offshore,  off  New  York,  in  July 
1929,  euphausiids  as  a  whole  altered  but  little  in  importance  from  June  to  July,  their 
respective  percentages  being  <  1  %  inshore  and  6%  offshore  for  the  former  month, 
<  1%  inshore  and  6%  offshore  for  the  latter,  in  the  year  in  question.  And  lep- 
tomedusae,  which  (in  1929)  had  formed  2-11%  of  the  catch,  at  a  few-  stations  in 
the  northeasternmost  sector  in  June  (p.  240),  had  practically  vanished  thence  by 
the  following  month. 


242  memoir:  museum  of  comparative  zoology 

On  the  other  hand,  the  siphonophores  {Muggiaea  kochii,  Lensia  conoidea,  and 
unidentifiable  agalmid  fragments)  may  locally  constitute  as  much  as  8-10%  mid- 
way out  on  the  shelf  in  the  southernmost  sector  as,  for  example,  in  1929  off 
Chesapeake  Bay  and  off  Winterquarter.  Ctenophores  (Pleurobrachia  pileus  and 
Mnemiopsis  leidyi),  also  may  become  the  dominant  group  inshore  and  to  the 
south  in  midsummer,  even  practically  monopolizing  the  water  there  at  that 
season  in  some  years,  e.g.,  1913,  as  described  below  (p.  369,  370),  though  they 
are  insignificant  in  importance  in  other  summers,  as  in  1929,  when  only  one  rich 
center  was  recorded  for  them.  And  it  appears  that  the  case  is  similar  for  at  least 
one  category  of  visitors  from  offshore,  namely,  the  salps,  which  were  dominant 
close  to  New  York  and  locally  elsewhere  well  inshore  by  the  first  of  August  in  the 
summer  of  1913, ^  though  curiously  enough,  they  only  occurred  sparsely  farther 
out  on  the  shelf  at  the  time  (Bigelow,  1915,  p.  270).  But  seemingly  it  is  only  in 
exceptionally  warm  years  that  this  occurs,  for  while  salps  formed  24%  of  the 
average  catch  offshore  in  July  1931,  and  swarmed  locally  near  the  200-meter 
line  in  that  month  of  1929,  they  were  recorded  only  occasionally  inshore  at  that 
season,  in  1930  or  in  1931,  and  not  at  all  on  the  shelf  in  the  cold  summer  of  1916, 
though  locally  abundant  out  beyond  the  continental  edge  at  the  time  (Bigelow, 
1922,  p.  156). 

The  scarcity  of  salps  inshore,  in  most  summers,  added  to  the  facts  that 
Rhincalanus  nasutus  showed  no  increase  in  relative  importance  from  June  to 
July  in  1929,  1930,  or  1931,  although  generally  distributed  in  the  latter  month, 
that  Doliolum  did  not  increase  relatively  from  June  to  July  in  1929  (the  only  year 
when  recorded  at  all  inside  the  200-meter  line) ,  and  that  other  oceanic  forms  have 
been  negligible  volumetrically,  in  on  the  shelf,  in  all  the  summers  of  record  is 
sufficient  e\idence  that  mass  invasions  of  the  northern  part  of  our  area  by  visitors 
from  warmer  waters  offshore,  are  exceptional  events,  even  at  the  warmest  time  of 
year.  In  fact  the  only  "tropicals"  ever  Hkely  to  be  of  volumetric  import  there, 
are  such  as  are  capable  of  very  rapid  multiplication — e.g.,  salps.  Contributions 
rom  mid-depths  along  the  slope,  of  the  category  represented  by  Eukrohnia 
hamaia  and  Paraeuchaeta  norvegica  have  also  been  insignificant  inside  the  200- 
meter  curve,  north  of  Delaware  Bay,  in  every  year  of  record  (p.  250),  at  all  seasons. 

In  normal  years,  in  short,  the  waters  in  the  northern  sector  are  as  strongly 
dominated  in  July  as  in  June  by  the  boreal  assemblage,  lead  by  Calanus,  Centro- 
pages,  and  Sagitta  elegans,  whether  judged  by  the  fact  that  these  three  species 

'  No  precise  quantitative  data  available  for  that  year. 


BIGELOW   AND    SEARS:    NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  243 

together  have  averaged  about  86%  in  all  July  hauls  combined  in  that  sector,  or  by 
their  prominence  in  the  rich  (  >  500  c.c.)  centers,  of  which  they  have  on  the  aver- 
age constituted  about  this  same  percentage  for  the  several  years  combined,  in 
that  month.  In  the  southernmost  sector,  however,  represented  by  the  Winter- 
quarter,  Hog  Island,  and  Chesapeake  Bay  profiles,  a  considerable  invasion  by 
warm  water  species  was  indicated  in  1929,  by  the  presence,  in  July,  of  3%  (on 
the  average)  of  the  southern  appendicularian,  Oikopleura  dioica,  5%  of  Muggiaea 
kochii,  10%  of  Doliolum,  3%  of  Lucifer  typus,  3%  of  the  phyllosome  larvae  of 
Palinuridae,  and  9%  of  sundry  oceanic  copepods  in  combination.  And  the  occur- 
rence of  phyllosome  larvae  there  is  especially  interesting  as  evidence  of  the  infil- 
tration of  a  neritic  species  from  far  to  the  southward,  no  doubt  by  the  offshore 
route,  because  the  northern  boundary  for  their  probable  parent,  the  spiny  lobster 
(Panuhrus)  is  Cape  Lookout  in  North  Carolina  in  Latitude  about  35°  N. 

Yet,  even  with  this  invasion  of  oceanic  forms,  the  list  of  species,  which,  while 
not  equahng  1  %  in  any  subdivision  of  the  area  on  any  July  cruise,  hence  not  dis- 
cussed above,  have  formed  1%  or  more  at  particular  stations,  is  considerably  less 
extensive  for  July  than  for  June  or  May,  as  follows : 

Acartia  longiremis,  2  stations,  4-9  c.c,  1-3%;  agalmids,  1  station,  9  c.c,  8%;  Calanus 
hyperboreus,  1  station,  6  c.c,  1%;  Candacia  armata,  6  stations,  3-22  cc,  2-17%; 
Eucalanus  attenuatus,  2  stations,  4-89  c.c,  4-36%;  Evadne  sp.,  1  station,  4  c.c,  1%; 
Plironima  sp.,  1  station,  13  c.c,  1%;  Podon  sp.,  1  station,  6  c.c,  7%;  stomatopod 
larvae,  1  station,  4  cc,  1%;  Temora  longiconiis,  4  stations,  2-26  c.c,  1-2%;  Teniora 
stylifera,  1  station,  26  cc,  9%. 

Autumn.  Quantitative  information  is  lacking  for  August  and  September. 
But,  if  October  data  for  the  one  year  of  record  (1931)  be  representative,  as  seems 
probable,  it  appears  that  early  autumn  sees  a  notable  decrease  in  the  relative 
importance  of  Calanus,  the  percentage  of  which  declined  in  the  offshore  belt'  in 
that  year  from  61%  in  July  to  8%  in  October  in  the  northern  sector,  and  from 
41  %  in  June^  to  7%  in  October  in  the  southern.  In  fact,  the  maximum  percentage 
of  Calanus  in  any  of  the  richer  ( >  300  c.c.)  October  hauls,  was  only  16%,  con- 
trasted with  frequent  percentages  of  80%  or  more  in  summer  and  spring,  while 
there  was  one  October  haul  without  Calanus.  The  average  percentage  of  salps 
also  decreased  in  the  northern  sector,  offshore,  from  24%  in  July  to  less  than  1% 
in  October,  though  lasis  zonaria  then  formed  51%  of  the  total  catch  of  112  c.c. 
at  one  station  ofif  Winterquarter  in  the  south.    On  the  other  hand,  Centropages 

'■  The  October  cruise  of  1931  was  confined  to  the  offshore  belt. 
'  The  July  cruise  was  confined  to  the  northern  sector. 


244  memoir:  museum  of  comparative  zoology 

typicus,  which  had  been  neghgible  in  June  and  July  of  the  year  in  question,  so 
greatly  increased  relatively  (as  well  as  absolutely)  during  late  summer,  or  early 
autumn,  that  by  October,  it  ranked  far  above  its  closest  rival  (Paracalanus)  in 
the  north  (47%)  and  slightly  so,  in  the  south  (14%).  Perhaps  even  more  striking 
in  that  year  was  the  autumnal  increase  in  the  percentage  of  Paracalanus,  which 
did  not  average  more  than  1%  in  any  considerable  subdivision  of  the  area  in 
any  summer  of  record,  but  which  formed  26%  offshore  in  the  north,  and  12%  in 
the  south,  in  the  October  in  question.  Metridia  lucens,  also,  increased  in  the 
north  from  1%  to  11%  between  July  and  October,  but  was  in  about  the  same 
percentage  in  the  south  in  the  latter  month  (2%)  as  it  had  been  in  June  (1%). 
Other  species  showing  smaller,  or  less  regular  increases  from  June  or  July  to 
October  are  Pseudocalanus  from  less  than  1%  (June)  to  4%  (October)  in  the 
south,  but  with  httle  change  in  the  north;  Sagitta  serratodentata,  from  less  than 
1  %  in  June  to  2  %  in  October  in  the  north  and  from  2  %  in  June  to  9  %  in  October 
in  the  south;  while  Sagitta  enflata,  Oikopleura  dioica,  and  Peniha,  which  did  not 
enter  into  the  picture  at  all  in  the  north  in  July  or  in  the  south  in  June,  averaged 
6%,  2%,  and  8%,  respectively,  of  the  catch  in  the  latter  sector  in  October.  How- 
ever, none  of  the  other  members  of  the  community  that  constituted  as  much  as 
1  %  of  the  average  catch  for  that  October  had  experienced  any  significant  change 
in  relative  importance  since  June  (in  the  south)  or  July  (in  the  north).  And  the 
list  of  species  forming  1%  or  more  at  individual  stations  had  not  lengthened  ap- 
preciably in  this  same  period: 

Acartia  sp.,  2  stations,  1-4  c.c,  4-6%;  agahnid,  1  station,  9  c.c,  4%;  Candacia  ar- 
mata,  2  stations,  3-14  c.c,  3-11%;  Centropages  violaceous,  1  station,  1  c.c,  1%; 
Corycaeus  sp.,  1  station,  3  c.c,  3%;  Mecynocera  clausi,  4  stations,  1-4  c.c,  1-4%; 
Oncaea  sp.,  1  station,  9  c.c,  11%;  Pleuromamma  gracilis,  1  station,  2  c.c,  1%; 
Scolecithrix  danae,  2  stations,  3-14  c.c,  2-12%;  Teynora  longicornis,  1  station,  1  c.c, 
1%;  Temora  stylifera,  2  stations,  1  c.c,  1%. 

These  mutual  changes  in  relative  abundance  result  in  a  reversal  in  relative 
ranking,  from  smmner  (Table  p.  267)  to  mid-autmnn,  when  Calanus  ranked  only 
fourth  instead  of  first,  but  Centropages  first  instead  of  third,  fourth,  or  fifth 
(Fig.  11),  with  second  place  falhng  to  Paracalanus,  the  highest  ranking  of  which 
was  not  above  fifth  on  any  July  cruise,  while  it  does  not  appear  at  all  in  the  lists 
of  dominant  species  for  April,  May,  or  June. 

No  data  as  to  the  relative  abundance  of  the  various  species  are  available,  for 
the  three  month  period,  November  to  January,  of  any  year. 


bigelow  and  sears:  north  atlantic  zooplankton  studies        245 

Annual  Differences 

It  was  a  fortunate  chance  that  so  short  an  observational  series  should  have 
included  one  year  (1932)  when  the  late  winter  was  unusually  warm  in  the  waters 
of  our  area,  one  (1913)  in  which  the  summer  was  warmer  than  normal,  and  one 
(1916)  in  which  it  was  colder,  making  it  likely  that  such  annual  variations  as  were 
recorded  cover  the  range  commonly  to  be  expected  from  year  to  year. 

It  is  not  astonishing  that  typically  boreal  species  such  as  Calanus  finmarchi- 
cus  and  Sagitta  elegans  were  relatively  less  prominent  at  the  end  of  a  winter  of  the 
type  represented  by  1932  than  in  the  other  years.  And  the  high  percentage  of 
Paracalanus  in  that  same  February  may  also  have  been  associated  with  tempera- 
ture. But  temperature  offers  no  apparent  explanation  for  the  facts  that  one  of 
the  two  years  when  Centropages  typicus  ranked  high  (1930)  is  to  be  classed  as 
cool,  the  other  (1932)  as  warm;  or  that  while  S.  elegans  was  the  dominant  form 
in  one  of  the  cool  winters  (1931),  it  was  of  only  very  minor  importance  in  the 
other  (1930).  And  we  can  merely  note  that  annual  variations  in  the  relative 
ranking  of  the  several  dominant  species  were  much  wider  in  the  offshore  belt  than 
inshore.  No  doubt  the  explanation  for  this  lies  in  the  fluctuations  of  conflicting 
water  masses  along  the  edge  of  the  continent. 

Following  an  abnormally  warm  winter  (to  judge  from  1932),  Calanus  con- 
tinues much  less  important  relatively  than  usual,  as  well  as  less  abundant,  through 
May  and  June,  even  though  the  temperature  anomaly  may  have  entirely  dis- 
appeared by  mid-spring.  On  the  other  hand,  Aglantha,  Limacina,  and  Sagitta 
elegans  ranked  much  higher  in  that  June  than  in  any  other.  The  chief  variation 
from  year  to  year  later  on  in  the  summer — and  one  of  considerable  significance 
from  the  standpoint  of  the  fertility  of  our  region  as  a  pasture  for  plankton-eating 
fishes — is  that  Calanus  and  its  companion  species  dominate  much  farther  south- 
ward in  cool  and  average  summers,  than  in  what  may  be  classed  as  "warm,"  at 
least  along  the  outer  part  of  the  shelf.  On  the  other  hand,  ctenophores  (Mnemiop- 
sis  and  Pleurobrachia),  combined  with  salps,  play  a  dominating  role  to  the 
southward,  especially  inshore,  at  least  in  some  warm  summers,  but  are  not 
volumetrically  significant  over  any  extensive  areas  in  cool.  The  one  extreme  in 
these  respects  may  be  illustrated  by  1929,  when  Calanus  finmarchicus,  Centro- 
pages typicus,  and  Sagitta  elegans  formed  even  a  larger  percentage  of  the  total 
catches  for  the  southern  sector  as  a  whole  in  July  (average,  58%)  than  they  had 
in  June  (34%)  or  in  May  (27%),'  except  at  one  station  close  to  the  200-meter 

1  93%  in  April,  34%  in  May,  50%  in  June,  and  64%  in  July. 


246  memoir:  MtisEUM  of  comparative  zoology 

contour  off  Cape  May,  where  salps  swarmed  (p.  241).  And  Calanus  was  also  the 
leading  species,  offshore,  southward  of  New  York  in  the  summer  of  1916 — like- 
wise a  cold  year — along  a  narrowing  belt  to  the  offing  of  Chesapeake  Bay  as  de- 
scribed elsewhere  (Bigelow,  1922,  p.  139).  The  opposite  extreme  was  illustrated 
by  the  summer  of  1913,  when  copepods,  which  were  in  rich  aggregations  over  the 
continental  shelf  south  of  Cape  Cod,  were  "counted  by  individuals"  south  of 
New  York  (Bigelow,  1915,  p.  285),  instead  of  by  hundreds  of  cubic  centimeters, 
while  in  some  of  the  southern  hauls  no  copepods  at  all  were  detected.  Sagiita 
elegans  was,  also,  so  scarce  south  of  New  York  in  that  summer  that  none  of  the 
19  subsurface  hauls  yielded  more  than  odd  specimens  of  it  there,  which  applies 
equally  to  Limacina,  to  Euthemisto,  and  to  euphausiids  as  a  group. 

It  is  an  interesting  question  whether  it  is  characteristic  of  warm  summers 
that  the  poverty  of  the  Calanus  community,  from  New  York  southward,  is  com- 
pensated for — especially  in  the  inshore  belt — by  swarms  of  ctenophores  (Mnemi- 
opsis  and  Pleurobrachia)  and  of  salps,  as  happened  in  1913  (Bigelow,  1915,  p. 
270).  Unfortunately,  no  data  are  available  for  July  for  1932,  the  year  of  the 
present  series  that  seems  the  most  pertinent  in  this  connection.  But  the  fact  that 
ctenophores  were  found  swarming  locally  in  May  (p.  237),  in  June  (p.  369),  and 
in  July  (p.  242),  combined  with  widespread  presence  of  salps  in  the  offshore  belt 
at  that  season,  and  occasionally  in  abundance  (p.  242),  makes  it  likely  that  our 
area  is  always  sufficiently  seeded  with  members  of  these  groups  throughout  the 
vernal  half  year,  for  rapid  multiplication  to  take  place  when  circumstances  favor. 

No  information  is  available  as  to  annual  \'ariations  in  the  relative  abundance 
of  different  species  for  autumn  or  early  winter. 

Sources  of  the  Local  Plankton 

The  planktonic  community  of  our  area,  like  that  of  the  Gulf  of  Maine,  con- 
sists in  great  majority  of  holoplanktonic  species.  The  only  benthonic  derivatives 
that  have  averaged  as  much  as  1  %  of  the  total  volume  in  either  subdivision  of  the 
area  at  any  season,  have  been  small  leptoUne  medusae,  which  may  form  up  to 
4%  for  the  area  as  a  whole  in  May  (Table,  p.  229),  and  the  larvae  of  crabs  or  of 
hermit  crabs,  which  may  swarm  locally  next  the  land  in  July  (p.  286),  and  form 
up  to  1-5%  offshore  as  well,  in  late  spring  and  early  summer  in  some  years 
(Table,  p.  229).  Perhaps  we  should  also  so  classify  (because  of  winter  eggs)  the 
cladoceran,  Penilia  schmackeri,  which  formed  8%  offshore  in  the  south  in  the  one 
October  of  record. 


BIGELOW   AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  247 

Indigenous  species  also  greatly  predominate,  in  volume,  over  immigrant,  at  all 
seasons,  having  (on  the  average)  formed  upwards  of  92%  of  the  total  volume  of 
plankton  for  the  area  as  a  whole  in  February,  95%  in  April  and  May,  89%  in 
June,  84%  in  July  (of  the  one  year  when  the  entire  area  was  surveyed)  and  89% 
on  the  one  October  cruise.  Immigrant  species  were  in  fact  negligible  in  February 
and  April,  except  for  salps  which  may  appear  in  small  percentage  in  the  offshore 
belt,  even  this  early  in  the  year  (p.  272).  With  the  advance  of  the  season,  other 
immigrants  enter,  however,  in  small  amount  into  the  "dominant"  list  (>  1%), 
some  from  the  colder  waters  from  the  east,  others  from  the  continental  slope  off- 
shore. The  members  of  the  first  of  these  categories,  represented  in  May  and 
June  by  Oikopleura  lahradoriensis,  by  Calanus  hyperboreus,  and  perhaps  by  the 
lobate  ctenophore  Bohnopsis,  have  never  in  our  experience  been  of  any  vol- 
umetric importance  even  in  the  easternmost  sector  which  is  most  open  to  their 
entry.  And  except  for  salps,  visitors  from  offshore  are  equally  insignificant  in  the 
vohmietric  total  in  the  northern  sector,  even  offshore,  in  spring.  But  they  play  a 
somewhat  more  important  role  by  midsummer  in  the  southern  sector,  where,  in 
1929,  tropical  decapods  and  copepods,  Doliolum,  salps,  and  siphonophores  to- 
gether formed  about  15%  of  the  average  catch.  And  the  state  is  similar  in  au- 
tumn, judging  from  the  fact  that  in  October  1931,  offshore  chaetognaths  and 
oceanic  copepods,  in  combination,  formed  18%  in  the  offshore  belt  in  the  south. 

The  lists  also  include  a  considerable  variety  of  other  immigrants,  besides 
those  just  mentioned,  which — if  never  plentiful  enough  to  be  of  any  volumetric 
importance  within  our  limits — promise  to  be  of  such  value  as  indicator-species 
that  they  deserve  some  discussion  from  that  ^^ewpoint,  even  though  this  be 
somewhat  foreign  to  the  main  thesis  of  the  present  report.  Indicator-species, 
from  whatever  source,  may  be  grouped  for  convenience  into  two  categories. 
The  one  group  comprises  those  species  that  either  fail  to  breed  in  exotic 
surroundings,  so  that  their  presence  there  endures  only  for  the  span  of  life 
of  the  entrant  individuals,  or  which — if  they  do  succeed  in  breeding — vanish 
shortly  after  the  immediate  effects  of  active  invasion  by  exotic  waters  have 
ceased.  The  locaUties  of  occurrence  of  the  members  of  this  group  yield  direct 
evidence  as  to  short  term  intrusions  of  waters  of  one  or  other  origin  into  our  area. 
The  second  category  comprises  the  considerable  variety  of  species  that,  while 
regularly  indigenous,  vary  periodically  in  abundance  as  water  of  one  or  another 
origin  is  in  the  ascendency.  In  the  long  run,  it  is  from  this  group  that  we  may 
expect  the  more  reliable  information  as  to  the  secular  variation  in  the  propor- 
tionate amounts  in  which  the  different  waters  mingle.  For  an  excellent  example 


248  memoir:  museum  op  comparative  zoology 

of  this  we  may  refer  to  the  information  that  the  mutual  fluctuations  in  abundance 
of  Sagitta  elegans  and  Sagitta  setosa  have  yielded  as  to  variations  in  the  propor- 
tionate amounts  of  Atlantic  and  of  Coastal  waters  in  the  English  Channel 
(Russell,  1935;  Kemp,  1938;  Furnestin,  1938)  and  of  oceanic  and  "Bank"  water 
off  the  east  coast  of  England  (Wimpennj^  1937).  But  trustworthy  inferences  in 
this  regard  must  await  much  more  detailed  knowledge  of  the  ecological  rela- 
tionships of  the  animals  concerned,  than  our  studies  can  yet  provide. 

In  the  particular  part  of  the  sea  with  which  we  are  concerned,  the  waters 
that  mingle  over  the  shelf,  to  maintain  the  comparatively  stable  state  (as  regards 
temperature  and  salinity)  that  exists  there,  draw  chiefly  from  land  drainage  on 
the  one  side,  from  the  highly  saline  band  along  the  continental  slope  on  the  other, 
and  from  intrusions  past  Cape  Cod,  of  shelf  water  from  the  east,  which  come 
chiefly  in  the  spring.  The  planktonic  animals  that  these  waters  bring  with  them 
are  subject  to  a  similar  classification.  But  it  is  doubtful  whether  any  considerable 
volume  of  water  of  shelf  origin  enters  our  area  past  Cape  Hatteras  from  the  south, 
for  no  direct  e\'idence  of  this  has  been  detected  in  the  distribution  of  temperature 
or  of  salinity  (Parr,  1933;  Bigelow,  1933;  Bigelow  and  Sears,  1935). 

Within  our  limits — even  in  the  most  southern  sector — immigrants  from  shelf 
waters  further  south  have  been  negligible,  as  was  to  be  expected  from  the  insig- 
nificance of  water  increments  from  this  source  that  might  convey  them.  In  fact, 
the  only  species,  palinurid  larvae,  certainly  belonging  to  this  category  that  has 
been  detected  in  such  examination  of  the  catches  as  has  yet  been  made,  may 
actually  enter  our  boundaries  via  the  offshore  route,  i.e.,  from  the  continental 
edge,  for  these  phyllosome  larvae  are  notoriously  subject  to  extensive  northerly 
drifts  along  the  American  slope.  Immigrants  of  the  opposite  category,  namely 
from  more  easterly  shelf  waters,  play  a  much  more  important  role,  due  to  the 
considerable  amounts  of  water  from  that  source,  that  may  enter  the  eastern 
sector  in  the  spring  in  some  years,  and  perhaps  to  some  extent  every  year. 
Calanus  hyperboreus  is  especially  interesting  in  this  respect,  its  existence  being 
so  brief  in  our  area  that  the  fluctuations  in  its  local  status  very  closely  follow  the 
waxing  and  waning  of  the  drifts  that  bring  it  past  the  offing  of  Cape  Cod.  Oiko- 
pleura  labradoriensis  appears  to  be  slightly  more  successful  as  a  colonist,  for  it 
may  breed  to  some  extent,  as  it  drifts  westward  and  southward.  But  its  annual 
presence  within  our  limits  is  sufficiently  brief  for  it,  too,  to  serve  as  a  reliable 
indicator  of  a  more  boreal  source.  So,  too,  for  Thysanoessa  longicaudata  and  per- 
haps for  Erythrops.  The  incidence  of  the  species  belonging  to  this  category  may 
indeed  prove  more  reliable  as  drift-indicators  in  this  particular  case,  than  is  the 


BIGELOW   AND    SEARS :   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  249 

distribution  of  temperature,  because  the  latter  is  so  nearly  alike  on  the  two  sides 
of  Cape  Cod  at  the  critical  season,  that  it  is  not  easy  to  follow  east-west  move- 
ments of  water  there,  by  their  thermal  effects. 

We  can  already  say  that  the  combined  records  for  these  species  show  the 
effects  of  such  indrafts  as  extending  widespread  over  the  shelf  down  at  least  to 
latitude  38°,  and  occasionally  to  36°,  in  one  year  or  another  in  the  months  of 
April,  May  and  June,  but  that  they  give  no  evidence  of  any  significant  increment 
from  this  source  in  the  later  summer,  in  autumn,  or  at  the  end  of  winter. 

Comparison  with  the  plankton  of  the  Gulf  of  Maine  brings  out  the  interesting 
difference  that  none  of  the  hauls  in  our  area  have  yielded  so  much  as  a  single  speci- 
men of  the  typically  subarctic  forms,  Ptychogena  ladea,  Mertensia  ovum,  Limacina 
helicina,  or  Oikopleura  vanhoffeni.  Neither  has  Metridia  longa  ever  been  detected 
west  of  Cape  Cod,  although  it  occupies  much  the  same  faunal  niche  in  the  Gulf 
of  Maine  to  the  east  that  Oikopleura  lahradoriensis  does  in  our  area,  becoming 
widely  distributed  there,  and  perhaps  succeeding  in  breeding  to  some  extent,  in 
years  when  it  passes  Cape  Sable  in  greatest  number  (Bigelow,  1926,  p.  61).  The 
offing  of  Cape  Cod  thus  appears  to  mark  as  definite  a  transition-zone  (probably 
thermal)  for  some  of  the  northern  planktonic  animals  as  it  does  for  many  ben- 
thonic  forms.  Another  interesting  regional  contrast  is  that  while  our  area  and  the 
Gulf  of  Maine  both  receive  immigrants  from  the  east,  these  tend  to  disperse  much 
more  generally  over  the  shelf  west  and  south  from  Cape  Cod,  than  is  the  case  in 
the  Gulf,  where  the  shorter-lived,  subarctic  immigrants  have  been  most  fre- 
quently encountered  either  in  the  eastern  side  or — if  further  west — within  about 
30  miles  or  so  of  the  land ;  in  other  words  in  the  track  of  the  peripheral  anticyclonic 
drift  (Bigelow,  1926,  p.  59;  Fish  and  Johnson,  1937,  p.  248). 

Little  can  be  said  as  to  the  magnitude  of  the  contribution  that  waters  com- 
ing from  the  east — i.e.,  from  George's  Bank  or  the  Gulf  of  Maine — makes  to  the 
volume  of  plankton  in  our  area,  because  much  the  same  assemblage  of  species 
usually  dominates  the  community  to  the  east  as  well  as  to  the  west  of  Cape  Cod; 
the  few  that  are  sure  indicators,  within  our  limits,  of  this  eastern  source  are  rela- 
tively unimportant  in  the  Gulf  also,  so  far  as  volume  is  concerned. 

The  basin  of  the  Gulf  of  Maine,  at  depths  greater  than  about  150  meters 
might  be  described — without  much  exaggeration — as  a  lateral  extension  of  the 
slope  water  habitat  via  the  Eastern  Channel,  so  far  as  temperature  and  salinity 
are  concerned.  And  this  is  reflected  in  the  fact  that  its  deeper  strata  constantly 
support  a  considerable  endemic  population  of  the  boreal  mid-water  copepod, 
Paraeuchaeta  norvegica,  and  of  the  decapod  genus,  Pasiphaea,  with  a  sparse  but 


250  memoir:  museum  of  comparative  zoology 

generally  distributed  immigrant  population  of  Eukrohnia  hamata.  A  scattering 
of  Dimophyes  arctica  of  similar  origin  also  occurs  there,  while  Sagitta  maxima  and 
*S.  lyra,  (perhaps  the  most  useful  animal  indicators  of  water  entering  the  Gulf  of 
Maine  at  lower  levels)  have  been  taken  repeatedly  in  the  deep  hauls  in  the  basin. 
No  topographic  parallel  to  this  particular  situation  exists  west  or  south  of  Cape 
Cod,  for  while  the  submarine  gorge  off  the  Hudson  River,  and  the  others  farther 
south  that  have  recently  been  mapped  by  the  U.  S.  Coast  and  Geodetic  Survey 
(Rude,  1938)  furrow  the  edge  of  the  continent  to  a  depth  much  greater  than  that 
of  the  basin  of  the  Gulf  of  Maine,  the  extent  of  area  occupied  by  their  troughs  is 
so  small  that  movements  of  water  up  their  slopes  would  not  be  expected  to  con- 
tribute much  to  the  communities  existing  over  the  neighboring  shelf.  On  the 
other  hand,  the  whole  frontage  of  the  latter  is  open  to  direct  influences  of  this 
sort  from  the  continental  slope. 

Actually,  Paraenchaeta  norvegica,  Sngittn  maxima,  and  Eukrohnia  hamata 
complete  the  list  of  immigrants  from  offshore  that  are  reliable  indicators  of 
water  from  the  deeper  levels  within  our  area,  for  we  have  no  record  at  all  of  S. 
lyra  or  of  Demophyes  arctica  within  our  Umits,  nor  did  the  towings  for  the  period, 
1929-1932,  yield  a  single  representative  of  the  bathypelagic  community  of  black 
fishes  that  Beebe  (1929)  found  so  abundant  in  the  mouth  of  the  Hudson  Gorge, 
of  red  prawns,  or  of  deep  water  medusae.  The  very  paucity,  however,  of  what 
we  may  name  the  "Paraenchaeta"  community  makes  them  (when  they  do  occur 
on  the  shelf) ,  the  more  reUable  indicators  of  highly  saUne  water,  expanding  shore- 
ward up  the  bottom  slope.  By  this  evidence,  water  movements  of  this  sort  have 
no  significant  effect  as  agents  of  mass  transport  beyond  the  100-meter  contour. 

Monthly  ratios  of  records  of  Paraenchaeta,  Eukrohnia,  and  S.  maxima  com- 
bined, to  total  number  of  offshore  stations,  of  1.1  to  1  for  February  (32  records) ; 
0.6  to  1  for  May  (55  records),  0.1  to  1  for  June  (8  records),  and  0.1  to  1  for  July 
(4  records),  with  no  records  at  all  for  October,  point  to  such  indrafts  as  decreasing 
somewhat  either  in  frequency  or  in  volume  from  late  winter  through  spring  to 
summer,  seemingly  to  cease  entirely  in  early  autumn. 

The  great  majority  of  the  records  for  this  group  lie  within  20  miles  or  so  of 
the  edge  of  the  continent,  though  occasionally  dispersed  farther  inshore,  as  dis- 
cussed under  the  individual  species  concerned.  But  they  are  scattered  all  along 
this  offshore  belt,  from  the  one  boundary  of  the  area  to  the  other,  without  ap- 
parent concentration  in  any  particular  sector. 

The  records  inside  the  200-meter  contour  for  species  which  (while  equally 
surely  of  offshore  origin)  may  have  come  in  with  the  upper  strata  of  water,  are 


BIGELOW    AND    SEARS:    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES 


251 


not  only  much  more  numerous  (Fig.  13),  but  this  group  is  qualitatively  much 
more  varied  for  it  includes  salps  and  Doliolum;  the  decapod,  Lucifer  typus;  the 
amphipod,  Phronima;  the  euphausiids,  Stylocheiron,  Thysanopoda,  and  Euphau- 
sia;  the  copepods,  Eucalanus,  Euchirella,  Pleuromamma,  Rhincalanus,  Centro- 
pages  violaceus,  Mecynocera  clausi,  Sapphirina,  Scolecithrix  danae,  Calanus 
minor,  Undinula  vulgaris;  the  pteropods,  Corolla calceola,  Creseis acicula,  C.  conica, 


76'                    73'                   74"                    TT                   IZ'                    7r                     w 

«" 

tr 

^^K^^^  \         .^^ 

2 

» 

^hMH^^ 

4 

12 

9 

22 

13 
16 
56.^' 

34 
47 

38- 

3r 

H 

f 

2 

8 

22 

26 

54 '/V    / 

/ 

t 

w 

3 

7 
27 

49 

/ 

5'  4o/ 
/ 

/ 

|BI4       21 

^%' 

H' 

K^   -> 

\ 
1 

J6" 

mV  '*'^'' 

^KHK' 

j 

76"                        75'                        74*                        ly                       W                       1 

r                   70- 

Fig.  13.  Numliers  of  records  at  different  localities,  for  tropical  oceanic  species  listed  on  this  page. 

C.  virgula ;  the  heteropod,  Firoloida ;  the  chaetognaths,  Sagitta  hexaptera,  Ptero- 
sagitta  draco,  and  Krohnita  subtilis;  the  medusae,  Niobia  and  Aglaura  (Bigelow, 
1915);  and  the  siphonophores,  Abylopsis,  Bassia,  Agalma  okeni,  Ceratocymba 
sagittata,  Eudoxoides  spiralis,  Chelophyes  appendiculata,  Lensia  fowleri,  Vogtia 
pentacantha,  Physophora,  and  PhysaUa.  Candacia  armata  and  Sagitta  enflata  also 
depend  on  immigration,  from  the  continental  slope,  for  their  continued  existence 
on  the  shelf,  though  perhaps  breeding  there  to  some  extent  locally  (pp.  318,  356), 
while  the  pahnurid  larvae  may  also  reach  our  southern  .sector  via  the  offshore 
route  (p.  243).  The  localities  of  capture,  for  this  group  as  a  whole,  have  been  most 


252  memoir:  museum  of  comparative  zoology 

numerous  at  the  outermost  stations,  i.e.,  within  10-15  miles  of  the  200-meter 
contour,  progressively  less  and  less  so  in  toward  the  land  (Fig.  14),  as  distance 
from  their  source  of  origin  increases,  though  occasional  members  of  the  group 
have  been  taken  even  close  in  to  the  coast  here  or  there  as  noted  elsewhere. 
The  most  interesting  feature  of  the  distributional  chart  is,  however,  that  the 
records  for  the  category  as  a  whole  have  been  about  as  frequent,  relatively,  off 
one  sector  of  the  coast  as  off  another  (apparent  concentrations  on  the  chart  re- 
sult from  the  geographic  locations  of  the  several  profiles  and  the  number  of  sta- 
tions on  each),  corresponding  to  the  fact  that  the  slope  water  with  its  oceanic 
fauna,  not  only  fronts  the  entire  length  of  our  area,  but  lies  as  close  to  the  200- 
meter  line  in  one  place  as  in  another,  and  with  successive  isotherms  tending  on 
the  whole  to  parallel  the  trend  of  the  continent.  In  fact,  a  majority  of  the  species 
here  used  as  indicators  of  that  source  have  long  been  known  to  occur  in  this  belt 
much  further  to  the  north  and  east. 

The  obvious  implication  is  that  the  mixture  of  offshore  oceanic  water  with 
inshore  shelf  water  takes  place  all  along  the  outer  edge  of  the  shelf  westward  and 
southward  from  the  offing  of  Martha's  Vineyard  past  that  of  Chesapeake  Bay. 
This  contrasts  with  the  situation  in  the  Gulf  of  Maine,  where  indrafts  of  this  sort 
enter  chiefly  in  the  eastern  side,  and  where  records  of  oceanic  indicators  have  not 
only  been  far  less  numerous  than  is  the  case  west  and  south  of  Cape  Cod,  but 
confined  for  the  most  part  to  the  peripheral  belt,  just  as  are  those  for  arctic  in- 
dicators (Bigelow,  1926,  Fig.  31). 

The  average  number  of  records  for  this  category  of  species  (omitting  Sagitta 
enflata  and  Candacia  which  may  reproduce  to  some  extent  within  our  limits)  was 
2.4  per  offshore  station  in  February,  2.4  in  April,  3.1  in  May,  2.6  in  June,  and  2.7 
in  July,  but  4.6  in  October,  for  the  period  1929-1932.  This  seasonal  distribution 
suggests  that  immigration  is  as  active  in  one  month  as  another  from  the  end  of 
winter  through  spring  and  summer,  but  somewhat  more  so  in  autumn.  In  this 
connection,  it  is  perhaps  appropriate  to  point  out  that  averages  of  about  3.4 
records  per  offshore  station  for  species  of  this  category  for  all  months  combined 
in  1930,  3.4  record  in  1931,  and  3.6  record  in  1932  do  not  suggest  any  wide  differ- 
ence from  year  to  year,  during  the  three  year  period.  And  while  the  average  for 
1929  was  somewhat  smaller  (about  1.5)  it  may  not  have  been  significantly  so,  in 
view  of  the  roughness  of  the  methods  employed. 

The  catches  have  yielded  but  few  species  the  production  of  which  is  known 
to  be  closely  confined  to  the  immediate  vicinity  of  the  land,  and  which  can  con- 
sequently be  classed  as  reliable  indicators  of  drifts  of  water  seaward  from  the 


BIGELOW   AND    SEARS:   NORTH    ATLANTIC    ZOOPLANKTON    STUDIES  253 

coast-line  out  over  the  .shelf.  Among  such  are  the  larvae  of  the  blue  crab  (Calli- 
nectes) ;  the  mysid,  Neomysis  americana;  the  copepods,  Acartia  clausii  and  A. 
tonsa;  and  the  cladoceran  genera,  Podon  and  Evadne.  Stomatopod  and  palinurid 
larvae  also  fall  in  this  same  category,  but  may  more  immediately  be  indicators  of 
offshore  water  in  the  particular  region  in  question  (p.  243).  And  the  bottom  habi- 
tats of  other  benthonic  derivatives  so  far  detected  in  the  catches,  such  as  the 
larvae  of  other  species  of  crabs,  the  decapod-shrimp,  Crago,  and  the  various 
leptoline  medusae,  extend  to  depths  so  great  that  the  contribution  of  their  pelagic 
stages  to  the  plankton  of  the  overlying  waters  may  take  place  well  out  on  the 
shelf.  We  may  refer  in  this  connection  to  the  hydroids  that  have  been  found 
floating — and  apparently  growing — near  the  surface  over  George's  Bank  in 
spring  and  summer  (Bigelow,  1914,  p.  414;  1926,  p.  379;  Fraser,  1915,  p.  306). 

In  the  case  of  the  blue  crab  larvae,  the  captures  extended  50  miles  out,  off 
Chesapeake  Bay  (Bigelow,  1915,  p.  271).  Captures  of  Acartia  clausii  and  A. 
tonsa  (the  latter  for  1916  only)  were  however  coastwise  (p.  303).  Most  of  the 
records  of  Neomysis  likewise  lie  within  a  few  miles  of  land.  In  short,  the  contri- 
bution, from  this  general  source,  to  the  planktonic  community  of  the  continental 
shelf,  even  in  the  inshore  belt — has  proved  negUgible,  except  locally  and  tempo- 
rarily, close  in  to  the  land,  which  parallels  the  situation  in  the  Gulf  of  Maine. 
Neither  have  we  any  record  out  at  sea  of  the  scyphomedusan  genus  Aurellia, 
anywhere  west  or  south  from  the  Martha's  Vineyard  profile,  though  it  is  plentiful 
at  Woods  Hole  at  the  one  boundary  of  our  area,  and  in  Chesapeake  Bay  at  the 
other  (Cowles,  1930),  or  of  Dactylometra,  which  swarms  in  summer  in  estuarine 
situations  along  our  middle  Atlantic  coast.  But  since  no  particular  watch  was 
kept  for  large  medusae  (as  has  been  done  in  the  Gulf  of  Maine) ,  the  inference  to 
be  drawn  from  their  failure  in  the  collections,  is  only  that  they  were  not  dispersed 
seaward,  in  any  summer  of  record,  in  sufficient  numbers  to  be  picked  up  in  the  nets. 

On  the  other  hand,  the  records  for  Evadne  were  widespread  across  the  whole 
breadth  of  the  shelf,  with  those  for  Podon  also  extending  well  out  (p.  346),  which 
suggests  considerably  wider  dispersal  offshore  for  these  cladocerans  within  our 
limits  than  in  the  Gulf  of  Maine  (Bigelow,  1926,  p.  307;  Fish  and  Johnson,  1937), 
where  the  dominant  drift  is  more  definitely  peripheral. 

THE  AREA  AS  A  FEEDING  GROUND  FOR 
PLANKTON-EATING  FISH 

It  is  common  knowledge  that  the  groups  of  animals  ordinarily  dominant  in 

the  plankton  of  our  area  are  staple  food  for  fishes  in  boreal  seas  generally,  and 


254  memoir:  museum  op  comparative  zoology 

that  the  particular  species  of  copepods,  pteropods,  amphipods,  and  sagittae, 
with  which  we  are  here  concerned,  are  leading  items  in  the  diet  of  the  mackerel 
within  our  limits  (Bigelow  and  Welsh,  1925,  p.  201),  as  well  as  in  that  of  the 
herring  in  higher  latitudes.'  In  fact,  the  only  occasions  when  any  considerable 
fraction  of  the  animal  plankton  of  our  area  is  not  what  we  may  term  "nutritive" 
fish  food,  are  when  ctenophores,  medusae,  or  salps  may  temporarily  dominate 
the  local  community. 

The  various  analyses  that  have  been  published  of  the  chemical  composition 
of  planktonic  animals  show  considerable  differences  in  the  percentage  of  proteins, 
carbohydrates,  and  fats,  not  only  between  different  groups  and  species,  but  even 
for  the  same  species  at  different  times  and  localities.  Brandt  (1898),  Rosenwald 
(1904),  and  Delff  (1912),  for  example,  long  ago  showed  that  the  fat  content  of 
the  dry  matter  in  copepod  plankton  at  different  locaHties  in  North  European 
waters,  may  be  as  high  as  12.5%  or  as  low  as  5%,  while  Wimpenny  (1929,  p.  19) 
records  9.3%-36%  of  fat  in  the  dry  matter  of  different  samples  of  plankton 
dominated  by  various  species  of  copepods,  sagittae,  Oikopleura,  and  other  organ- 
isms. Especially  pertinent  in  the  present  connection  is  Orr's  (1934)  observation 
that  adult  Calanus  may  have  from  10.5-29.6%  of  fat  in  the  female  and  18-33.7% 
in  the  male,  at  a  given  locaUty  at  different  seasons.-  And  the  presence  of  about 
30%  of  ether  extract  in  dried  Calanus  collected  off  New  York  in  May  1929,  as 
determined  by  Dr.  Th.  von  Brand  at  the  Woods  Hole  Oceanographic  Institu- 
tion, even  after  long  preservation  in  formahn,  .shows  that  this  copepod  may  be  at 
least  as  rich  in  fat  in  the  western  side  of  the  North  Atlantic  as  it  is  in  the  eastern. 
Five  per  cent  of  ether  extract  in  the  dry  matter  of  Limacina  preserved  in  formalin, 
similarly,  confirms  Rosenfeld's  (1904)  early  report  of  7.3%  fat  for  this  pteropod. 
So  far  as  we  are  aware,  sagittae  have  not  previously  been  studied  from  this  point 
of  view,  although  they  enter  largely  into  the  dietary  of  various  fishes.  But  recent 
analyses  by  Dr.  von  Brand  of  the  dry  matter  of  freshly  caught  Sagitta  elegans 
showed  about  16%  of  ether  extract  (fats  plus  lipoids).  Dr.  von  Brand  also  in- 
forms us  that  in  samples  preserved  in  formalin  (and  hence  comparable  to  the 
plankton  volumes  with  which  we  are  dealing),  the  dry  weight  is  about  12%  of 
the  wet  weight  in  Calanus  finmarchicus,  a  value  falling  well  within  the  range  re- 
ported by  various  authors  for  copepod  plankton,  and  about  11%  of  the  wet 
weight  in  Limacina,'  but  only  about  7%  in  Sagitta  elegans. 

'  For  a  recent  study  of  the  relative  importance  of  the  different  species  in  the  diet  of  the  herring  in  the 
North  Sea,  see  Savage,  1937. 
'  For  summaries  of  early  analyses,  see  Steuer,  1910,  pp.  656-659. 
'  No  doubt  the  shells  of  the  latter  had  lost  some  of  their  lime  in  the  preservative. 


BIGELOW   AND    SEARS :   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  255 

The  foregoing  suggests  that  while  Calanus  in  good  condition  ranks  far  above 
most  of  the  other  dominant  species,  if  judged  by  fat  content,  the  other  crusta- 
ceans may  be  grouped  with  Limacina  and  with  Sagitta  elegans,  while  the  high 
ratio  of  wet  weight  to  dry  in  the  latter  is  perhaps  balanced  by  the  high  percentage 
of  chitin  in  crustaceans,  and  of  lime  in  Limacina.  Comparisons  of  this  sort  fail, 
however,  to  include  any  estimate  of  the  relative  digestibility  of  these  different 
groups,  which  no  doubt  differs  for  different  species  of  marine  fishes,  a  vital  matter 
as  to  which  we  still  remain  wholly  in  the  dark.  Lacking  information  in  this  re- 
gard, we  base  the  following  discussion  on  the  assumption  that  the  crustacean, 
moUuscan,  and  chaetognath  fractions  can  be  justly  combined  as  being  of  high 
nutritive  value,  leaving  the  salps  and  the  watery  ctenophores  and  medusae  out 
of  account  altogether,  as  being  of  low.  Subtraction  of  the  latter  does  not  ap- 
preciably affect  the  picture  for  February  or  for  April,  when  medusae,  ctenophores, 
and  salps  combined,  formed  less  than  1%  of  the  total  catch,  in  each  year's 
record.  And  these  may  continue  negligible  (not  more  than  1  %)  right  through  the 
later  spring  and  summer,  as  happened  in  1930.  But  they  formed,  on  the  average 
9%  of  the  total  plankton  in  May  and  June,  and  10%  in  July  of  the  other  years 
combined,  with  a  maximum  of  22%  for  the  area  as  a  whole  in  June  1932,  per- 
centages large  enough  to  be  of  some  significance  in  relation  to  the  nutritional 
value  of  the  general  community. 

The  distributional  pictures  for  the  volumes  of  plankton  surface  to  bottom 
after  omission  of  the  non-nutritive  group,  would  represent  what  may  be  termed 
the  potential  richness  of  the  area  as  a  feeding  ground  for  fishes.  But  it  is  obvious 
that  some  levels  will  be  richer  than  others,  some  poorer,  unless  the  vertical  distri- 
bution of  the  plankton  be  uniform.  Average  volumes  as  calculated  for  the  water 
column  as  a  whole  may  therefore  be  a  considerable  understatement  of  actual 
richness  as  this  affects  the  fish  population.  For  example,  a  given  locahty  might 
well  be  much  richer,  at  some  one  level,  than  might  be  suggested  by  an  average 
volume  derived  from  the  combination  of  a  poor  catch  shoal,  with  a  large  catch 
deep,  or  vice  versa.  The  ratios  of  the  richest  catches  (irrespective  of  depth)  to  the 
average  volumes  for  the  column  as  a  whole,  in  the  months  when  tows  were  made 
at  two  or  more  levels,  do,  in  fact,  show  that  such  averages  considerably  minimize 
the  actual  richness,  at  the  most  productive  depth.  This  is  true  even  at  the  end  of 
winter  when  the  plankton  is  most  nearly  uniform  vertically,  while  in  midsummer, 
the  ratio  has  approached  the  maximum  that  is  possible  for  cases  where  an  average 
has  been  derived  from  two  values  only. 


256 


memoir:  museum  of  comparative  zoology 


Ratio  of  volume  at  level  of  maximum  abundance  to  average  volume  for  the 
water  column  as  a  whole,  for  the  more  nutritive  plankton 


Month  and 

year 

North 

South 

Inshore 

Offshore 

General 
average 

Feb.  1932 

1.4  to  1 

1.4  to  1 

1.2  to  1 

1.4  to  1 

1.35  to  1 

Apr.  1929 

1.5  to  1 

1.6  to  1 

1.5  to  1 

1.4  to  1 

1.5  to  1 

May  1929 
1931 
1932 

1.3  to  1 

1.3  to  1 

1.4  to  1 

1.7  to  1 
1.2  to  1 
1.2  to  1 

1.6  to  1 
1.4  to  1 
1.2  to  1 

1.5  to  1 

1.3  to  1 

1.4  to  1 

1.38  to  1 

June  1929 
1931 
1932 

1.6  to  1 
1.5  to  1 
1.5  to  1 

1.4  to  1 
1.4  to  1 
1.6  to  1 

1.3  to  1 

1.5  to  1 

1.6  to  1 

1.7  to  1 
1.5  to  1 
1.5  to  1 

1.5  to  1 

July  1929 
1931 

1.8  to  1 

1.9  to  1 

1.7  to  1 

1.7  to  1 
1.9  to  1 

1.7  to  1 
1.74  to  1 

1.8  to  1 

Discussion  as  to  the  relative  richness  of  different  parts  of  the  area,  at  different 
seasons  should  therefore  be  based  primarily  on  the  volumes  existing  at  the  level 
of  greatest  abundance.  Information  in  this  respect  is  available  for  the  great 
majority  of  the  stations  of  1929,  likewise  for  most  of  the  offshore  stations  of  1931 
and  1932.  But  the  hauls  at  many  of  the  inshore  stations  in  the  two  latter  years 
extended  from  near  the  bottom  to  the  surface,  hence  yielded  minimal  values  only. 
And  these  have  been  adjusted,  in  the  following  tabulation,  according  to  the  par- 
ticular ratio  that  prevailed  at  the  time  in  the  general  subdivision  in  question. 


Average  volumes  at  level  of  maximum  abundance,  observed  or  adjusted 
as  above,  for  the  more  nutritive  plankton 


Month 

Year 

Inshore 

Offshore 

North 

South 

Total  area 
surveyed 

February 

1931 
1932 

264 
660 

153 
116 

74 
74 

220 
519 

199 

478 

April 

1929 

304 

479 

440 

324 

376 

May 

1929 
1931 
1932 

265 
522 
299 

441 
801 
497 

439 
640 
371 

178 
481 
365 

329 
608 
361 

June 

1929 
1931 
1932 

232 

784 
251 

447 

1124 

431 

379 
981 
264 

26 
809 
410 

312 
908 
301 

July 

1929 
1931 

480 
1294 

347 
1178 

730 
1322 

313 

415 

BIGELOW   AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


257 


If  the  ratio  averaged  about  the  same  in  1930  (when  only  one  haul  was  made 
at  each  station),  as  it  did  in  the  other  years  combined,  the  volume  of  nutritive 
phmkton  at  the  level  of  maximum  abundance  may  be  assumed  to  have  been 
about  as  follows,  in  that  year : 


Area 

Month 

Inshore 

Offshore 

North 

South 

surveyed 

February 

299 

59 

144 

193 

187 

April 

362 

451 

594 

390 

279 

May 

607 

1436 

896 

970 

928 

June 

645 

467 

630 

363 

512 

July 

1080 

580 

787 

— 

— 

At  the  end  of  the  winter,  according  to  these  tabulations,  the  amount  of  nutri- 
tive plankton  averages  smallest  in  the  north  and  along  the  offshore  belt,  where, 
in  fact,  the  supply  may  be  so  poor  (less  than  75  c.c.)  as  to  make  it  doubtful 
whether  any  considerable  population  of  plankton-eating  fishes  could  subsist,  if 
actively  feeding.  And  while  volumes  have  averaged  somewhat  larger  in  the 
south  near  shore,  the  only  winter  (1932)  when  even  that  subdivision  could  be 
classed  as  "rich"  (more  than  400  c.c.)  was  one  when  the  water  was  abnormally 
warm. 

In  every  year,  the  volume  of  nutritive  plankton  considerably  increased  in 
the  northern  sector  by  April  (1929)  or  by  May,  at  latest.  This  vernal  augmenta- 
tion on  a  broad  scale  involved  the  area  as  a  whole,  in  one  year  (1931),  but  it  was 
confined  to  the  northern  sector  in  each  of  the  other  two  years  with  average  vol- 
umes decreasing  somewhat  in  the  southern  sector  in  each  of  these  cases  from 
February  or  April  to  June  or  July,  resulting  in  a  roughly  stationary  or  slightly 
downward  trend  for  the  area  as  a  whole  (Table,  p.  229).  It  follows,  in  most  years, 
that  when  volumes  reach  their  peak  in  the  northern  sector,  which  may  be  as 
early  as  May  (1930)  or  not  until  July  (1929,  1931),  they  average  considerably 
larger  there  than  they  do  in  the  southern  sector  at  any  time  of  year,  a  regional 
contrast  illustrated  graphically  by  the  concentration  of  very  large  ( >  1000  c.c.) 
catches  north  of  the  Cape  May  profile,  irrespective  of  the  season  (Fig.  15B).  And 
while  the  center  of  summer  abundance  may,  on  the  contrary,  lie  in  the  south 
after  an  abnormally  warm  winter,  (e.g.,  1932)  when  large  amounts  of  nutritive 
plankton  exist  there  in  February,  the  combined  evidence  of  the  series  as  a  whole 
is  that  the  richest  pasture  for  fishes  usually  develops  somewhere  within  the  area 
outlined  in  Figure  15;  and  in  July,  by  which  month  the  average  volume  of  nutri- 
tive plankton  at  the  levels  of  greatest  abundance  had  risen  there  to  about  1300 


258  memoir:  museum  of  comparative  zoology 

c.c.  in  1931,  to  about  800  c.c.  in  1930,  and  to  about  700  c.c.  in  1929,  with  occa- 
sional concentrations  as  rich  as  2000  c.c. 

The  famiUar  fact  that  the  mackerel  are  thin  when  they  first  appear  near  our 
coasts  in  spring,  but  soon  gain  fat  on  the  diet  afforded  by  the  local  plankton,  is 
sufficient  evidence  that  average  volumes  of  this  general  order  of  magnitude,  at 
the  level  of  greatest  abundance,  corresponding  to  1.2-2.2  c.c.  per  cubic  meter  of 
water,  are  more  than  sufficient  for  the  maintenance  and  growth  requirements  of 
this  particular  fish.  They  contrast  with  an  average  of  only  about  300  c.c,  and 
maximum  of  914  c.c.  for  June  and  July,  inshore  in  the  southern  sector. 

By  the  evidence  of  1931,  the  amount  of  nutritive  plankton — however  esti- 
mated— greatly  decreases  all  along  the  ofTshore  belt  between  July  and  October, 
to  an  average  so  low  (perhaps  200-300  c.c.)  that  autumnal  feeding  conditions 
must  be  classed  as  moderate  at  best.  But  we  have  no  information,  under  the 
present  heading,  for  the  late  autumn  or  early  winter. 

Consideration  of  the  relative  frequency  with  which  the  plankton  averaged 
more  than  twice  as  voluminous  shoal  (<  10  meters)  as  deep  (>  18-20  meters) 
or  the  reverse,  in  different  months,  shows  that  mackerel,  or  other  fish  would  find 
the  best  feeding  conditions  as  often  at  one  depth  as  at  another  during  February, 
April,  and  May,  while  more  often  still,  there  is  no  strong  gradient  of  either  order 
at  this  season.  By  July,  however,  when  the  plankton  as  a  whole  averages  most 
abundant  in  mid-depths  (p.  218),  the  best  feeding  conditions  would  be  found  15-20 
times  as  often  at  some  depth  greater  than  20  meters  as  between  10  meters  and 
the  surface.  And  on  the  few  occasions  (total  cases,  24)  when  a  very  strong  con- 
centration was  recorded  at  any  particular  depth  (volume  more  than  10  times  as 
great  at  one  level  as  at  another),  this  occurred  about  5  times  as  often  deep  as 
shoal,  for  the  months,  February- June,  but  at  least  20  times  as  often  deep  as  shoal 
in  July.  Thus,  there  is  nothing  in  the  vertical  distribution  of  the  more  nutritious 
plankton  that  might  tend  to  hold  the  fishes  subsisting  upon  the  latter  at  any  one 
depth  more  than  at  any  other,  from  the  end  of  winter  through  spring  and  into 
the  first  month  of  summer.  But  if  plankton-eating  fishes  are  able  to  find  their 
way  to  the  zones  where  food  is  most  plentiful — which  it  is  likely  that  they  can 
vertically — they  would  naturally  tend  to  desert  the  superficial  10-20  meters  of 
water  in  midsummer  (with  corresponding  efTect  upon  the  fisheries),  unless  this 
descent  were  to  bring  them  into  temperatures  unfavorably  low.  And  this  would 
hardly  be  the  case  for  the  mackerel,  conamercially  the  most  important  fish  that 
subsists  on  the  larger  zooplankton  in  our  region,  for  water  of  12-14°  appears  to 
be  entirely  suitable  for  it. 


BIGELOW   AND    SEARS:   NORTH   ATLANTIC   ZOOPLANKTON   STUDIES  259 

The  frequency  with  which  the  hauls  have  yielded  odd  specimens  of  mysids 
(chiefly  Neomysis  americana  and  Erythrops  erythophthalma) ,  bottom  living 
shrimps  (Crago),  gammarids,  and  cumaceans,  occasionally  in  volumes  as  great 
as  20-25  c.c.  or  more,  further  indicates  that  recruitment  of  the  plankton  near  the 
sea  floor  from  this  source  provides  an  important  additional  food  supply  for  any 
plankton-eating  fishes  that  may  forage  close  to  the  bottom,  a  category  to  which 
the  local  species  of  hake  (Urophycis) — well  known  "shrimp  eaters" — doubtless 
belong.  And  we  find  a  still  more  arresting  example  of  this  sort  in  the  Gulf  of 
Maine  in  a  great  abundance  of  the  edible  shrimp,  Pandalus  borealis,  over  muddy 
bottoms  (Hjort  and  Ruud,  1938;  Bigelow  and  Schroeder,  1939). 

There  has  been  much  discussion  (with  wide  disagreement)  of  the  extent  to 
which  fishes  of  various  species  that  subsist  on  animal  plankton  direct  their  wan- 
derings to  follow  the  richest  aggregations  of  available  feed.  The  opportunity 
afforded  by  the  records  of  1930  to  compare  the  location  of  the  chief  mackerel 
fishery  from  month  to  month  (supplied  by  Mr.  O.  E.  Sette,  of  the  U.  S.  Bureau 
of  Fisheries),  with  the  areas  where  nutritive  plankton  was  most  abundant  is, 
therefore,  timely,  this  having  been  a  season  when  both  mackerel  and  plankton 
were  at  least  moderately  abundant,  and  when  the  latter  was  composed  almost 
entirely  of  the  more  nutritious  groups.  In  that  particular  year,  mackerel  were 
first  caught  in  abundance  in  the  last  week  of  April,  off  Delaware  Bay  and  south- 
ward, where  the  average  amount  of  plankton  had  recently  risen  from  about  180 
c.c.  to  about  500  c.c.  for  the  water  column  as  a  whole  (Fig.  14).  Prior  to  the  first 
of  that  May,  the  northward  extension  of  the  rich  belt  of  plankton  seems  to  have 
been  barren  of  mackerel  or  nearly  so.  But  the  fishery  (following  the  schools  of 
mackerel  sighted  at  the  surface)  shifted  northward  by  the  middle  of  May  to  a  belt 
extending  all  along  from  New  York,  eastward  to  Nantucket,  suggesting  that  the 
mackerel  that  had  first  struck  in  well  to  the  south,  had  migrated  northward  and 
eastward  along  the  belt  of  rich  feeding  (  >  500  c.c.  of  plankton),  leaving  behind 
the  richest  pool  of  all.  And  by  July,  the  main  body  of  mackerel — or  at  least  that 
supporting  the  seine-fishery — ^had  moved  still  farther  east,  to  the  region  of  Nan- 
tucket Shoals  and  the  neighboring  parts  of  the  Gulf  of  Maine,  although  volumes  of 
1100-1600  c.c.  of  plankton  for  the  water  column  as  a  whole  still  existed  locally, 
in  the  region  which  they  had  recently  deserted,  with  a  strong  probability  that 
the  whole  sector  between  the  New  York  and  Martha's  Vineyard  profiles  still 
supported  an  average  volume  of  at  least  700  c.c.  at  the  most  productive  level. 

At  this  point,  we  should  remind  the  reader  that  the  purse-seine  fishery  for 
mackerel,  to  which  the  present  discussion  is  Umited,  draws  only  from  such  schools 


260 


memoir:  musexjm  of  comparative  zoology 


as  are  close  enough  to  the  surface  to  be  seen,  which  leaves  open  the  possibility 
that  considerable  bodies  of  mackerel  may  have  remained  in  the  eastern  sector  of 
our  area  that  sunmier,  after  the  surface  schooling  fish  had  moved  farther  east- 


40 


40 


Fig.  14.  Locations  of  areas  where  plankton  was  most  abundant  (hatched);  and  where  the  mackerel 
fishery  was  chiefly  concentrated  (dotted),  in  1930:  A,  April  22-May  1;  B,  May  12-23;  C,  June 
24-July  I;  D,  July  10-18. 

ward,  but  living  deep  down  in  the  water  where  we  have  good  reason  to  believe 
that  the  plankton  was  most  abundant.  Nevertheless,  the  evidence  outlined  above 
seems  to  us  strong,  that  at  least  a  large  proportion  of  the  mackerel  stock  travelled 
horizontally  along  the  belt  of  abundant  plankton,  seemingly  without  reference 
to  the  precise  localities  where  the  latter  was  richest,  to  waters  where  we  have  no 


BIGELOW   AND   SEARS:   NORTH   ATLANTIC   ZOOPLANKTON   STUDIES  261 

reason  to  suppose  that  feeding  conditions  averaged  any  better;  a  movement  for 
which  the  quantitative  distribution  of  the  plankton  does  not  offer  any  apparent 
explanation  and  which  must  therefore  be  credited  to  some  impulse  to  migration 
of  a  different  sort. 

In  this  respect,  the  mackerel  may  differ  from  the  herring,  for  Wimpenny 
(1929,  p.  10)  remarks  that  the  "view  that  herring  seeks  and  perhaps  depends 
upon,  the  fattest  food  seems  to  me  to  have  much  to  commend  it." 

CONCLUSIONS 

The  Plankton  as  a  Whole 
Horizontal  Distribution 

The  preceding  discussion  may  be  summarized  as  follows: 

In  years  when  the  shelf  waters  as  a  whole  experience  a  pronounced  winter 
chilling,  the  zooplankton  is  volumetrically  at  a  low  ebb  in  February,  though  we 
have  yet  to  learn  whether  the  annual  minimum  falls  in  that  month  or  earlier  in 
the  winter.  The  plankton  then  averages  richest  next  the  coast,  and  in  the  south, 
poorest  in  the  northeastern  sector  and  along  the  edge  of  the  continent.  Ex- 
perience in  1930  and  1931 — presumably  the  situation  was  essentially  similar  in 
1929 — suggests,  as  a  reasonable  expectation  for  a  normal  February,  volumes 
ranging  from  about  100  to  500  c.c,  for  the  water  column  as  a  whole  (occasionally 
as  rich  as  1000  c.c.)  in  the  belt  marked  A  on  Fig.  2,  with  averages  of  150-200  c.c, 
of  50-100  c.c,  and  of  10-120  c.c.  in  the  belts  B,  C,  and  D,  respectively.  And 
available  data  show  that  the  water  is  notably  barren,  then,  along  the  continental 
slope — a  maximum  of  only  35  c.c,  a  minimum  of  13  c.c,  for  all  years  combined. 

After  a  warm  winter,  however,  such  as  that  of  1932,  the  late  February 
plankton  may  average  from  two  to  three  times  as  plentiful  in  the  southern  sector 
as  in  cooler  years,  but  not  significantly  richer  in  the  northern  sector,  so  that  the 
regional  contrast  at  this  season  is  widest  in  years  of  that  type. 

A  marked  augmentation  in  the  volume  of  plankton  takes  place  during  the 
spring  in  some  years  (illustrated  by  1930  and  1931),  but  not  in  others,  e.g.,  1929, 
1932,  but  we  still  lack  a  convincing  explanation  for  the  existence  of  these  two 
contrasting  states.  In  years  when  this  augmentation  does  occur,  it  appears  first 
at  scattered  rich  centers,  as  a  result  of  which  the  plankton  may  vary  widely  in 
amount  between  neighboring  localities.  These  centers  may  soon  merge,  so  that 
the  productive  areas  become  rather  sharply  demarked  from  the  less  productive, 
as  in  1930,  or  they  may  continue  more  or  less  independent,  resulting  in  a  de- 
cidedly complex  areal  pattern,  as  in  1931  (Fig.  4). 


262  memoir:  musetjm  of  comparative  zoology 

In  years  when  vernal  augmentation  occurs,  it  is  first  centered  chiefly  in  the 
northern  and  northeastern  sector,  expanding  eastward  at  some  time  during  the 
spring  to  the  hmit  of  our  area  (Martha's  Vineyard  profile)  in  years  of  high  pro- 
duction (e.g.,  1930,  1931).  And  it  also  tends  to  spread  progressively  from  north 
to  south  with  the  advance  of  spring,  most  markedly  along  the  mid-belt  of  the 
shelf,  but  also  involving  the  waters  seaward  to  some  extent  out  beyond  the  conti- 
nental edge.  Vernal  augmentation  of  this  type  may  either  culminate  in  May, 
followed  by  some  decrease  in  early  June,  with  little  further  alteration  through 
July  (as  in  1930),  or  the  plankton  may  continue  to  increase  slowly  in  amount 
through  June  and  into  July,  as  in  1931.  Contrasting  with  this  enrichment  to  the 
north,  and  ofTshore  to  the  south,  the  plankton  of  the  coastal  belt  south  of  New 
York,  at  most  increases  slightly  during  the  spring,  as  happened  in  1931,  or  it 
may  even  decrease  somewhat,  as  in  1930.  Neither  have  rich  pools  such  as  may 
exist  close  to  the  mouth  of  Chesapeake  Bay  or  southward  of  the  latter  in  Feb- 
ruary or  in  April,  ever  been  found  there  in  May  or  later. 

Various  lines  of  evidence  indicate  that  vernal  augmentation  in  our  area  re- 
sults chiefly  from  local  reproduction,  with  mass  immigration  from  the  east  con- 
tributing very  little;  indrafts  of  water  from  that  source  may  even  tend  toward 
impoverishment. 

In  years  of  the  opposite  type  (e.g.,  1929,  1932),  the  plankton  alters  but  little 
in  average  volume  (continuing  relatively  poor)  from  late  winter  or  early  spring 
through  June,  as  in  1932,  or  even  through  July,  as  in  1929.  Conditions  in  1932 
suggest  that  a  poor  production  of  plankton  is  to  be  expected  after  an  abnormally 
warm  winter — even  if  this  thermal  abnormality  be  obliterated  during  the  spring, 
and  if  summer  temperatures  be  about  normal.  But — by  the  evidence  of  1929 — 
production  may  also  be  poor  after  a  normally  cold  winter. 

Since  the  observational  series  included  as  many  years  of  the  one  type,  as  of 
the  other,  so  far  as  production  of  plankton  is  concerned,  the  following  monthly 
averages,  for  all  years  combined,  are  probably  a  fair  representation  of  the  charac- 
teristic state.  On  this  basis,  the  volume  of  plankton  may  be  expected  perhaps  to 
double  over  the  area  as  a  whole  between  February  and  May  or  June,  and  to 
multiply  more  than  six-fold  in  the  northern  sector  between  February  and  July, 
in  an  average  year,  while  in  a  year  of  good  production,  it  may  multiply  nearly 
four-fold  over  the  area  as  a  whole,  and  more  than  14-fold  in  the  north,  between 
the  end  of  winter  and  the  date  of  the  summer  peak.  Peaks  of  equal  abundance 
may  perhaps  develop  locally  in  August  and  September  when  quantitative  data 
are  lacking.  But  the  plankton  then  decreases  again  by  October,  to  a  volume  about 
equalling  the  average  for  April. 


BIGELOW   AND    SEARS!    NORTH    ATLANTIC   ZOOPLANKTON    STUDIES  263 

Average  volumes,  c.c.  per  standard  haul 


Area 

Month 

Inshore 

Offshore 

North 

South 

surveyed 

February 

339 

80 

75 

282 

222 

April 

252 

335 

354 

234 

231 

May 

342 

576 

451 

425 

443 

June 

343 

421 

388 

330 

372 

July 

551' 

4921 

505 

— 

— 

October  1931 

— 

234 

236= 

23  P 

— 

'  North  onlv. 


'■  Offshore  only. 


The  tabulation  further  shows,  first,  that  regional  contrasts  are  much  the  widest 
in  February,  when  the  plankton  has  averaged  some  four  times  as  rich  inshore  and 
south,  as  offshore  and  north,  and,  second,  that  a  reversal  tends  to  take  place 


Fig.  15.  Volumes  of  plankton:  A,  averages  in  different  areas,  mid-May  to  mid-June,  of  all  years  com- 
bined; with  locations  of  volume  greater  than  900  cc;  B,  stations  where  the  volume  of  Plankton 
was  larger  than  1000  cc.  at  the  richest  level,  after  subtraction  of  medusae  and  salps. 

early  in  the  spring,  from  the  winter  state,  to  a  slight  contrast  of  the  opposite 
sense  (richest  offshore  and  north),  which  prevails  generally  through  April,  May, 
and  June.  And  the  majority  of  the  chief  centers  of  abundance  (  >  900  c.c.)  have 
similarly  been  concentrated  on  the  outer  part  of  the  shelf  to  the  north  and  east 


264  memoir:  muset.tm  of  comparative  zoology 

of  the  Hog  Island  profile  (Fig.  15A).  But  there  have  been  considerable  varia- 
tions in  this  respect  among  individual  cruises. 

It  is  probable  that  the  average  volume  of  plankton  existing  in  the  offshore 
belt  in  the  north  in  July  1931  (912  c.c.)  approximates  the  maximum  that  is  to  be 
expected  over  any  considerable  part  of  our  area,  at  any  season,  in  normal  years, 
though  the  richest  catch  of  all  (67109  c.c,  mostly  salps  off  Delaware  Bay,  July 
18,  1929)  was  made  at  the  edge  of  the  continental  shelf.  At  the  other  extreme, 
the  region  that  has  proved  least  productive  in  late  spring  and  summer  is  the  coast- 
wise belt  southward  from  Barnegat,  where  volumes  from  mid-May  to  June  have 
averaged  only  about  193  c.c.  Very  rich  centers  ( >  900  c.c.)  may,  however,  de- 
velop locally,  in  this  barren  zone,  as  they  do  elsewhere  (Fig.  15A). 

In  October,  to  judge  from  1931,  volumes  average  about  as  large  north  as 
south.'  But  we  have  no  information  as  to  the  regional  gradients  later  in  the 
autumn,  or  in  early  or  mid-winter. 

Vertical  Distribution 

At  the  end  of  the  winter,  the  plankton  averages  considerably  richer  in  the 
upper  25-30  meters  than  deeper,  but  it  is  distributed  with  comparativ^e  uni- 
formity, vertically,  within  that  stratum,  nor  do  diurnal  migrations  greatly  affect 
the  mass  distribution  at  that  season,  when  turbulence  is  still  active,  and  illumi- 
nation relatively  weak.  Migrations  of  this  type,  of  the  community  as  a  whole, 
upward  by  night  and  downward  by  day  have,  however,  proved  widespread  from 
April  through  June.  The  center  of  population  also  tends — though  quite  inde- 
pendent of  this  phenomenon — to  sink  to  the  mid-depths  by  April,  there  to  con- 
tinue through  the  later  spring  and  summer.  And  this  type  of  vertical  stratifica- 
tion greatly  intensifies  from  June  to  (probably)  a  maximum  in  July,  when  the 
20-30  meter  level  stratum  has  averaged  from  10-26  times  as  productive  of  plank- 
ton as  the  superficial  10  meters  in  different  years.  This  enrichment  of  the  deeper 
waters,  relative  to  the  surface,  which  is  confined  to  the  upper  40  meters  or  so, 
until  June,  also  extends  downward  below  60  meters  by  midsummer. 

It  is  probable  that  the  development  of  this  type  of  vertical  stratification  in 
spring  results  chiefly  from  the  alteration  usual  at  this  season  from  dominance  of 
the  community  by  Centropages  and  Limacina,  to  dominance  by  Calanus.  But 
the  intensification  of  stratification  that  takes  place  from  June  to  July  results 
from  the  fact  that  by  midsummer,  the  surface  stratum  warms  to  a  temperature 

'  The  one  October  cruise  was  confined  to  the  offshore  belt. 


BIGELOW   AND    SEARS:    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES  265 

that  is  unfavorably  high  for  the  members  of  the  boreal  community  that  are  the 
most  prominent  relatively  at  the  season. 

Annual  Differences 

The  fact  that  the  plankton,  over  the  region  as  a  whole  averaged  about  twice 
as  voluminous  in  the  richest  year  as  in  the  poorest  in  February  and  about  three 
times  as  voluminous  in  midsummer,  within  a  period  of  only  four  years,  marks  our 
area  as  one  subject  to  fluctuations  in  this  respect,  perhaps  wide  enough  seriously 
to  affect  the  welfare  of  plankton-eating  fishes.  At  the  end  of  the  winter,  the 
plankton  was  richest  in  the  year  (1932)  when  temperatures  were  highest,  but 
subsequent  production  poor,  whereas  vernal  augmentation  was  most  pronounced 
and  summer  volumes  largest  in  a  year  when  the  plankton  had  been  very  scarce 
in  February  and  when  winter  temperatures  had  been  normally  low.  But  it  is 
clear  that  this  is  not  a  case  of  simple  control  by  temperature,  for  one  of  the  years 
of  this  same  thermal  type  (1929)  was  one  of  poor  plankton  production. 

Comparison  With  Other  Areas 

By  available  data  our  area  on  the  one  side  of  the  Atlantic  and  the  southern 
part  of  the  North  Sea,  on  the  other,  rank  together  in  volume  of  plankton,  with 
averages  of  about  0.5-0.8  c.c.  per  cubic  meter  (as  estimated  by  the  displacement 
method)  at  the  season  of  maximum  production,  somewhat  surpassed  by  more 
northerly  European  waters,  but  in  turn  ranking  ahead  of  the  Gulf  of  Maine,  the 
Gulf  of  St.  Lawrence,  the  Nova  Scotian  shelf  waters,  the  English  Channel,  and 
the  Baltic. 

Relative  Importance  of  Different  Species 

Recent  data  corroborate  earlier  observations  that  the  planktonic  community 
of  our  area,  though  quantitatively  varied,  is  dominated  by  a  small  number  of 
boreal  species,  among  which  the  only  ones  that  have  individually  formed  as 
much  as  15%  of  the  total  catch  over  the  area  as  a  whole  in  any  one  month  have 
been  Calanxis  finmarchicus,  Centropages  typicus,  Sagitta  elegans,  Sagitta  serrato- 
dentata,  and  Limacina  retroversa.  Inclusion  of  species  that  have  formed  15%  for 
one  or  other  subdivision  (though  not  of  the  whole  area)  on  at  least  one  survey, 
would  add  to  this  list  euphausiids  as  a  group,  Euthemisto  sp.,  Metridia  lucens, 
Paracalanus  parvus,  Pseudocalanus  minutus,  Aglantha  digitale,  ctenophores,  and 
salps,  the  latter  being  the  only  warm  water  derivatives  that  have  been  found  to 


266  memoir:  museum  of  comparative  zoology 

form  any  considerable  part  of  the  total  catch,  in  either  subdivision  as  a  whole, 
on  any  cruise.  The  members  of  these  categories  (combined)  have  formed  89-93% 
of  the  total  volume  in  February,  91-95%  in  April,  80-93%  in  May,  81-91%  in 
June,  and  80-95%,,  and  78%  in  October. 

At  the  end  of  winter,  either  Centropages  typicus,  Sagitta  clegans,  or  Limacina 
may  be  the  leading  item  in  the  community  (Table,  p.  229).  Centropages,  however, 
and  Sagitta  serratodentata  then  decline  so  greatly  in  relative  abundance,  that  they 
are  of  but  minor  importance  in  the  community  in  late  spring  and  early  summer, 
while  Limacina,  which  ranks  high  as  late  as  June  in  some  years  (1930,  1932),  is 
neghgible  later  in  the  summer.  On  the  other  hand,  Calanus  which  occupies  a 
minor,  or  at  most  an  intermediate  position  in  February  (8%  on  the  average)  so 
greatly  increases  in  importance  shortly  thereafter  that  it  was  by  far  the  most 
prominent  species  in  each  April  of  record,  ranking  also  first  or  second  (in  which 
case  it  closely  rivalled  the  leader)  in  May,  in  June,  and  in  July  of  each  year. 
Since  the  observational  series  included  one  year  (1932)  when  Calanus  was  rela- 
tively scarce,  but  others  (1930,  1931)  when  it  was  abundant,  its  combined 
monthly  ranking  for  the  series  as  a  whole  may  be  accepted  as  approximately 
representative  of  normal  years.  On  this  basis,  Calanus  with  a  combined  per- 
centage of  about  35%  for  all  cruises  combined  (for  the  regions  surveyed),  is  on 
the  whole  responsible  for  the  production  of  a  larger  volume  of  plankton  than  any 
other  one  species,  its  closest  rivals  being  Sagitta  elegans,  Limacina,  and  Centra- 
pages  typicus,  each  with  11-12%.  Calanus  may,  in  fact,  be  expected  to  form 
nearly,  or  quite  ^2  of  the  total  volume  of  plankton,  whether  for  the  area  as  a 
whole  (47%),  for  the  northern  sector  (50%),  or  offshore  (50%),  from  April 
through  July.  In  the  south,  however,  it  reaches  its  maximum  importance  in 
April  (47%),  decUning  thereafter  through  May  and  June  (18-22%)  to  July 
(9%)  in  1929  (perhaps  10-20%  in  1916  and  neghgible  in  1913).  Calanus  also 
decreased  greatly  in  relative  importance  during  the  early  autumn — at  least  in  the 
offshore  belt — in  the  one  year  of  record  (1931),  whereas  Centropages  so  increased 
that  it  ranked  first  in  that  October,  and  Calanus  only  third  or  fourth  (see  Table, 
p.  229),  which  taken  with  conditions  in  February,  suggests  that  Centropages  prob- 
ably continues  much  the  more  important  of  the  two  through  later  autumn  and 
winter.  The  salps  and  ctenophores,  Pleurobrachia  and  Mnemiopsis, — insignif- 
icant in  amount  earlier  in  the  season — may  practically  monopoUze  the  water 
over  considerable  areas  in  the  mid-sector,  in  warm  summers  such  as  that  of  1913, 
and  be  abundant  there  locally,  even  in  cool  (e.g.,  1916,  p.  370). 

In  the  cases  of  the  other  members  of  the  dominant  group,  the  trend  in  rela- 


BIGELOW    AND    SEARS:    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES 


267 


tive  importance  is  upward  tiirough  the  spring  and  summer  in  some  years,  but 
downward  in  others  or  alternately  up  and  down. 

The  following  tabulation  of  monthly  percentages  for  the  dominant  species 
in  the  total  community  for  the  series  as  a  whole  may  be  taken  as  representative 
of  the  relative  rankings  to  be  expected  in  normal  years. 


o 


a 

o 


a 
a 
o 


O 


CO 

3 

03 

•^^ 

-^ 

C/3 

3 

3 

03 

CO 

> 

a 

0) 

Oj 

fH 

> 

-a 

c 

o3 
3 

3 
C 
c3 

O 

s 

M 

o 
2 

m 

n 

.2 

"5 

03 
o 
O 

03 

03 

c3 

o 

2 

-a 

o3 

o 

■*^ 

P 

'Sb 

bC 

OJ 

03 

'ji 

•  ^« 

cj 

c3 

^ 

(1h 

IX, 

)-] 

m 

m 

O 

03 


February 

8% 

April 

51 

May 

32 

June 

42 

July! 

51 

Octobcr- 

t 

8 

4 

4 

17 

34 


7% 

3% 

5% 

13% 

15% 

13% 

9 

5 

9 

5 

6 

5 

4 

22 

20 

1 

3%. 

4 

2 

/ 

15 

<1 

2 

1 

1 

13 

1 

7 

17 

2 

<1 

5 

5 

<1% 

2 

5 

<1 


'  North  only. 


'  OfTshore  only. 


The  chief  seasonal  contrasts,  apart  from  the  ups  and  downs  for  individual 
species  as  just  outUned,  is  that  monopohzation  of  the  waters  of  our  area  by  the 
regularly  dominant  community  is  most  nearly  complete  in  April,  May,  and  June ; 
least  so  in  February,  on  the  one  hand,  and  in  October,  on  the  other;  also  in  the 
south  in  July. 

Previous  explorations  had  shown  that  one  need  journey  out  only  a  few  miles 
from  the  continental  edge  to  encounter  the  typical  warm-oceanic  communities 
in  full  strength.  And  occasional  representatives  of  a  considerable  number  of 
species  of  warm  water  tunicates,  copepods,  pteropods,  amphipods,  etc.,  were  re- 
ported widespread,  here  and  there,  over  our  area  in  the  summers  of  1913  and 
1916  (Bigelow,  1915;  1922),  as  well  as  on  most  of  the  cruises  for  the  period,  1929- 
1932,  most  frequently  in  the  offshore  belt,  as  was  to  be  expected.  But  it  appears 
that  a  definite  barrier  exists — perhaps  in  low  saUnity — to  mass  invasion  of  the 
shelf  from  offshore,  anywhere  to  the  north  of  the  latitude  of  Delaware  Bay,  for 
oceanic  species  of  all  species  combined,  have  never  formed  as  much  as  1  %  of  the 
plankton  in  the  northern  sector,  at  any  season,  even  in  the  offshore  belt,  with  the 
exception  of  salpae,  which  may  multiply  so  rapidly  in  the  high  temperatures  of 


268  memoir:  museum  of  comparative  zoology 

summer  that  the  volumes  that  may  be  found  on  the  shelf  on  any  particular 
occasion  give  no  indication  of  the  numbers  that  may  previously  have  drifted 
thither.  And  the  dominant  boreal  community  equally  monopolizes  the  southern 
sector  down  past  the  offing  of  Chesapeake  Bay,  in  April,  May,  and  June.  But  it 
becomes  relatively  less  important  in  the  southernmost  sector  by  July  of  normal 
summers,  and  a  wide  variety  of  oceanic  and  southern  species  considerably  more 
so  there,  at  that  time  of  year.  This,  in  fact,  and  the  north-south  gradient  for 
Calanus  are  the  most  striking  regional  contrast  that  exists  within  our  limits  at 
any  season.  And  the  situation  in  the  late  winter  of  1931  suggests  that  tropical 
communities  may  be  expected  to  dominate  the  shelf  waters  from  Cape  Hatteras 
southward,  the  year  around. 

It  is  difficult  to  determine  to  what  extent  the  plankton  of  our  area  may  draw 
from  the  shelf  waters  to  the  eastward — George's  Bank  and  the  Gulf  of  Maine — 
with  the  invasion  of  the  cold  water  that  often  comes  from  that  direction  in  spring, 
because  the  qualitative  composition  of  the  dominant  communities  is  much  the 
same,  west  as  east  of  Cape  Cod.  But  the  distributional  pictures  for  the  leading 
species  lead  to  the  conclusion  that  even  if  there  be  some  recruitment  from  this 
more  eastern,  and  more  strictly  boreal  source,  the  maintenance  of  the  local 
plankton  depends  chiefly  on  local  production.  It  is  especially  instructive  in 
this  connection  that  species  definitely  to  be  classed  as  northern  indicators  in  our 
area,  such  as  Calanus  hyperboreus  and  Oikopleura  labradoriensis,  have  invariably 
formed  a  small  part  of  the  catches  (an  average  of  6%  at  most)  even  on  occa- 
sions when  they  have  become  dispersed  far  and  wide.  And  with  visitors  from  the 
coastwise  water  to  the  south  equally  negligible  in  average  amount  even  in  the 
southern  sector  at  all  seasons,  with  the  unique  exception  of  the  invasion  of  Pahnu- 
rus  larvae  in  July  1929  that  is  described  elsewhere  (p.  243),  our  area  is  primarily 
self-contained  so  far  as  its  plankton  is  concerned. 


Feeding  Conditions  for  Plankton-Eating  Fishes 

It  has  long  been  known  that  the  major  items  in  the  diet  of  the  local  mackerel 
and  probably  of  other  fishes  feeding  on  zooplankton,  are  the  species  of  crusta- 
ceans, of  pteropods,  and  of  sagittae  that  dominate  the  plankton  of  our  area. 
These  differ  in  chemical  composition  as  discussed  on  page  254.  But  it  is  not  pos- 
sible to  assign  relative  weights  to  them,  as  fish  food,  without  knowledge  of  their 
relative  digestibiUty,  for  different  fishes.  The  present  discussion  of  the  richness 
of  the  area  as  a  feeding  ground  is  therefore  based  on  the  assumption  that  they 


BIGELOW   AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


269 


can  be  grouped  together  as  "nutritive,"  the  medusae,  ctenophores,  and  salpae 
being  left  out  of  consideration  as  of  low  food  value. 

A  better  index  to  richness  from  the  feeding  standpoint,  is  afforded  by  the 
volume  at  the  richest  level,  than  by  the  average  for  the  water  column  as  a  whole, 
because  the  former  to  some  extent  expresses  the  relative  accessibility  for  fishes  of 
the  available  supply.  Information  in  this  regard,  obtained  in  1929,  1931,  and 
1932,  when  two  or  more  hauls  were  made  at  most  of  the  stations,  combined  with 
adjusted  values  for  1930  (see  p.  257  for  the  method  of  adjustment)  yields  the 
following  monthly  volumes  of  the  more  nutritive  plankton  at  the  richest  level, 
for  all  years  combined: 


Average  monthly  volumes  (c.c.)  of  the  more  nutritive  plankton 
in  the  different  subdivisions  at  the  richest  level 


Month 

Inshore 

Offshore 

North 

South 

Area 
Surveyed 

Per 
Tow 

Per 

M' 

Per 

Tow 

Per 
M' 

Per 
Tow 

Per 

M' 

Per 

Tow 

Per 

M^ 

Per 

Tow 

Per 

M' 

February 

April 

May 

June 

July 

408 
333 
423 
478 
951 

0.7 
0.6 
0.8 
0.8 
1.6 

109 
465 
794 
617 
701 

0.2 
0.8 
1.3 
1.0 
1.2 

114 
292 
586 
563 
791 

0.2 
0.5 
1.0 
1.0 
1.3 

311 

258 
498 
447 

0.5 
0.4 

0.8 
0.8 

288 
327 
556 
508 

0.5 
0.6 
0.9 
0.9 

This  tabulation  (and  the  data  for  the  separate  years  on  which  it  is  based. 
Tables  pp.  256,  257)  shows  the  northern  sector,  offshore,  as  averaging  poor  in  fish 
food  at  the  end  of  winter  (in  some  years  very  barren  indeed),  but  the  southern 
sector,  inshore,  as  moderately  productive,  especially  if  winter  chilUng  has  been 
less  extreme  than  usual  (e.g.,  1932).  In  years  of  this  latter  type,  the  center  of 
abundance  may  even  continue  in  the  south  until  early  summer.  Usually,  how- 
ever, the  volume  of  nutritive  plankton  so  greatly  increases  in  the  northern  sector 
during  the  early  spring  as  to  make  this  the  richest  part  of  the  area  by  May,  at 
latest,  so  to  continue  through  July.  In  some  years — those  of  high  production — 
the  vernal  augmentation  involves  the  other  parts  of  the  area  as  well,  but  in 
others  (represented  with  equal  frequency  in  the  observational  series),  it  is  con- 
fined to  the  northern  sector.  The  volume  in  the  richest  area  at  the  peak  season 
has  been  found  to  average  nearly  two  to  three  times  as  great  in  a  rich  year 
(1930)  as  in  a  poor  (illustrated  by  1932).  But  these  annual  differences  are  not 
wide  enough  to  obscure  the  general  rule  that  the  largest  volumes  usually  develop 


270  memoir:  museum  of  comparative  zoology 

within  the  area  outlined  in  Fig.  15  and  in  July,  when  the  average  there  has  been 
about  1300  c.c.  for  the  richest  year  and  about  900  c.c.  for  all  years  combined, 
with  local  centers  of  abundance  as  rich  as  2000  c.c. 

A  decided  decrease  is  then  to  be  expected — by  the  evidence  of  the  one  year 
of  record — from  this  peak,  to  perhaps  Yi  as  much  nutritive  plankton  in  the  off- 
shore belt  in  October.  But  we  have  no  corresponding  information  for  the  inshore 
belt  for  that  month,  or  for  any  part  of  the  area  in  later  autumn  or  winter. 

In  winter,  in  spring,  and  in  the  first  month  of  summer,  the  most  productive 
level  for  the  nutritious  plankton  is  about  as  often  shoal  as  deep,  while  more  often 
still  there  is  no  pronounced  stratification  of  either  order  at  this  season.  But  by 
July,  much  the  largest  volumes  occur  about  15-20  times  as  often  at  depths  greater 
than  20  meters  as  within  10  meters  of  the  surface,  so  that  feeding  conditions 
average  much  the  best  then  in  the  mid-depths.  But  we  have  no  evidence  as  to  the 
extent  to  which  mackerel  or  other  fish  are  able  to  profit  by  the  opportunity  pro- 
vided by  this  type  of  stratification. 

In  the  one  year  (1930)  when  the  location  of  the  chief  mackerel  fishery,  from 
month  to  month  was  compared  with  the  volumetric  distribution  of  the  more 
nutritious  plankton,  it  appeared  that  the  fish  first  struck  in  at  the  southerly  end 
of  the  productive  belt,  and  then  moved  northward  and  eastward  along  the  latter 
(apparently  indifferent  to  the  precise  locaUties  where  plankton  was  richest) 
until  by  July,  the  surface  schooUng  portion,  at  least,  of  the  mackerel  stock  had 
passed  out  of  our  area,  leaving  behind  feeding  conditions  probably  as  rich  as 
those  of  the  Gulf  of  Maine,  Nantucket  Shoals,  and  George's  Bank,  to  which  they 
had  repaired.  But  it  is  possible  that  a  part  of  the  mackerel  may  have  remained 
west  of  Cape  Cod,  but  Uving  deep. 

PART  II.    VOLUMETRIC  DISTRIBUTION  OF  INDIVIDUAL  SPECIES 

CHORDATES 

DOLIOLUM  Sp. 

Doliolum  was  taken  in  abundance  midway  of  the  shelf  off  Delaware  Bay 
in  the  summer  of  1913,  with  occasional  specimens  near  land  to  the  southward 
and  at  scattered  stations  along  the  continental  edge  (Bigelow,  1915).  In  1929,  a 
large  catch  (80  c.c.)  was  made  off  Currituck  in  May,  and  Doliolum  had  become 
generally  dispersed  through  the  southern  sector  by  that  June,  so  to  continue 
through  July  (50%  of  the  stations.  Fig.  16A),  in  quantity  varying  from  less 
than  1  c.c.  to  82  c.c,  or  sufficient  to  make  it  of  some  importance  locally  in  the 
plankton.    But  the  most  northerly  record  for  it  on  the  shelf  during  this  period 


BIGELOW   AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES         [271 

was  off  Atlantic  City,  proving  that  it  was  almost  entirely  confined  to  the  southern 
sector  at  the  time.  Unfortunately,  we  have  no  information  as  to  how  late  into 
the  autumn  Doliolum  may  have  persisted  on  this  occasion,  other  than  that  it 


Fig.  16.  Locality  records:-A,  Fritillaria,  and  Doliolum  contour  marking  southern  limit  of  Fritillaria;  B, 
Oikopleura  dioica;  C,  Salps;  D,  volumes  of  Clione  limacina  larger  than  4  c.c. 

had  disappeared  at  some  time  prior  to  the  following  February.  But  the  fact  that 
we  have  no  other  record  of  Doliolum  within  our  limits  during  the  four  year 
period,  1929-1932,  shows  that  invasions  by  it  on  a  broad  scale  are  rather  unusual 
events,  the  effects  of  which  are  limited  for  the  most  part  to  the  southern  sector. 
During  the  height  of  vernal  invasion  in  1929  (May-July),  the  average 


272  memoir:  museum  of  comparative  zoology 

volume  of  Doliolum  was  about  9  c.c.  for  the  southern  sector  as  a  whole,  the 
maximum  about  82  c.c.  And  it  is  interesting  that  catches  should  have  averaged 
rather  higher  inshore  (4-5  c.c.)  than  offshore  (1  c.c),  for  Doholum  undoubtedly 
comes  to  our  area  from  the  continental  slope. 

Salps 
It  has  long  been  known  that  salps  of  one  species  or  another  are  plentiful 
along  the  continental  slope  abreast  of  our  area.  And  the  records  for  1931  show 
that  they  are  to  be  expected  in  small  numbers  close  in  to  the  coast  south  of  Cape 
Hatteras,  even  at  the  coldest  time  of  year,  while  they  may  also  occur  here  and 
there  well  inshore  considerably  farther  north  at  the  end  of  a  warm  winter,  such 
as  that  of  1932,  when  lasis  zonaria  was  taken  at  three  stations  on  the  Cape  May 
profile.  But  salps  are  either  wholly  wanting  on  the  shelf  north  of  Latitude  36° 
at  this  season  in  more  normal  years  (e.g.,  1930),  or  at  least  confined  then  (and 
in  small  volumes)  to  the  immediate  vicinity  of  the  200-meter  contour  as  in  1931. 
And  by  available  evidence,  their  status  is  essentially  the  same  in  April,  when  the 
only  inshore  record  was  south  of  Latitude  36°,  though  we  have  record  of  them  at 
10  April  stations  near  the  200-meter  Une  in  1930,  in  volumes  ranging  from  1  c.c. 
to  30  c.c,  with  one  catch  of  5000  c.c.  off  Montauk  on  the  23rd  of  the  month  in 
1929.  But  the  records  for  May,  while  equally  closely  confined  to  the  continental 
edge  in  the  south,  are  dispersed  well  in  on  the  shelf  in  the  east  (Fig.  16C),  in 
amount  up  to  150  c.c  By  June,  they  have  been  recorded  widespread  in  the  off- 
shore belt,  south  as  well  as  north,  on  one  cruise  or  another,  averaging  about  14  c.c. 
in  the  offshore  belt  (all  years  combined)  with  a  maximum  of  261  c.c.  And  by  July 
salpae  may  not  only  occur  in  enormous  quantities  along  the  edge  of  the  continent 
— witness  a  volume  of  some  66  hters  (by  rough  estimate),  just  inside  the  200-meter 
contour  on  the  Cape  May  profile,  in  1929,  and  another  of  1046  c.c.  at  about  the 
same  relative  situation  on  the  Martha's  Vineyard  profile  in  1931 — but  may  also 
invade  the  inshore  belt  as  well  at  this  season,  to  the  southward  of  New  York  in 
great  abundance,  if  the  summer  be  a  warm  one.  This,  for  example,  happened  in 
1913,  when  they  were  in  swarms,  along  the  coasts  of  New  Jersey,  Maryland,  and 
Virginia  (Bigelow,  1915,  p.  275,  Fig.  67),  and  were  recorded  at  every  station 
westward  and  southward  from  the  Montauk  profile.  Small  numbers  of  salps 
were  even  taken  close  in  to  New  York  in  the  cold  summer  of  1916 — when  tropical 
immigrants  were  at  a  minimum — as  well  as  at  several  stations  offshore,  in  the 
south  (Bigelow,  1922,  p.  156,  Fig.  52).  Conditions  vary  widely,  however,  from 
year  to  year,  in  this  respect,  apart  from  temperature,  for  the  record  of  salps, 


BIGELOW   AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON    STUDIES  273 

inshore  for  1929  is  confined  to  scattered  specimens  at  three  stations,  all  south  of 
New  York  (maximum,  5  c.c).  And  we  have  no  reason  to  suppose  that  they  ever 
occur  in  abundance  inshore,  east  of  New  York,  even  in  a  July  when  they  are 
general  and  abundant  farther  out  at  sea,  as  they  were  in  1931,  when  the  average 
catch  of  salps  offshore  in  the  north  was  167  c.c.  (maximum,  1046  c.c). 

Salps  may  persist  in  small  or  moderate  numbers  here  and  there,  well  in  on  the 
shelf,  until  October  (maximum,  103  c.c.)  in  some  years,  as  exemplified  by  1931, 
most  abundant  and  most  frequent  in  the  south  as  was  to  be  expected — or  even 
until  November,  when  many  lasts  zonaria  were  taken  at  two  stations  off  Martha's 
Vineyard  in  1916  (Bigelow,  1922,  p.  157).  But  their  status  in  February  (p.  272) 
makes  it  Ukely  that  they  vanish  wholly  from  the  shelf,  to  the  north  of  Lat.  36°, 
with  falUng  temperatures,  at  the  onset  of  winter. 

Salps  may  be  regarded  strictly  as  oceanic  immigrants  in  our  region,  some- 
times developing  local  swarms  there  under  the  favorable  conditions  of  summer, 
but  unable  to  maintain  themselves  anywhere  inshore  from  the  200-meter  contour 
through  the  cold  season. 

Fritillaria  sp. 

Representatives  of  Fritillaria  were  recognized  at  localities  widely  scattered 
across  the  whole  breadth  of  the  continental  shelf  in  the  northern  sector,  and 
southward  along  the  edge  of  the  continent  as  far  as  the  Currituck  profile  (Fig. 
16A).  And  while  the  condition  of  the  material  did  not  allow  specific  identifica- 
tion, i.e.,  whether  belonging  to  the  wide-ranging  F.  borealis  or  to  the  warm  water 
F.  venusta  (Lohman,  1901;  1911),  this  distributional  picture  at  least  suggests  a 
more  northern  source  of  supply.  The  seasonal  distribution  of  the  catches  (11% 
of  the  stations  for  February,  2%  for  April,  7%  for  May,  0%  for  June,  3%  for 
July,  and  0%  for  October)  would  not  of  itself  suggest  any  relationship  to  the 
vernal  indrafts  of  water  from  the  east.  But  the  fact  that  the  only  catch  of  meas- 
urable volume  (9  c.c.)  was  made  in  May,  is  at  least  compatible  with  temporary 
recruitment  from  the  region  of  the  Gulf  of  Maine,  where  Fish  and  Johnson  (1937) 
found  F.  borealis  in  considerable  frequency.  Neither  have  we  any  evidence  of 
reproduction  of  appreciable  magnitude  anywhere  within  our  area,  for  all  the  other 
local  records  of  Fritillaria  were  of  occasional  specimens  only. 

OiKOPLEURA   DIOICA 

Records  for  this  appendicularian  have  been  confined  to  the  months  of  May 
(2  stations),  June  (4  stations),  July  (7  stations),  and  October  (4  stations),  and 


274  memoir:  museum  of  comparative  zoology 

so  strictly  limited  to  the  south  (Fig.  16)  that  it  can  be  as  safely  regarded  as  a 
warm  water  indicator  in  our  area,  as  is  0.  labradoriensis  of  water  from  the  east  and 
north.  In  the  southern  sector,  however,  the  localities  of  capture  have  been  wide- 
spread, from  close  inshore  out  to  the  continental  edge.  And  the  fact  that  the 
catches  ranged  from  1-14  c.c.  (average,  about  8  c.c.  for  the  stations  of  record) 
makes  it  Ukely  that  some  local  reproduction  of  0.  dioica  takes  place  as  far  north 
as  Delaware  Bay  during  the  warm  half  of  the  year.  But  it  is  unhkely  that  this 
is  ever  on  a  scale  large  enough  to  make  it  an  important  item  in  the  plankton. 
And  we  have  yet  to  learn  whether  it  vanishes  from  within  our  Umits  during  the 
cold  half  of  the  year  as  completely  as  our  failure  to  find  it  in  February  or  in  April 
would  suggest  (implying  that  reestablishment  later  in  the  spring  depends  on  re- 
newed invasion) ,  or  whether  a  scattering  of  the  species  actually  survives  the  winter. 

OiKOPLEURA    LABRADORIENSIS 

Oikopleura  labradoriensis  deserves  attention,  among  the  species  that  are 
usually  of  minor  importance,  both  because  of  the  wide  variation  in  its  abundance 
from  year  to  year,  and  as  an  indicator  of  water  from  the  north  and  east.  It  has 
been  recorded  at  only  three  stations  in  February,  at  five  in  April  (all  in  1930), 
always  in  minimal  numbers,  and  it  may  continue  very  scarce  in  some  years  right 
through  the  spring  and  early  summer,  as  in  1930,  when  it  was  recorded  at  only 
one  of  the  27  stations  for  May,  not  at  all  in  June  and  July.  But  it  may  increase 
considerably  in  abundance,  in  spring,  in  other  years  as  in  1931,  when  (not  found 
at  all  in  February)  it  was  widespread  in  May,  over  the  outer  parts  of  the  shelf 
as  a  whole,  southward  to  the  ofhng  of  Delaware  Bay,  averaging  5  c.c.  in  volume, 
for  the  whole  region,  with  a  maximum  of  G7  c.c.  And  it  was  still  more  abundant  in 
1932,  when — similarly  lacking  in  February — it  had  become  general  throughout  all 
but  the  southern  part  of  the  region  by  the  beginning  of  May,  averaging  about 
29  c.c.  by  the  second  week  of  the  month,  with  a  maximum  of  122  c.c.  This,  how- 
ever, represented  the  peak  for  0.  labradoriensis  in  each  case,  for  its  southern 
boundary  shifted,  in  1931,  from  the  ofhng  of  Cape  May  to  (roughly)  the  offing  of 
New  York  between  mid-May  and  mid-June,  while  in  1932,  the  percentage  of 
stations  at  which  it  occurred  decUned  in  the  northern  sector  from  95%  at  the  end 
of  May  to  53%  by  the  first  week  in  June,  and  to  15%,  by  the  third  week  of  that 
month,  by  which  date  it  was  entirely  confined  to  the  extreme  northeastern 
corner  of  our  area,  while  its  average  volume  similarly  decreased  in  the  northern 
sector,  to  an  average  of  4  c.c.  by  the  end  of  May,  and  to  1  c.c.  in  June.  We  have 
occasional  record  of  it  only  in  July  (8  stations,  all  years  combined)  and  none  at 
all  in  autumn. 


BIGELOW   AND   SEARS :   NORTH   ATLANTIC    ZOOPLANKTON    STUDIES  275 

Its  status,  thus,  appears  primarily  to  be  that  of  an  immigrant  entering  our 
area  in  spring,  to  muU-iply  there  temporarily,  if  conditions  favor,  which  happens 
most  often  in  May,  but  disappearing  altogether  by  mid-summer. 

MOLLUSCS 
Clione  limacina 

Frequency.  The  records  for  individual  cruises  have  shown  that  Clione  may 
appear  anywhere  on  the  shelf,  from  the  coast  line  out  to  the  edge  of  the  continent, 
at  any  time  between  February  and  October,  but  so  irregularly  and  sporadically 
that  no  dependence  can  be  placed  on  its  presence  or  absence  in  any  particular 
region,  or  at  any  particular  time  of  year.  Combination,  however,  of  the  records 
for  the  several  cruises  suggests  that  it  is  usually  least  frequent  at  the  end  of 
winter  and  in  early  spring,  and  that  it  tends  to  become  much  more  so  with  the 
advance  of  the  season,  for  it  was  taken  at  1 1  %  only  of  the  stations  for  February 
and  April,  but  at  46%  in  May,  53%  in  June,  and  32%  in  July,  with  maximum 
frequencies  of  84%,  78%,  and  50%,  respectively  for  the  last  three  months.  And 
this  is  in  Une  with  Rathbun's  (1889)  report  of  it  in  about  30%  of  his  towings  be- 
tween the  offings  of  New  York  and  of  Chesapeake  Bay  in  late  April,  and  May  of 
1887.  An  increase  in  frequency  through  the  spring  was  even  recorded  in  the  year 
(1932)  when  Clione  was  most  common  in  February,  namely  from  48%  of  the  sta- 
tions in  that  month  to  54%  in  May  and  70%  in  June.  And  the  data  for  1929, 
when  it  was  not  taken  at  all  in  April,  but  at  15%  and  21%  of  the  stations  in  May 
and  June,  respectively,  and  for  1931,  when  it  was  lacking  in  February,  but  was 
at  37%  of  the  stations  in  May,  similarly  suggest  that  in  some  years  it  may  not 
exist  at  all  in  our  area  until  late  in  the  spring. 

These  lines  of  evidence  mark  May  and  June  as,  on  the  whole,  the  months  in 
which  CUone  is  most  frequent,  with  July  faUing  but  Uttle  behind.  If  1931  can 
be  taken  as  representative,  the  species  then  tends  to  decline  in  frequency  during 
the  autumn,  and  perhaps  to  disappear  altogether  before  the  beginning  of  winter, 
for  it  was  taken  at  15%  only  of  the  stations  in  that  October  and  not  at  all  in 
November  1916. 

On  one  cruise  or  another,  Clione  has  been  found  most  frequent  inshore,  off- 
shore, in  the  north,  and  in  the  south,  apparently  independent  of  the  time  of  year. 
It  tends,  however,  to  be  somewhat  more  frequent  offshore  than  inshore,  for  this 
was  not  only  the  case  in  eight  individual  months  with  the  reverse  true  in  five  only 
(February,  1930,  1932;  June,  1929,  1930;  July,  1929),  but  it  was  also  recorded  at 
a  sUghtly  greater  percentage  of  stations  offshore  (33%)  than  inshore  (25%)  for  the 
series  as  a  whole. 


276 


memoir:  museum  of  comparative  zoology 


We  might  perhaps  have  reasonably  expected  to  find  a  species  of  presumably 
boreal  origin — such  as  Clione — occurring  more  frequently  in  the  northern  sector 
of  our  area  than  in  the  southern.  Actually,  it  was  taken  at  38%  of  the  stations 
in  the  south,  but  at  34%  only  in  the  north,  on  all  cruises  combined.  The  fact 
that  it  was  wholly  lacking  in  the  northern  sector  in  five  of  the  individual  months 
(February,  1931,  1932;  April,  May,  June,  1929;  October,  1931),  but  only  in  two 
months  (February,  1930,  1931)  in  the  southern,  is  in  Une  with  the  foregoing.  It 
was,  again,  most  frequent  in  the  south  in  ten  of  the  months  when  both  sectors 
were  surveyed,  but  in  two  only  (April,  1930;  May,  1931)  was  it  most  frequent  in 
the  north.  And  the  maximum  frequency  in  any  one  month  has  been  considerably 
higher  south  (90%,,  May  1930  and  June,  1932)  than  north  (66%,  June  1932). 
Thus,  there  seems  no  escape  from  the  conclusion  that  in  our  area,  Chone  must  be 
classed  as  a  southern  species,  even  though  this  places  its  center  of  frequency  close 
to  the  southern  boundary  to  its  regular  occurrence.  And  the  distribution  of  the 
richer  catches  is  in  hne  with  this,  volumes  larger  than  4  c.c.  having  been  entirely 
restricted  to  the  sector  southward  from  the  New  York  profile  (Fig.  16,  D),  with 
some  slight  concentration  of  the  largest  volumes  off  Virginia  and  off  Chesapeake 
Bay,  while  Rathbun  (1889)  found  it  "common"  and  "abundant"  to  the  south, 
only,  of  Lat.  39°  N.,  in  the  spring  of  1887. 


Percentage  of  stations  at  which  Clione  Umacina  was  taken 


Month 

Year 

Offshore 

Inshore 

North 

South 

Area 
Surveyed 

February 

1930 
1931 
1932 

0% 
44 

9% 
67 

10% 
0 

0% 
75 

5% 
48 

April 

1929 
1930 

20 

6 

19 

9 

16 

May 

1929 
1930 
1931 
1932 

15 
91 
70 
62 

7 
66 
13 
45 

0 
66 
50 
45 

22 
91 
13 
71 

15 

77 
37 
54 

June 

1929 
1930 
1931 
1932 

0 
10 
63 
90 

16 
25 

54 
60 

0 
24 
48 
66 

33 

0 

83 

90 

21 
19 
67 
70 

July 

1929 
1930 
1931 

0 
20 
75 

25 
28 
23 

10 
23 
38 

22 

14 
23 
38 

October 

1931 

15 

— 

0 

20 

— 

BIGELOW  AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


277 


Average  volumes  of  Clione  limacina 

Month 

Year 

Inshore 

Offshore 

North 

South 

Area 
Surveyed 

Maxi- 
mum 

February 

1930 
1931 
1932 

<1 
<1 

0 
<1 

<1 
0 

0 
<1 

<1 
<1 

<1 
<1 

April 

1929 
1930 

<1 

<1 

<1 

<1 

<1 

<1 

May 

1929 
1930 
1931 
1932 

<1 
<1 
<1 

<1 

<1 

<1 

<1 

2 

<1 
<1 
<1 

<1 

<1 

<1 

<1 

3 

<1 
<1 
<1 

1 

6 
<1 

4 
75 

June 

1929 
1930 
1931 
1932 

5 

<1 

3 

5 

<1 
<1 

2 
21 

0 

<1 
<1 

3 

8 

0 

6 

31 

3 

<1 

3 

12 

79 

2 

50 

235 

July 

1929 
1930 
1931 

<1 

7 
<1 

0 
<1 
<1 

<1 

3 

<1 

<1 

<1 

3 

<1 

<1 

50 

<1 

October 

1931 

— 

<1 

0 

<1 

<1 

<1 

Abundance.  The  records  of  Clione  in  February  and  April,  have,  in  every 
case,  been  based  on  volumes  smaller  than  1  c.c.  And  even  in  May,  when  this 
pteropod  is  approaching  its  maximum  frequency,  the  average  catch  for  the  area 
as  a  whole  still  continued  less  than  1  c.c.  in  three  of  the  years  (1929,  1930,  1931), 
and  was  only  1  c.c.  in  the  fourth  (1932).  But  the  maximum  had  risen  to  4-6  c.c. 
by  that  month  in  1929  and  1931;  1932  yielded  six  May  catches  of  5-10  c.c.  with 
one  of  75  c.c,  while  the  average  volume  of  Clione  was  about  5  c.c,  for  the  area 
as  a  whole  in  June  of  the  several  years  combined,  with  maxima  of  50  c.c  in  1931, 
of  79  c.c.  in  1929,  and  of  235  c.c  in  1932.  Thus,  it  appears  that  this  pteropod  may 
be  expected  to  experience  a  considerable  vernal  augmentation  in  our  waters  in 
most  years,  either  accompanying  or  briefly  succeeding  the  seasonal  increase  in  its 
frequency  of  occurrence.  If  it  be  scarce  in  June,  it  may  considerably  increase 
soon  after,  as  happened  in  1930,  when  the  averages  and  maxima  rose  from  less 
than  1  c.c.  and  2  c.c.  in  that  month  to  3  c.c.  and  50  c.c  in  July.  It  may  even  be 
rather  numerous  locally,  as  late  as  August  in  cool  summers — e.g.,  1916  (Bigelow, 
1922,  p.  156).  It  seems  more  usual,  however,  for  it  to  be  in  very  small  volume, 
anywhere  west  or  south  from  Cape  Cod  by  midsummer,  even  if  widely  dispersed 
then,  as  was  the  case  in  1931;  or  even  lacking  altogether,  as  seems  to  have  been 


278  memoir:  museum  of  comparative  zoology 

the  case  in  1913.  And  the  fact  that  the  maximum  catch  for  October  was  less  than 
1  c.c,  added  to  om-  failure  to  find  it  at  all  in  November  (1916),  suggests  that  this 
is  equally  true  in  autumn. 

The  rapidity  with  which  the  volume  of  Clione  may  alter,  emphasizes  the 
danger  of  generahzing,  as  to  what  is  to  be  expected  in  the  oft  quoted  "normal" 
year.  In  1932,  for  example,  the  average  fell  from  27  c.c.  in  the  first  week  in  June, 
to  less  than  1  c.c.  a  few  days  later,  but  then  ro.se  to  8  c.c.  in  the  third  week  of  the 
month,  with  a  corresponding  fluctuation  in  the  maximum  catch.  But  the  data 
suggest  that  a  maximum  catch  of,  say,  upwards  of  200  c.c.  may  be  regarded  as 
exceptionally  rich  for  Chone,  at  any  season,  or  an  average  of  more  than  20  c.c, 
whether  for  the  area  as  a  whole,  or  for  any  considerable  subdivision  of  the  same. 

Source  of  tlie  local  stock.  The  abruptness  with  which  fluctuations  take  place 
in  the  abundance  and  frequency  of  occurrence  of  CUone,  added  to  the  fact  that 
one  year  may  be  richest  in  this  pteropod  in  one  month,  another  year  in  another, 
introduces  the  question,  whether  maintenance  in  our  area  depends  more  on  local 
reproduction  or  on  waves  of  immigration.  The  probabiUty  has  already  been 
mentioned  (Bigelow,  1922,  p.  174),  that  local  centers  for  this  species  are  the 
result  of  temporary  breeding  activity  "of  such  few  specimens  as  from  time  to  time 
stray  southward  past  Cape  Cod."  And  if  this  pteropod  does  actually  vanish  as 
completely  from  our  scene  in  autumn  as  now  appears  to  be  the  case,  this  early 
suggestion  is  no  doubt  correct.  Failure  to  detect  any  larvae,  on  the  February  or 
April  cruises,  makes  it  unlikely  that  breeding  takes  place  in  our  area  in  winter, 
or  in  early  spring.  It  is  certain,  however,  that  widespread  reproduction  may  occur 
in  late  spring  and  early  summer,  because  larvae  were  taken  at  27%  of  the  sta- 
tions for  May  and  38%  for  June  in  1929,  75%,  in  May  in  1930,  and  50%  in  May 
and  28%  in  June  in  1932.  But  the  situation  in  this  respect  evidently  varies 
widely  from  year  to  year,  for  larvae  were  detected  at  three  stations  only  (all  in 
May)  in  1931.  Nor  have  we  any  evidence, — in  the  occurrence  of  larvae — of  re- 
production later  than  June  in  any  year,  anywhere  west  of  Cape  Cod. 

In  good  breeding  years,  such  as  1929,  1930,  and  1932,  the  production  of 
larvae  is  widespread  over  the  area  as  a  whole,  at  the  height  of  the  season,  with- 
out apparent  concentration  in  any  particular  region,  except  perhaps,  from  the 
New  York  profile  southward,  as  contrasted  with  the  sector  farther  to  the  east. 
But  it  appears  that  the  offing  of  Chesapeake  Bay  is  roughly  the  southern  bound- 
ary to  their  presence. 

In  most  instances,  the  records  for  larvae  have  been  based  on  less  than  1  c.c. 
per  catch,  except  during  the  first  June  cruise  of  1932,  when  half  a  dozen  catches 


BIGELOW   AND   SEARS :   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  279 

were  made  larger  than  15  c.c,  with  a  maximum  of  73  c.c.  bringing  the  general 
average  for  the  area  up  to  27  c.c.  It  is  particularly  interesting — if  difficult  to 
explain — that  the  largest  catch  for  this  cruise  (for  the  entire  series  for  that 
matter)  was  made  in  the  south,  off  Chesapeake  Bay. 


LiMACINA   RETROVERSA 

Frequency.  It  seems  that  the  southern  boundary  to  the  regular  occurrence 
of  this  well  known  boreal  pteropod  is  not  far  from  Latitude  36°  N.,  for  it  was  not 
found  near  Cape  Hatteras,  on  the  one  cruise  (February  1931)  that  extended  so 
far  in  that  direction.  But  it  has  been  taken  widespread  throughout  our  area  to 
the  northward  of  Chesapeake  Bay,  on  one  occasion  or  another.  At  any  given 
time,  its  range  may  cover  all  parts  of  the  area  indifferently — as  was  the  case  in 
May  1932 — it  may  be  confined  to  a  definite  pool  or  pools,  leaving  other  exten- 
sive areas  bare,  as  in  that  same  month  in  1929,  or  it  may  fail  altogether  at 
one  station,  but  prove  extremely  numerous  at  another,  only  a  few  miles  distant, 
with  rich  and  barren  centers  complexly  intermingled.  But  its  presence  at  58% 
of  the  stations  inshore,  50%  offshore,  56%  in  the  north,  and  59%  in  the  south, 
shows  it  as  about  as  frequent  (relatively)  in  one  subdivision  as  in  another. 

Seasonally,  however,  a  rather  definite  succession  has  appeared  from  highest 
frequency  in  February  (81%  of  the  stations)  through  April  and  May  (70%), 
June  (66%)  and  July  (21%),  for  all  years  combined.  In  each  year  of  record 
furthermore,  Limacina  was  either  lacking  in  the  easternmost  sector  throughout 
the  season  of  observation  (e.g.,  1929),  or  showed  a  general  tendency  to  disappear 
thence  during  the  spring,  either  locally  (1931,  1932),  or  completely  (1930).  It 
also  disappeared  from  the  south  between  February  or  April  and  May  or  June, 
in  three  of  the  years  of  record,  either  from  the  inshore  belt  alone  (1929,  1932),  or 
across  the  whole  breadth  of  the  shelf,  although  in  the  fourth  year  (1930),  it  con- 
tinued general  in  the  southern  sector  right  through  June. 

The  general  implication  of  the  foregoing  is  that  in  the  case  of  Limacina, 
throughout  the  spring  and  early  summer,  a  comparatively  barren  belt  usually 
separates  one  distributional  center  in  the  Gulf  of  Maine,  from  another  in  the 
waters  west  of  Cape  Cod,  centering  chiefly  in  the  general  offing  of  New  York. 

If  the  data  for  October  1931,  and  for  November  1916  can  be  taken  as  repre- 
sentative, two  alternative  explanations  are  open:  either  that  adult  Limacina 
vanishes  entirely  from  the  offshore  belt — hence  presumably  from  the  area  as  a 
whole — in  late  summer  and  early  autumn  (it  was  not  found  at  all  in  October), 


280 


memoir:  museum  of  comparative  zoology 


to  reappear  widespread  as  far  south  as  Delaware  Bay  by  November,  much  as 
Redfield  (1939)  reports  for  the  Gulf  of  Maine,  or  else  that  a  stock  of  adults  per- 
sists right  through  the  autumn  in  some  years,  but  not  in  others. 

No  information  is  available  in  our  area  through  the  first  two  months  of 
winter. 


Average  volumes  of  Limacina 

Month 

Year 

Inshore 

Offshore 

North 

South 

Area 
surveyed 

Maxi- 
mum 

February 

1930 
1931 
1932 

2 
<1 

228 

4 
<1 

7 

5 

<1 

1 

1 
<1 

195 

3 
<1 
138 

33 

2 

1833 

April 

1929 
1930 

18 
29 

2 
43 

12 

49 

11 
25 

12 
36 

138 
409 

May 

1929 
1930 
1931 
1932 

44 
111 

48 

77 

20 

577 

27 

54 

49 
243 

18 
74 

17 

411 

91 

35 

35 

318 

39 

68 

969 

2666 

400 

887 

June 

1929 
1930 
1931 
1932 

13 

84 

6 

12 

9 
16 

4 
40 

13 

74 

<1 

19 

8 

<1 

16 

42 

12 

59 

6 

26 

220 

1296 

63 

221 

July 

1929 
1930 
1931 

6 
18 
24 

<1 

13 
0 

2 
13 
15 

5 

3 
13 
15 

55 
128 
233 

October 

1931 

— 

<1 

<1 

<1 

<1 

<1 

Abundance.  The  great  irregularity  of  distribution  just  emphasized,  for 
Limacina,  shows  the  danger  of  generaUzation,  as  to  the  normally  seasonal  cycle 
of  abundance,  or  as  to  the  average  volumes  of  this  pteropod  that  may  be  ex- 
pected in  different  months.  As  an  extreme  illustration  of  variation  in  abundance, 
at  a  given  season,  we  may  cite  February,  when  Limacina  averaged  less  than  1  c.c. 
(maximum,  less  than  2  c.c.)  in  any  subdivision  of  our  area  in  1931,  but  when  it 
averaged  138  c.c.  for  the  area  as  a  whole,  in  1932,  with  a  maximum  catch  at  the 
rate  of  1833  c.c.  A  rather  definite  progression  does,  however,  appear  in  the  fact 
that  the  volumes  of  Limacina  for  the  area  as  a  whole  averaged  definitely  higher 
in  May  of  three  of  the  four  years  (1929,  1930,  1931)  than  either  earlier  in  the 
season  or  later,  this  seasonal  peak  being  most  clearly  outlined  in  1930,  when  it 
averaged  five  or  six  times  more  abundant  in  May  (318  c.c.)  than  in  April  (36  c.c), 
on  the  one  hand,  or  in  June  (59  c.c.)  on  the  other  (Fig.  17).  Again,  in  1931,  the 


BIGELOW   AND    SEARS :    NORTH   ATLANTIC    ZOOPLANKTON    STUDIES 


281 


average  May  catch  (39  c.c.)  was  more  than  39  times  that  for  February,  six  times 
that  for  June  (6  c.c.)  and  more  than  twice  that  for  July  (15  c.c).  Rich  (  >  100  c.c.) 
catches  have  also  been  made  most  frequently  in  May  (17%  of  the  stations),  much 


c.  c 


300- 


250- 


MAR.         APR-  MAY  JUNE  JULY 

Fig.  17.  Average  monthly  volumes  of  LAmacina  retroversa,  in  different  years,  for  the  area  as  a  whole. 

less  SO  in  February  (6%  of  the  stations),  April  (10%),  June  (6%),  or  July  (3%), 
while  mid-  and  late  summer  scarcity  has  also  been  recorded  for  1913  and  1916 
(Bigelow,  1915;  1922).  But  a  different  succession  is  illustrated  by  the  data  for 
1932,  when  one  very  large  catch  (1833  c.c.)  near  Currituck  was  responsible  for 


282  memoir:  museum  of  comparative  zoology 

an  average  of  195  c.c.  in  the  south,  and  of  138  c.c.  for  the  area  as  a  whole  in  Feb- 
ruary, but  when  the  general  average  fell  to  68  c.c.  for  May  and  to  26  c.c.  for  June. 

Thus,  it  appears  that  it  is  usual  for  Limacina  in  our  area  to  multiply  after  a 
normally  cool  winter  to  a  pronounced  vernal  peak  of  abundance  in  May,  then  to 
decline  as  rapidly,  through  the  summer,  but  that  the  peak  may  fall  as  early  as 
February,  after  an  abnormally  warm  winter,  to  be  followed  by  progressive  de- 
crease through  the  spring  and  early  summer.  Present  indications  are  that 
Limacina  either  disappears  altogether  for  a  time  in  early  autumn,  or  at  least  con- 
tinues decidedly  scarce  through  that  quarter  of  the  year  and  presumably  through 
the  first  two  months  of  winter. 

The  volumes  of  Limacina  have  averaged  about  as  large  in  one  subdivision 
of  the  area  as  in  another,  for  all  cruises  combined  (average,  north,  29  c.c;  south, 
37  c.c. ;  inshore,  48  c.c.  offshore,  39  c.c).  And  moderately  large  catches  have  been 
so  widespread  (Fig.  18)  at  the  season  of  maximum  abundance  (May)  as  to  sug- 
gest that  one  part  of  the  area  is  as  likely  to  support  a  considerable  population  as 
any  other.  Successive  cruises  have,  however,  shown  that  it  is  characteristic  for 
the  richest  centers  of  Limacina  to  be  confined  to  small  areas,  and  for  these  to 
shift  position  and  extent  much  more  rapidly  from  month  to  month  than  is  usual 
with  most  of  the  other  dominant  members  of  the  planktonic  community. 

This  was  illustrated  in  1930  by  the  fact  that  a  rich  center  existing  near 
Martha's  Vineyard  early  in  April,  expanded  southwestward,  along  the  mid-belt 
of  the  shelf,  as  far  as  the  offing  of  Delaware  Bay,  during  the  next  two  weeks,  a 
second  rich  zone  having  meantime  developed,  offshore  and  to  the  south  of  Chesa- 
peake Bay,  to  coalesce  with  the  more  northerly  center  by  May,  followed  by  a 
contraction  eastward  of  the  resultant  rich  area  through  June  (Fig.  18).  Two  rich 
centers  again  developed  in  1931,  between  February  and  May,  a  larger  offshore, 
a  smaller  inshore,  the  former  to  be  dissipated  by  June,  while  the  latter  (though 
not  encountered  that  month)  may  actually  have  persisted  until  July,  when  a  rich 
catch  was  made  at  a  neighboring  locality.  And  the  seasonal  succession  proved 
still  more  complex  in  1932,  when  the  four  separate  centers  that  were  encountered 
during  the  first  week  of  May  had  given  place  in  the  second  and  third  weeks  to 
three  centers  (Fig.  18),  which  then  dispersed  by  the  last  of  the  month,  to  be  re- 
placed early  in  June  by  a  fourth  center,  which  in  its  turn  had  entirely  dispersed 
two  weeks  later. 

It  would  require  a  much  more  extensive  series  of  observations  to  fit  altera- 
tions as  complicated  and  as  seemingly  sporadic  as  these  into  a  regularly  seasonal 
schedule,  if  indeed,  any  such  applies  in  this  case. 


BIGELOW   AND   SEARS :   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  283 


•4  0 


40 


■ 

K^ 

'%j 

mSt^^^^  ■■'■■'■■■ 

^^^^1 

Ef^^ 

4  0'^ 

H 

MMm 

— , 

mm  -/ 

V 

mm/     E 

/ 

^^^H 

f 
,.'' 

'li^HP 

,/ 

7, 

/ 

1/  #'/ 


.^-^ 

't; 

r^ 

^ 

1/ 

»'        •■•••.  •■:-' 

^^ 

f    y 

D 

f^     ^nS 

/ 

^      li 

L,'     jJSa 

y 

r 

/ 

70" 


Fig.  18.  Areas  where  the  volume  of  Lvmacina  retroversa  averaged  greater  than  100  cc,  per  standard  haul, 
in  different  months:  A,  April  3-11,  1930  and  April  22-May  1,  1930;  B,  May  12-23,  1930;  C, 
June  7-18,  1930;  D,  May  2-6,  1932;  E,  May  9-16,  1932:  F,  May  19-23,  1932. 


284  memoir:  museum  of  comparative  zoology 

Vertical  distribution.  Among  the  stations  where  tows  were  made  at  two 
levels,  the  shoal  catch  of  Limacina  was  twice  the  larger  of  the  pair  slightly  more 
often  (55  cases)  than  the  reverse  (47  cases),  though  rhore  frequently  (97  cases), 
the  two  did  not  differ  significantly,  one  from  the  other.  The  volumes  also  aver- 
aged slightly  larger  (average,  53.5  c.c,  200  ca.ses),  for  hauls  centering  shoaler 
than  10  meters  than  for  those  centering  deeper  than  20  meters  (average,  37.8  c.c, 
201  cases). 

Perhaps  more  significant  is  the  fact  that  five  of  the  seven  catches  greater  than 
400  c.c,  that  were  made  at  pertinent  stations,  were  from  the  shoaler,  only  two 
from  the  deeper  haul,  suggesting  that  centers  for  Limacina  are  more  subject  to 
transport  by  temporary  wind-drifts  than  in  the  cases  of  species  for  which  the 
centers  of  abundance  lie  deeper. 

Relation  to  tem-pcrature.  The  data  do  not  warrant  any  general  comparison 
between  average  catch  and  the  temperature  of  the  water.  It  appears,  however, 
that  values  higher  than  about  18°  are  not  favorable  for  Limacina,  for  while  the 
catch  of  the  shoal  hauls  averaged  about  27  c.c.  in  June  and  July  as  a  whole,  it 
averaged  only  about  5  c.c.  (including  one  catch  of  81  c.c),  at  the  group  of  sta- 
tions where  the  upper  10  meters  was  warmer  than  18°. 

Annual  variations.  Average  volumes  for  the  period,  February-June,  were 
5-6  times  as  great  in  the  most  productive  years  of  the  series  (average,  about  100 
c.c.  in  1930,  and  about  71  c.c.  in  1932),  as  in  the  poorest  (14  c.c,  1931).  And  it  is 
not  unlikely  that  a  longer  term  might  not  have  shown  still  wider  variation,  there 
being  no  warrant  for  assuming  that  any  one  of  the  four  years  illustrated  an  ex- 
treme state  in  either  direction.  Limacina  retroversa  is,  in  short,  an  extremely 
variable  species,  so  far  as  prevailing  abundance  is  concerned,  not  only  from  season 
to  season,  and  from  place  to  place,  but  also  from  year  to  year.  And  a  similar 
variability  is  recorded  for  its  frequency  of  occurrence,  for  while  it  was  recorded 
at  83%  of  the  stations  in  one  of  the  years  (1932),  it  was  found  at  51%,  only,  in 
another— 1929. 

Source  of  the  local  stock.  Redfield's  (1939)  recent  demonstration  that  the 
stock  of  Limacina  in  the  Gulf  of  Maine  draws — at  least  largely — on  immigration 
from  Nova  Scotian  shelf  waters,  and  that  the  quantitative  distribution  within 
the  Gulf  is  determined  by  the  drifts  undergone  by  the  entering  shoals  and  their 
offspring,  opens  the  question  to  what  degree  this  may  also  apply  to  the  waters 
west  of  Cape  Cod.  Two  Unes  of  evidence  are  available,  first,  the  presence  or 
absence  of  very  young  stages,  the  imphcation  of  which  is  obvious,  and  second, 
the  time  intervals  that  intervened  between  the  development  of  different  centers 


BIGELOW   AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON    STUDIES  285 

of  abundance,  relative  to  their  geographic  locations,  and  to  their  distances  apart. 

The  year  1930  is  the  member  of  the  series  in  which  a  Ciulf  of  Maine  source 
might  most  plausibly  be  argued  for  an  increase  that  took  place  between  February 
and  early  April  in  the  abundance  of  Limacina  in  the  most  easterly  sector  of  our 
area,  since  the  temperatures  gave  evidence  of  a  coincident  drift  of  cold  water 
from  the  east,  past  Cape  Cod  (Bigelow,  1933,  p.  30).  But  we  cannot  credit  to  this 
source  the  extension  of  the  rich  belt  southward  along  the  mid-zone  of  the  shelf, 
past  the  offing  of  Chesapeake  Bay,  a  distance  of  about  360  miles,  (Fig.  18A), 
that  took  place  between  the  first  and  third  weeks  of  the  month,  unless  we  admit 
the  prevalence,  meantime,  of  a  corresponding  drift  at  the  rate  of  at  least  10  miles 
per  day,  which  is  forbidden,  not  only  by  navigational  experience,  but  by  the 
progress  of  vernal  warming  at  the  time  (Bigelow,  1933,  p.  30).  And  the  develop- 
ment of  rich  centers,  in  the  years,  1929,  1931,  and  1932,  centered  chiefly  in  the 
mid-sector  of  our  area,  with  httle  evidence  of  prevaihng  drift,  whether  north  or 
south.  It  thus  appears  that  if  any  mass  immigration  did  take  place  into  our 
area  from  the  eastward  in  either  year,  this  must  have  happened  prior  either  to 
April  (1929,  1930),  or  at  least  prior  to  May  (1931,  1932),  i.e.,  at  a  season  when 
the  stock  of  Limacina  in  the  southwestern  part  of  the  Gulf  of  Maine  and  on 
George's  Bank  is  low. 

The  e\ddence  of  the  frequency  of  young  stages  among  the  population  in 
April,  1930,  also  argues  for  a  local  source — or  at  least  for  sources  not  far  distant, 
for  rich  centers  that  developed  during  that  month,  for  such  of  the  catches  as  were 
greater  than  100  c.c.  contained  57-86%,  by  number,  of  juveniles  less  than  0.6 
mm.  in  diameter ;  they  even  contained  2-26%  of  specimens  smaller  than  0.3  mm., 
although  the  meshes  of  the  nets  used  were  large  enough  (about  0.5  mm.)  to  have 
allowed  these  to  pass  through.  And  rough  examination  has  shown  that  consider- 
able percentages  of  young  specimens  are  also  included  in  the  rich  catches  for 
other  years,  which  have  not  yet  been  examined  in  detail. 

We  have,  in  short,  as  good  a  reason  for  regarding  Limacina  as  regularly  and 
primarily  endemic  southward  past  Delaware  Bay,  and  perhaps  southward  past 
Chesapeake  Bay,  as  for  so  regarding  any  other  member  of  the  planktonic  com- 
munity. 

Invasions  of  water  past  Cape  Cod,  would  of  course,  add  to  the  local  popu- 
lation, if  at  a  season  when  Limacina  is  abundant  in  the  neighboring  parts  of  the 
Gulf  of  Maine  area— as  would  indeed,  happen  for  any  other  member  of  the  boreal 
assemblage.  But  there  is  no  need  to  invoke  such  invasions  to  explain  the  con- 
tinued presence  of  this  pteropod  within  our  area. 


286  memoir:  museum  of  comparative  zoology 

Other  Molluscs 

During  July-August,  1913,  the  warm  water  pteropods,  Corolla  calceola, 
Criseis  acicula,  C.  conica,  and  C.  virgula,  and  the  heteropod,  Firoloida  desmarestia, 
were  taken  at  scattered  localities  on  the  shelf,  inshore  as  well  as  offshore,  from 
the  Atlantic  City  profile  southward:  Corolla,  in  fact,  in  some  abundance  at  one 
station,  as  described  elsewhere  (Bigelow,  1915,  p.  302,  Fig.  72).  And  warm  water 
species  of  Limacina  were  again  recognized  at  three  stations,  along  this  same  sec- 
tor, in  the  summer  of  1916  (Bigelow,  1922,  p.  155,  Fig.  51).  But  our  only  record 
of  this  group  during  the  period  1929-1932,  was  for  odd  specimens  of  tropical 
pteropods  at  one  station  off  Bodie  Island  in  February  1930,  at  one  off  Currituck 
in  April  of  that  same  year,  and  at  two  on  the  New  York  profile  in  February  1932. 
These  data  show  that  it  is  an  unusual  event  for  members  of  this  category  to  in- 
vade the  shelf  from  offshore  in  numbers  sufficient  to  be  picked  up  in  the  tow  nets. 
And  the  fact  that  they  were  most  strongly  represented  in  on  the  shelf  in  the  sum- 
mer of  1913  and  again  in  1916,  but  apparently  not  at  all  at  that  season  in  1929, 
or  in  October  1931  (years  in  which  the  southern  sector  was  surveyed  in  those 
months),  is  evidence  that  their  temporary  presence  within  our  limits  (resulting 
from  transport  by  indrafts  of  water  from  the  slope)  is  independent  of  precise 
conditions  of  temperature  at  the  particular  place  and  time  where  they  may  be 
encountered. 

DECAPODS 
Crab  and  hermit  crab  larvae 

Crab  or  hermit  crab  larvae  (not  yet  identified)  were  recorded  at  1/3-1/5  of 
the  stations  for  February,  in  each  of  the  years  of  record,  mostly  over  the  outer 
half  of  the  shelf,  but  indifferently  from  north  to  south,  with  a  maximum  catch 
of  less  than  1  c.c.  They  were  slightly  more  general  in  April,  when  they  were 
recorded  at  most  of  the  stations  in  the  southern  sector  in  1929,  and  scattered 
all  along  from  south  to  north  in  1930,  in  which  year  a  catch  of  82  c.c.  near  Dela- 
ware Bay  on  the  24th,  showed  that  important  contributions  from  this  source, 
are  to  be  expected  locally,  close  inshore  at  this  season  (Fig.  19).  And  by  May,  crab 
larvae  may  be  present  in  such  abundance  next  the  land  as  to  jdeld  catches  as 
large  as  200  c.c,  as  happened  in  1932,  though  in  other  years  (1931),  their  num- 
bers may  be  insignificant  in  that  month,  even  at  inshore  localities. 

The  catches  made  in  successive  cruises  in  May  1932  illustrate  the  wide  fluctu- 
ations that  may  take  place  in  their  abundance  within  short  intervals  of  time,  for 


BIGELOW   AND    SEARS:   NORTH    ATLANTIC    ZOOPLANKTON    STUDIES  287 


Fig.  19.  Locality  records,  all  cruises  combined;  A,  crab  and  hermit-crab  larvae,  volumes  larger  than 
10  cc;  B,  Crago,  contour  marking  offshore  boundary;  C,  euphausiid  larvae,  volumes  larger  than 
25  cc;  D,  Lucifer  typus,  palinurid  larvae  and  stomatopod  larvae. 


288  memoir:  museum  of  comparative  zoology 

they  practically  disappeared  from  the  whole  area  between  the  first  and  middle 
of  the  month,  but  soon  reappeared  in  the  southern  sector  in  such  abundance  that 
catches  up  to  144-200  c.c.  were  made  there  during  the  third  and  fourth  weeks, 
another  center  of  abundance  (maximum,  90  c.c.)  havdng  meantime  developed 
near  the  northeastern  boundary  of  the  area.  Wide  variation  has  also  been  re- 
corded from  year  to  year  in  the  order  of  change,  from  May  through  June,  in  the 
case  of  this  heterogeneous  category.  The  one  extreme  is  illustrated  by  1931,  when 
they  were  recorded  at  4  stations  only  in  May,  but  had  become  general  throughout 
the  area  by  mid-June,  the  other  extreme  by  1929,  when  100%  frequency  in  May 
gave  place  to  only  about  50%  frequency  in  June,  while  the  other  two  years  showed 
intermediate  states,  namely,  a  peopUng  of  the  coastal  belt  eastward  from  New 
York  between  mid-May  and  the  first  of  June  in  1929,  contrasted  with  a  decrease 
in  frequency  in  the  northern  sector  in  1932  from  100%  of  the  stations  in  May,  to 
only  37%  in  June.  Until  the  several  constituent  species  are  studied  in  detail,  it 
will  be  premature  to  postulate  the  order  that  exists  in  these  fluctuations,  beyond 
the  two  outstanding  facts,  (a)  that  crab  larvae,  on  the  whole,  appeared  in  about 
twice  as  great  frequency  in  June  (67%  of  the  stations),  as  in  May  (34%  of  the 
stations)  for  the  term  of  years  as  a  whole,  and  (b)  that  most  of  the  June  catches 
that  were  larger  than  4  c.c,  were  made  within  35  miles  of  the  coast  Une. 

The  frequency  of  occurrence  of  crab  larvae  changed  but  Httle  from  June  to 
July  in  1929  or  1930.  But  in  1931,  they  had  disappeared  entirely  by  July  from 
the  northern  sector,  where  they  had  been  taken  at  84%  of  the  stations  in  the  pre- 
ceding month.  The  following  tabulation  also  shows  that  they  considerably  de- 
creased in  volume  from  June  to  July  in  each  of  the  years  (1929,  1930)  when  they 
occurred  in  more  than  minimal  amount  in  either  of  these  months.  The  catches 
have  also  averaged  so  small  in  the  offshore  belt  even  at  the  peak  season  (average, 
6  c.c,  June  1931)  as  to  show  that  the  contribution  made  to  the  plankton  above, 
by  crabs  living  in  depths  greater  than  about  50  meters  is  usually  negligible.  And 
while  experience  in  1913  had  already  shown  that  larvae  of  the  blue  crab  (CalU- 
nectes)  may  swarm,  close  in  to  the  land  near  Chesapeake  Bay  in  July  (Bigelow, 
1915,  p.  271),  we  have  no  e\'idence  that  the  product  of  this  particular  species  ever 
adds  appreciably  to  the  plankton,  for  more  than  a  few  miles  out  from  the  land, 
though  its  breeding  range  extends  northward  at  least  to  Woods  Hole.  No  crab 
larvae  were  detected  during  October  1931  in  the  offshore  belt,  though  they  may 
have  been  present,  in  unknown  number  at  the  time,  in  the  inshore  belt,  which 
was  not  visited  during  that  cruise,  nor  have  we  any  information  for  the  months 
November-January. 


BIGELOW  AND   SEARS :   NORTH   ATHANTIC    ZOOPLANKTON   STUDIES 


289 


The  volume  of  crab  larvae  averaged  about  10  times  as  large,  for  the  area 
as  a  whole,  in  the  most  productive  year  (1931,  10  c.c),  as  in  the  least  productive 
(1930,  1  c.c),  in  June — on  the  whole  the  peak  month —  while  a  maximum  catch 
of  226  c.c.  in  1932,  contrasts  with  68  c.c.  in  1930. 


Average  volumes  of  crab  and  hermit  crab  larvae 

Month 

Year 

Inshore 

Offshore 

North 

South 

Area 
surveyed 

Maxi- 
mum 

February 

1930 
1931 
1932 

<1 

0 

<1 

<1 
<1 
<1 

<1 

0 

<1 

<1 
<1 
<1 

<1 
<1 
<1 

<1 
<1 
<1 

April 

1929 
1930 

<1 
3 

<1 
<1 

<1 

<1 

<1 
4 

<1 
2 

7 
82 

May 

1929 
1930 
1931 
1932 

6 
5 

1 
13 

<1 
1 
0 
3 

2 

4 
1 
2 

6 

3 

0 

19 

4 

3 

<1 

8 

31 
36 

18 
226 

June 

1929 
1930 
1931 
1932 

7 

3 

12 

4 

1 

<1 

6 

1 

7 

<1 

11 

3 

3 

7 
8 
2 

5 

1 

10 

2 

87 
68 
70 
38 

July 

1929 
1930 
1931 

2 
<1 

<1 
0 

<1 

<1 

1 

1 
<1 

10 
<1 

October 

1931 

— 

Crago  sp. 

The  records  for  Crago,  in  late  stages  of  development — picked  up  no  doubt 
when  the  nets  were  working  closest  to  bottom — are  sufficiently  general  (Fig.  19B) 
to  show  that  a  considerable  population  of  this  famihar  benthonic  genus  of  shrimp 
inhabits  the  inshore  belt,  as  a  whole,  from  the  one  boundary  of  our  area  to  the 
other.  But  one  only  of  the  records  was  more  than  40  miles  out  from  the  coast,  the 
great  majority  lying  within  30  miles  of  the  latter.  Thus,  it  appears  that  the  off- 
shore boundary  to  the  regular  occurrence  of  Crago  Hes  not  far  from  the  50-meter 
contour,  depth  rather  than  distance  from  land  being  in  all  probability  the  de- 
termining factor. 

The  capture  of  Crago  at  35%  of  the  inshore  stations  in  February,  and  27% 
in  April,  but  only  10%  in  May,  6%  in  June,  and  5%  in  July,'  suggests  that  it 
rises  (or  is  swept  upward)  from  the  sea  floor  in  significant  numbers,  most  commonly 

'  No  hauls  were  made  inshore  in  October. 


290  memoir:  museum  of*  comparative  zoology 

in  winter,  when  turbulent  movements  of  the  water  are  the  most  active,  and  that 
it  does  so  less  and  less  commonly,  with  the  advance  of  spring  and  summer. 

As  a  rule,  odd  specimens  only  were  taken.  Once,  however,  a  volume  of 
35  c.c.  of  juveniles  (13%  of  the  total  catch)  was  recorded  (off  New  York,  June  26, 
1930),  showing  that  on  occasion  the  j'oung  stages,  at  any  rate,  of  this  genus 
may  occur  in  the  mid-depths  in  such  abundance  as  to  be  an  important  item  in  the 
general  planktonic  community.  And  the  fact  that  adults  were  so  often  picked 
up  in  our  nets,  suggests  that — being  of  considerable  size — they  may  be  an  im- 
portant source  of  food  for  fish  foraging  near  the  bottom  well  out  on  the  shelf,  as 
has  long  been  known  to  be  true  in  shallow  water  near  shore. 

Palinurid  larvae 
The  northern  boundary  to  the  normal  range  of  the  adult  spiny  lobster 
{Panulirus  argus)  along  the  American  coast,  lies  some  120  miles  south  of  the 
southern  limits  of  our  area.  But  it  has  long  been  known  that  the  curious  leaf-Uke 
"Phyllosome"  larvae,  probably  of  this  parentage,  often  stray  far  to  the  northward 
with  the  general  drift  of  the  so-called  Gulf  Stream,  as  do  many  other  tropical 
animals.  Consequently,  it  would  not  be  astonishing  should  odd  specimens  be 
carried  here  and  there  across  the  continental  edge,  in  summer,  when  the  surface 
waters  are  at  their  warmest.  Actually,  however,  we  have  record  of  only  one  such 
incursion,  namely,  in  July  1929,  when  the  larvae  were  found  in  amounts  varying 
from  1  c.c.  to  9  c.c.  at  6  out  of  the  9  stations  that  were  occupied  south  of  Delaware 
Bay  (Fig.  19D),  but  when  they  seem  entirely  to  have  been  confined  to  the  south- 
ernmost sector. 

Lucifer  typus 

The  only  decapod,  apart  from  crab  larvae,  Crago,  and  palinurid  larvae 
that  has  been  detected  in  the  catches,  is  Lucifer  typus,  a  visitor  from  offshore. 
Lucifer  was  taken  at  two  stations  in  February  (1930,  1932),  at  one  station  in 
April  (1930),  three  stations  in  June  (1932),  and  five  stations  in  July  (1929),  al- 
ways in  the  southern  sector  as  accords  with  its  warm  water  origin  (Fig.  19D). 
One  catch  of  23  c.c.  was  recorded  in  July  1929,  one  of  8  c.c.  in  that  same  month, 
and  another  of  8  c.c.  in  April,  the  other  records  being  based  on  odd  indi\'iduals 
only.  There  is  nothing  in  this  record  to  suggest  that  Lucifer  is  ever  of  volumetric 
importance  within  our  area,  even  in  the  southernmost  sector. 

STOMATOPODS 
Occasional  stomatopod  larvae  were  recorded  in  February  (1  station).  May 
(1  station),  June  (3  stations),  and  July  (7  stations)  at  the  localities  shown  on 


BIGELOW   AND    SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


291 


Fig.  19D ;  also  near  the  outer  edge  of  the  shelf,  on  the  Winterquarter  profile,  in 
July,  1913  (Bigelow,  1915,  p.  271). 

EUPHAUSllDS 
Meganyctiphanes  norvegica 
Meganyctiphanes  norvegica  is  an  offshore  and  northern  species  in  our  area 
(Table,  p.  292)  as  was  to  be  expected.   In  February  and  April,  its  area  of  occur- 


Fig.  20.  Locality  records,  all  cruises  combined:  A,  Nematoscelis  megalops;  B,  Meganyctiphanes  norvegica, 
February- June;  C,  Meganyctiphanes  norvegica,  July  and  October;  D,  Thysanoessa  inermia. 

rence  southward  from  the  offing  of  Atlantic  City  has  in  fact  been  definitely  con- 
fined to  the  outer  part  of  the  shelf  (Fig.  20),  while  it  was  found  only  once  inshore 


292 


memoir:  museum  of  comparative  zoology 


in  May  of  either  year.  And  though  some  slight  dispersal,  toward  the  land,  seems 
to  have  taken  place,  in  June  in  1931  and  1932  it  was  wholly  lacking  inshore  or  in 
the  south  in  June  and  July  1929,  though  present  then  at  about  50%  of  the  stations 
offshore,  in  the  north. 

In  the  northern  sector  offshore,  it  has  averaged  about  as  frequent  in  one 
month  as  in  another,  from  February  to  June  over  the  term  of  years.  Its  status 
there  varies  widely,  however,  in  midsummer,  from  year  to  year,  the  one  extreme 
being  illustrated  by  1929,  1930,  and  1932,  when  it  was  about  as  frequent  in  July 
as  in  June,  the  other  by  1913  and  1916,  when  it  was  not  found  at  all  in  July- 
August  inside  the  200-meter  contour.  Neither  is  any  correlation  apparent  be- 
tween its  presence  or  absence  at  this  season,  and  the  prevaihng  temperature,  for 
one  of  the  summers  when  it  failed  (1916),  was  notably  cold  in  this  part  of  the 
sea,  the  other  (1913),  warm.  And  the  danger  of  confusing  annual  variations  with 
seasonal  makes  us  cautious  in  drawing  from  the  tabulation  of  monthly  averages 
the  obvious  inference  that  Meganyctiphanes  tends  to  spread  southward  along 
the  offshore  belt,  and  inshore,  in  May  and  June,  to  vanish  thence  with  the  ad- 
vance of  summer. 

Percent  of  stations  at  which  Meganyctiphanes 
was  taken,  all  years  combined 


Month 

Offshore 
North 

Offshore 
South 

Inshore 
North 

Inshore 
South 

February 

April 

May 

June 

July 

October 

33% 

25 

27 

40 

55 

25 

0% 

0 
44 
60 

0 
20 

0% 
0 
7 
13 

7 

0% 
0 
0 
22 
0 

The  records  of  capture  of  Meganyctiphanes  in  October  of  1931  (Table,  p. 
293)  suggest  that  in  a  year  when  it  is  widespread  (at  least  in  the  northern  sector) 
in  summer,  a  scattering  population  may  be  expected  to  persist  all  along  the  off- 
shore belt  through  the  autumn. ^  But  it  appears  that  in  years  when  it  is  absent 
in  late  summer,  reestablishment  is  not  to  be  expected  during  the  subsequent 
autumn,  and  perhaps  not  until  the  following  spring,  for  in  1916 — a  year  of  that 
type — it  was  not  found  at  all  in  November  whether  inshore  or  offshore.  It  is 
unfortunate,  in  this  connection,  that  no  information  is  available  as  to  the  status 


'  No  inshore  data  for  that  cruise. 


BIGELOW   AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


293 


of  this  euphausiid  during  the  autumn  of  1929,  a  year  when  it  was  widespread  off- 
shore in  July. 

The  following  tabulation  suggests  that  it  is  characteristic  (if  not  invariable) 
for  Meganyctiphanes  to  increase  somewhat  in  average  abundance,  and  still  more 
in  maximum  abundance,  from  a  minimum  in  February  and  April,  to  a  maximum 
in  late  spring  or  early  summer,  though  perhaps  never  to  an  extent  sufficient  to 
make  it  of  more  than  a  very  minor  item  in  the  general  community  of  our  area. 
In  some  years  this  vernal  augmentation  may  culminate,  as  a  more  or  less 
definite  peak  in  June  (1929,  1931),  with  a  subsequent  impoverishment  by  July. 
And  it  is  possible  that  1913  and  1916  represent  the  extreme  in  this  direction, 
i.e.,  entire  disappearance  in  summer,  but  our  lack  of  information,  earlier  in  the 
season,  for  either  of  these  years  leaves  this  an  open  question.  In  other  years,  e.g., 
1930,  Meganyctiphanes,  while  averaging  somewhat  less  abundant  in  June  than 
in  May,  may  average  about  as  abundant  in  July  as  in  June,  at  least  in  the  north- 
ern sector,  or  again,  there  may  be  no  significant  alteration  in  this  respect  between 
May  and  June  (e.g.,  1932). 

In  any  case,  the  small  volumes  recorded  for  October  1931,  combined  with 
the  fact  that  Meganyctiphanes  was  not  taken  at  all  in  November  1916,  suggest 
that  this  euphausiid  may  be  expected  to  decrease  in  abundance  through  the 
autumn,  or  even  to  disappear  entirely  from  our  waters  by  that  time,  if  it  has  not 
already  done  so  earher  in  the  season. 

Average  and  maximum  volumes  of  Meganyctiphanes, 
all  cruises  combined 


Offshore 

Offshore 

Inshore 

Inshore 

North 

South 

North 

South 

Aver- 

Maxi- 

Aver- 

Maxi- 

Aver- 

Maxi- 

Aver- 

Maxi- 

age 

mum 

age 

mum 

age 

mum 

age 

mum 

February 

<1 

<1 

0 

0 

0 

0 

0 

0 

April 

<1 

<1 

0 

0 

0 

0 

0 

0 

May 

<1 

18 

1 

39 

<1 

<1 

<1 

<1 

June 

5 

89 

1 

16 

<1 

18 

1 

43 

July 

1 

10 

0 

0 

<1 

5 

0 

0 

October 

1 

1 

1 

1 

— 



^"^ 

Nematoscelis  megalops 
Records  of  the  occurrence  of  Nematoscelis  megalops  on  the  continental  shelf 
are  interesting  chiefly  because  this  euphausiid  may  safely  be  regarded  as  an  in- 
dicator of  water  from  the  continental  slope.   The  fact  that  48  of  the  59  stations 


294  memoir:  museum  of  comparative  zoology 

where  it  has  been  taken  within  our  Hmits  on  one  cruise  or  another  have  been 
more  than  30  miles  out  from  the  land,  and  most  of  them  near  the  continental  edge 
(Fig.  20A),  accords  with  its  oceanic  origin.  And  it  occurred  about  as  frequently- 
offshore  in  the  one  sector  as  in  the  other  (18%  of  the  stations  north,  25%  south, 
all  cruises  combined). 

Nematoscelis  was  taken  at  20%  of  the  stations  on  the  outer  half  of  the  con- 
tinental shelf  in  February,  15%  in  April,  15%  in  May,  23%  in  June,  6%  in  July, 
and  25%  in  October,  suggesting  some  tendency  toward  temporary  withdrawal 
in  midsummer,  corresponding  to  which  it  was  not  found  at  all  inside  the  200-meter 
contour  in  July-August  of  1913  or  of  1916,  though  it  was  recorded  over  the  con- 
tinental slope  in  both  these  summers.  And  seemingly  it  is  more  likely  to  stray 
inshore  in  June,  than  either  earlier  or  later  in  the  season,  for  the  distribution  of 
inshore  records  (1929-1932)  was  February,  0;  April,  0;  May,  1;  June,  4;  July,  1. 

In  the  great  majority  of  cases  the  catches  of  Nematoscelis  in  on  the  continen- 
tal shelf  have  been  smaller  than  1  c.c,  the  only  notable  exceptions  being  the 
following : 

February,  1930,  average,  offshore,  5-6  c.c,  maximum,  34  c.c, 

April,  1929,  average,  offshore,  1-2  c.c,  maximum,  23  c.c. 

May,  1932,  1  station,  60  c.c. 

June,  1929,  average  offshore,  1  c.c,  maximum,  13  c.c, 

June,  1932,  average  offshore,  2  c.c,  maximum,  28  c.c. 

Catches  larger  than  10  c.c.  were  made  at  two  offshore  stations  in  February, 
none  in  April,  two  in  May,  five  in  June,  and  none  in  July  and  October,  a  distribu- 
tion suggesting  that  when  scattered  centers  of  moderate  abundance  do  develop 
for  this  species,  this  is  more  apt  to  happen  in  summer  or  early  autumn  than  in 
winter,  spring,  or  late  autumn.  And  the  largest  volumes  of  all  (34  c.c,  February 
11,  1930;  60  c.c.  May  9,  1932)  were  encountered  in  the  southernmost  sector  off 
Bodie  Island,  and  off  Chesapeake  Bay. 

Nematoscelis — hke  most  other  planktonic  species — varies  considerably  in 
its  status  in  our  waters  from  year  to  year,  catches  of  10  c.c.  or  larger  having  been 
made  at  two  offshore  stations  in  1929,  one  in  1930,  none  in  1931,  and  six  in  1932. 

Thysanoessa  inermis 

Frequency.  Discussion  of  the  status  of  Th.  inermis  is  hampered  by  the  fact 
that  in  many  cases,  the  catches  of  euphausiids  were  so  damaged  that  it  was  not 
possible  to  carry  identifications  of  the  members  of  Thysanoessa  farther  than  to 
their  genus.  The  following  estimate  of  the  local  abundance  of  Th.  inermis  must 


BIGELOW   AND    SEARS :   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  295 

therefore  be  regarded  as  minimal — actually  its  importance  was  no  doubt  greater. 
Even  allowing  for  this,  it  is,' however,  clear  that  adult  Th.  inermis  occurs  much  less 
regularly  in  our  area  (detected  at  only  13%  of  the  stations)  than  in  the  Gulf  of 
Maine,  where  it  has  been  recorded  at  about  50%  of  the  stations  where  euphausiids 
were  identified,  summer  and  winter  alike  (Bigelow,  1926,  p.  136).  Nevertheless, 
the  locaUties  of  record  are  so  widespread  (Fig.  20D)  that  Th.  inermis  is  evidently 
to  be  expected  anywhere  within  our  area,  at  one  time  or  another,  south  even  past 
the  offing  of  Chesapeake  Bay. 

Averages  for  different  months  indicate  considerably  greater  frequency  for 
July  (33%  of  the  stations)  and  October  (25%)  than  for  February  (9%),  April 
(4%),  May  (8%),  or  June  (10%).  But  we  must  remind  the  reader  that  this 
applies  only  to  the  adults; — inclusion  of  the  juveniles  (which  have  not  yet  been 
identified)  might  result  in  quite  a  different  seasonal  picture.  Th.  inermis  has  also 
averaged  more  frequent  in  the  north  (15%  of  the  stations)  than  in  the  south 
(5%),  as  was  to  be  expected.  In  fact,  it  was  recorded  only  twice  south  of  Chesa- 
peake Bay.  And  it  also  proved  more  frequent  offshore  (13%  of  the  stations)  than 
within  30  miles  of  the  land  (9%  of  the  stations). 

Abundance.  The  catches  of  adult  Th.  inermis  have  invariably  been  in- 
significant compared  with  those  of  the  volumetric  ally  more  important  species, 
the  maximum  being  only  20  c.c.  (Station,  Shinnecock  II,  June  12,  1931),  with 
1.5  c.c.  as  the  largest  average  for  any  one  cruise  (July  1930),  while  the  species  was 
not  detected  at  all  in  the  catches  on  two  of  the  cruises  (February  1932  and  June 
1929).  This  prevaiUng  scarcity  within  our  limits  of  adults  of  this  species  is  an 
interesting  contrast  to  the  important  role  in  the  planktonic  community  that  it 
often  plays  in  higher  latitudes,  in  both  sides  of  the  Atlantic. 

Th.  inermis  was  about  20  times  as  frequent  in  the  richest  year  (1931,  40% 
of  the  stations),  as  in  the  poorest  (1929,  2%).  But  the  volumes  have  invariably 
been  too  small  to  warrant  any  statement  as  to  annual  variations  in  abundance. 

Thysanoessa  gregaria 

Occasional  adult  specimens  of  Th.  gregaria  were  taken  in  February  1932  at 
stations  scattered  from  the  northern  boundary  of  the  area  to  the  southern  (Fig. 
21A).  Other  than  this  the  recent  record  of  the  species  in  our  area  is  confined  to 
one  station  off  New  York  in  May,  1929,  and  a  second  off  Block  Island  in  February, 
1931),  both  near  the  200-meter  line.  These  localities  show  a  distinctly  southern 
distribution,  as  was  to  be  expected.  And  the  numbers  have,  in  every  case  been  so 
small  (invariably  less  than  1  c.c.)  that  it  would  evidently  be  exceptional  for  Th, 


296 


memoir:  museum  of  comparative  zoology 


gregaria  to  form  any  appreciable  proportion  of  the  plankton,  at  least  during  the 
vernal  half  year. 


Fig.  21.  Locality  records,   all  cniises  combined:  A,    Thysanocssa  gregaria  and    Th.  longicaudala;  B, 
Euphausia,  Stylocheiron  and  Thysanopoda;  C,  Erythrops;  D,  Neomysis  americana. 

Thysanoessa  longicaudata 
The  few  records  for  this  cold  water  species — 3  for  February  and  3  for  June 
of  1930,  and  one  for  February,  1932 — have  all  been  to  the  northward  of  Latitude 
38°  (Fig.  21  A).  The  largest  catch  (Station,  Cape  May  V,  June  11,  1930)  was  at 
the  rate  of  7  c.c,  all  other  records  being  based  on  odd  individuals  only. 


BIGELOW   AND    SEARS :   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


297 


EUPHAUSIID    LARVAE 

Specific  identification  of  the  euphausiid  larvae  has  not  been  attempted,  but 
these  have  formed  a  sufficient  proportion  of  the  catches  in  some  months  (Table, 
p.  229)  to  justify  some  notes  as  to  their  frequency  of  occurrence  and  local  abun- 
dance. 

None  were  taken  in  February,  though  cruises  were  made  in  that  month  in 
three  different  years,  hence  we  may  conclude  that  there  is  no  reproduction  in 
significant  amount  by  any  of  the  local  species  of  this  group,  in  late  winter.  But 
the  frequent  occurrence,  in  April,  May,  June,  and  July,  of  juveniles  large  enough 
to  be  caught  in  our  nets  shows  that  one  euphausiid  or  another  may  be  breeding 
within  our  Uniits  from  early  spring  until  midsummer.  The  monthly  succession 
does  not  suggest  any  definitely  seasonal  gradient  in  the  frequency  of  occurrence 
for  larval  euphausiids  between  April  and  July,  other  than  the  irregularities 
(regional  and  secular)  that  are  to  be  expected  in  the  distribution  of  a  mixed 
plankton  population,  but  they  were  found  at  one  station  only  in  October. 

On  different  occasions,  these  larvae  have  been  most  generally  distributed  in- 
shore, offshore,  in  the  north,  and  in  the  south.  But  they  have  averaged  consider- 
ably more  frequent  offshore  than  inshore  in  each  month,  April-July,  for  the 
several  years  combined,  also  more  frequent  in  the  north  than  in  the  south,  in 
April,  May,  June,  and  October,  for  all  years  combined,  suggesting  that  pro- 
duction in  the  spring  and  autumn  is  chiefly  by  boreal  species,  probably  Th. 
inermis  and  Meganyctiphanes  in  combination.  In  July,  however,  of  the  one  year 
when  the  midsummer  survey  covered  the  entire  area,  they  were  most  frequent 
in  the  south. 

Percentage  of  stations  at  which  euphausiid  larvae  were  taken 


Year 

April 

May 

June 

July 

1929 

31% 

40% 

30% 

56% 

1930 

53 

7 

15 

12 

1931 

— 

40 

27 

5 

1932 

— 

54 

62 

— 

Percentages  of  stations  with  euphausiid  larvae,  in  the  different 
subdivisions,  all  years  combined 


Month 

Inshore 

Offshore 

North 

South 

April 
May 
June 
July 

24% 
30 
33 
30 

64% 
62  . 
43 
30 

54% 
54 
42 
30 

35% 
25 
22 
66 

298  memoir:  museum  of  comparative  zoology 

In  the  years  1930  and  1931,  the  catches  of  euphausiid  larvae  averaged  less 
than  1  c.c.  for  the  area  as  a  whole  on  11  out  of  the  12  cruises,  and  only  5  c.c.  (with 
a  maximum  of  60  c.c.)  on  the  twelfth  (April  22-May  1,  1930),  stocks  so  small  that 
they  did  not  contribute  significantly  to  the  general  volume  of  plankton  for  that 
pair  of  years.  They  were,  however,  considerably  more  abundant  both  absolutely 
and  relatively  in  the  other  two  years  of  the  series,  with  average  and  maximum 
catches  of  3  c.c.  and  47  c.c.  in  April,  8  c.c.  and  169  c.c.  in  May,  2  c.c.  and  27  c.c. 
in  June,  and  6  c.c.  and  118  c.c.  in  July,  in  1929;  also  8-10  c.c.  and  121  c.c.  in 
May  and  June,  1930.  These  values  combined  with  the  absence  of  euphausiid 
larvae  in  February  and  their  great  scarcity  in  the  one  October  of  record  is  e\idence 
that  in  their  years  of  abundance,  production  takes  place  chiefly  from  May  into  July. 

The  volumes  of  euphausiid  larvae  have  also  averaged  14  times  as  great  off- 
shore (average,  14  c.c.)  as  inshore  (average,  1  c.c),  and  somewhat  greater  in  the 
north  (8  c.c.)  than  in  the  south  (about  5  c.c),  in  the  8  surveys'  (combined)  for 
which  the  average  was  greater  than  1  cc,  in  either  subdivision:  evidence  that 
the  production  of  euphausiids  in  our  area  is  centered  chiefly  along  the  outer  belt 
of  the  shelf.  And  a  similar  segregation  appears  in  the  distribution  of  catches 
richer  than  25  cc.  (Fig.  19C). 

Other  euphausiids 

Odd  specimens  of  Euphausia,  Stylocheiron,  and  Thysanopoda  (strays  from 
offshore)  were  taken  along  the  outer  half  of  the  shelf  at  the  localities  shown  on 
Fig.  21B,  but  never  in  as  large  an  amount  as  1  c.c. 

MYSIDS 
A  considerable  proportion  of  the  hauls  yielded  a  scattering  of  adults  of  two 
species  of  mysids,  Neoinysis  americana  and  Erythrops  erythrophlhalma.  The  cap- 
tures of  the  former  (Fig.  21D) — occasional  specimens  only — show  that  it  is 
rather  closely  confined  to  the  inshore  belt,  as  was  to  be  expected,  since  it  is  com- 
mon in  seaweed  and  swimming  free  in  the  water  alongshore.  And  the  fact  that 
it  was  recorded  both  in  the  extreme  south  and  in  the  extreme  north  of  our  area 
accords  with  its  known  distribution,  coastwise,  for  it  is  plentiful  near  Woods  Hole, 
on  the  one  hand  (Sumner,  Osborne,  and  Cole,  1913),  and  in  Chesapeake  Bay,  on 
the  other  (Cowles,  1930).  The  fact  that  the  monthly  percentage  of  stations  in  the 
inshore  belt,  where  Neomysis  was  taken,  varied  only  between  5%  and  6%  from 
February  to  July,  shows  it  as  continuing  about  equally  frequent  from  late  winter 

>  AprU,  May,  June,  July,  1929;  April,  June,  1930;  May,  June,  1932. 


BIGELOW   AND    SEARS:    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES  299 

through  summer.  In  Chesapeake  Bay,  however,  Cowles  found  it  most  plentiful 
in  December  and  January.  And  this  may  be  equally  the  case  along  the  open 
coast,  where  we  have  no  information  for  early  or  mid- winter. 

In  the  case  of  Erythrops,  the  stations  of  record  are  generally  distributed 
over  the  inner  and  mid-belts  of  the  shelf;  all  but  two,  however,  of  the  41,  are  in 
the  northern  sector,  with  evident  concentration  toward  the  northeast  (Fig.  21C). 
In  fact,  the  discovery  of  this  northern  species,  at  all,  to  the  west  of  Cape  Cod, 
considerably  extends  its  known  range  in  that  direction,  for  it  had  not  previously 
been  recorded  south  of  Massachusetts  Bay,  on  the  North  American  coast,  though 
long  known  to  be  widespread  along  the  coasts  of  Europe,  southward  to  the  Irish 
Sea  (Zimmer,  1909).  And  the  fact  that  it  was  much  more  frequent  in  the  north- 
ern sector  in  February  and  April  (32%  and  20%  of  the  stations,  respectively) 
than  in  May  (8%),  June  (6%),  July  (8%),  or  in  October  when  it  was  not  taken 
at  all,  is  evidence  that  its  presence  within  our  limits  depends  chiefly,  if  not  wholly 
on  immigration.  The  largest  catch  of  Erythrops  was  4  c.c,  near  Shinnecock  on 
June  8,  1930. 

AMPHIPODS 

EUTHEMISTO   COMPRESSA 

Until  comparatively  recently,  it  was  generally  accepted  that  the  two  named 
representatives  of  Euthemisto,  that  have  long  been  known  to  abound  off  the 
northeastern  coasts  of  the  United  States,  compressa  Goes,  and  bispinosa  Boeck, 
were  well  defined  species.  Stephensen  (1924,  p.  103)  has,  however,  shown  from 
examination  of  extensive  series  from  different  parts  of  the  Atlantic  and  Mediter- 
ranean that  while  typical  specimens  of  the  two  are  easily  separable,  intermediates 
occur  so  often  that  he  has  definitely  classed  them  as"formae"  of  the  one  species 
compressa. 

Frequency.  The  genus  Euthemisto  (treated  here  as  a  unit)  rivals  Calanus 
finmarchicus  in  frequency  of  occurrence,  for  it  was  taken  at  every  station  on 
eight  of  the  cruises,  and  at  38%  of  the  stations  on  all  cruises  combined,  though  it 
may  be  only  irregularly  represented  on  occasion,  as  in  May  and  June  1929  (38% 
and  27%  of  the  stations  respectively).  No  preponderance  is  indicated  either 
north  or  south,  but  slightly  greater  frequency  is  probably  characteristic  for  it  off- 
shore (92%)  than  inshore  (77%),  which  accords  with  what  is  known  of  its  occur- 
rence in  the  Gulf  of  Maine  (Bigelow,  1926)  and  with  previous  records  for  the 
genus  in  the  region  now  under  consideration  (Bigelow,  1915;  1922). 

Relative  monthly  percentages  perhaps  warrant  the  generalization  that 
Euthemisto  tends  in  most  years  to  be  sUghtly  less  frequent  in  early  or  mid-sum- 


300 


memoir:  museum  of  comparative  zoology 


mer,  than  in  late  winter  or  spring,  it  being  especially  suggestive  that  an  alteration 
of  this  sort  took  place  in  1930,  for  Euthemisto  was  universal  in  that  year  up  to 
June.  And  sharper  fluctuations  may  occur  in  other  years,  as  illustrated  by  1929, 
when  the  frequency  decreased  progressively  by  more  than  Y2  from  April  through 
June,  but  then  rose  again  by  July  to  a  level  higher  than  had  existed  in  April. 

Abundance.  With  Euthemisto  so  nearly  universal,  throughout  the  area  in 
most  years,  it  is  interesting  to  find  the  catches  averaging  only  about  30  c.c,  at 
most,  in  any  month  (May  1930),  while  the  average  for  the  whole  area  (all  cruises 
combined)  was  only  about  11  c.c.  for  the  years  1930-1932,  and  2.5  c.c.  for  1929. 
Among  509  recorded  catches  of  Euthemisto,  only  26  were  larger  than  40  c.c,  with 
a  maximum  of  320  c.c.  at  a  station  on  the  inner  part  of  the  shelf  off  Atlantic  City, 
June  27,  1930. 

The  seasonal  gradient  in  abundance  has  varied  too  widely  from  year  to  year 
for  generahzation  as  to  the  normal  cycle,  except  that  the  catches  averaged  very 
small  in  February  and  in  April,  in  each  year.  In  two  of  the  years  (1931,  1932),  the 
catches  of  Euthemisto  continued  to  average  small,  right  through  the  season  in 
spite  of  an  occasional  rich  catch,  e.g.,  63  c.c.  at  Station  21415,  May  25,  1932. 
Considerable  augmentation  was,  however,  recorded  in  the  other  two  years, 
culminating  in  June  or  July,  as  appears  from  the  following  tabulation : 

Average  volumes  of  Euthemisto 


Area 

Maxi- 

Month 

Year 

Inshore 

Offshore 

North 

South 

surveyed 

mum 

February 

1930 

2 

1 

1 

1 

2 

9 

1931 

3 

3 

1 

5 

3 

17 

1932 

6 

8 

7 

6 

7 

40 

April 

1929 

<1 

<1 

<1 

<1 

<1 

2 

1930 

7 

5 

6 

8 

7 

50 

May 

1929 

<1 

<1 

<1 

<1 

<1 

5 

1930 

23 

33 

24 

23 

27 

106 

1931 

<1 

<1 

<1 

<1 

<1 

<1 

1932 

<1 

<1 

<1 

<1 

<1 

63 

June 

1929 

6 

1 

1 

1 

3 

84 

1930 

36 

9 

22 

41 

25 

320 

1930 

2 

<1 

<1 

3 

1 

48 

1932 

<1 

6 

<1 

7 

3 

47 

July 

1929 

4 

2 

2 

5 

3 

19 

1930 

37 

3 

27 

— 

27 

94 

1931 

1 

4 

1 

— 

1 

27 

October 

1931 

— 

1 

<1 

3 

1 

15 

Regional  volumes  also  show  that  Euthemisto  may  average  most  abundant 
either  inshore  or  offshore,  or  be  about  as  abundant  in  the  one  belt  as  in  the  other, 


BIGELOW   AND    SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  301 

at  any  given  time.   But  the  inshore  belt  has  most  frequently  been  the  more  pro- 
ductive of  the  two  on  occasions  when  any  significant  inshore-offshore  gradient  has 


Fig.  22.  Locality  records,  all  cruises  combined:  A,  large  volumes  of  Eulhemislo  compressa;  B,  Phronima; 
C,  Acartia  longiremis,  inshore  and  offshore;  D,  Anomalocera  pattersoni  and  Labidocera. 


existed,  resulting  in  a  general  average  about  twice  as  great  there  as  offshore 
(Table,  p.  300).  And  the  few  catches  larger  than  100  c.c,  as  well  as  the  majority 
of  those  larger  than  40  c.c.  have  all  been  within  45  miles  of  land  (Fig.  22A),  which 


302  memoir:  museum  of  comparative  zoology 

contrasts  sharply  with  conditions  eastward  from  Cape  Cod  (Bigelow,  1926,  p.  157, 
Fig.  55). 

Euthemisto  has  averaged  about  equally  abundant  south  as  north,  February- 
May,  but  considerably  more  so  south  than  north,  in  June,  on  the  three  occasions 
when  any  significant  gradient  existed  in  this  respect  (Table,  p.  300).  But  we  have 
some  e\'idence  that  the  center  of  population  tends  to  shift  northward,  as  summer 
advances,  in  the  fact  that  relatively  large  catches  were  recorded  only  in  the  most 
easterly  sector  of  the  area  in  July-August  of  1913,  and  northward  from  the  Cape 
May  profile  in  the  latter  month  of  1916  (Bigelow,  1915;  1922).  A  shift  in  this 
direction  would,  indeed,  be  the  natural  expectation  for  a  boreal  species,  with  the 
seasonal  warming  of  the  waters. 

Conditions  in  October  1931  when  Euthemisto  averaged  3  c.c.  in  the  southern 
sector,  but  less  than  1  c.c.  in  the  northern — if  characteristic — suggest  that  the 
center  of  population  again  shifts  southward,  early  in  the  autumn.  And  this  is 
corroborated  by  the  fact  that  rich  catches  were  made  near  Delaware  Bay  and  off 
Chesapeake  Bay  in  November  1916  (Bigelow,  1922). 

Annual  variations.  Little  annual  difference  in  the  average  volumes  of 
Euthenusto  has  been  recorded  for  February,  April,  or  May.  But  in  June  and 
July,  the  catches  averaged  about  27  times  as  great  in  the  richest  year  (1930, 
average,  27  c.c.)  as  in  the  poorest  (1931,  average,  1  c.c),  evidence  that  while  the 
status  of  this  species  at  the  end  of  the  winter  and  late  spring  is  comparatively 
constant  from  year  to  year,  very  wide  differences  in  this  respect  may  develop  in 
summer,  even  within  a  short  series  of  years. 

Other  amphipods 

Other  than  Euthemisto,  and  specimens  of  one  gammarid  or  another  picked 
up  when  the  tows  were  made  close  to  the  bottom,  the  only  amphipods  detected 
in  the  catches  were  Phronima  and  Hyperia.  The  former  has  been  recorded 
rather  frequently  offshore  in  the  north,  rather  less  so  offshore  in  the  south,  and 
once  close  inshore  off  New  York  (Fig.  22B).  The  fact  that  Phronima  was  re- 
corded at  2-5%  of  the  stations  from  February  to  June,  but  at  15%  in  July  and 
13%  in  October  suggests  that  this  visitor  from  offshore  is  to  be  expected  in  on 
the  shelf  more  often  in  midsummer  and  autumn  than  earlier  in  the  year.  The 
maximum  catch  (13  c.c.)  was  also  made  in  July,  all  other  records  being  based  on 
occasional  specimens  only. 

Hyperia  was  recorded  at  two  stations — one  in  April  1930  off  Currituck  at  the 
continental  edge,  the  other  at  a  similar  locality  off  Atlantic  City  in  June  1931. 


BIGELOW   AND    SEARS :    NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  303 

COPEPODS 

ACARTIA 

In  the  Gulf  of  Maine,  where  A.  longiremis  is  endemic  and  widespread,  though 
never  very  abundant,  it  appears  to  be  confined  to  the  shoaler  waters  (including 
the  offshore  banks  as  well  as  the  coastal  zone)  during  the  cold  half  of  the  year,  to 
disperse  out  over  the  deep  basin  in  spring,  summer,  or  autumn,  when  its  numbers 
increase  (Bigelow,  1926,  p.  180).  And  its  distributional  status  is  essentially  the 
same  to  the  west  and  south  of  Cape  Cod,  for  while  it  is  abundant  in  Chesapeake 
Bay  throughout  the  year — most  so,  in  fact,  in  March — (Wilson,  1932a,  p.  20), 
we  have  only  two  records  of  it  out  on  the  shelf  in  February  and  one  in  April, 
whereas  in  May,  June,  and  July,  it  was  found  about  as  frequently  offshore  (7% 
of  the  stations)  as  inshore  (9%),  for  the  several  years  combined  (Fig.  22C),  and 
at  20%  of  the  offshore  stations  during  the  only  October  of  record. 

The  records  of  occurrence  do  not  suggest  any  regularly  latitudinal  gradient 
during  the  season  of  its  offshore  dispersal,  between  the  offings  of  Martha's  Vine- 
yard and  of  Delaware  Bay.  But  it  was  taken  at  one  station,  only,  south  of  the 
Winterquarter  profile,  a  failure  in  the  extreme  south  that  is  difficult  to  explain 
in  the  face  of  its  constant  and  abundant  occurrence  in  Chesapeake  Bay  at  the 
same  times  of  year. 

Within  Chesapeake  Bay  and  probably  in  other  similar  situations  along  our 
mid- Atlantic  coast,  A.  longiremis  ranks  second  only  in  abundance  to  A.  clausii 
among  the  copepod  fauna  (Wilson,  1932a).  Out  over  the  continental  shelf,  how- 
ever, our  maximum  catch  was  at  the  rate  of  but  16  c.c,  while  11  only  out  of  the 
60  odd  catches  were  as  voluminous  as  5  c.c.  However,  the  fact  that  about  1/3  of 
the  catches  were  1  c.c.  or  larger,  shows  that  a  sufficient  population  of  it  exists 
(and  widespread)  in  the  shelf  waters  for  local  concentrations  to  be  expected 
(though  not  actually  encountered  as  yet)  should  a  happy  combination  of  circum- 
stances favor  its  reproduction. 

A.  clausii  is  described  by  Wilson  (1932a)  as  the  chief  copepod  constituent  of 
the  plankton  in  Chesapeake  Bay.  We  have  but  one  record  of  it,  however,  out  on 
the  shelf,  evidence  that  it  is  far  more  strictly  neritic  within  our  limits  than  in  the 
Gulf  of  Maine,  where  it  is  widespread,  inshore  and  offshore  aUke,  and  may  con- 
stitute even  up  to  30-50%  of  the  copepods  by  number  locally,  in  the  coastwise 
belt,  at  the  season  when  it  is  most  plentiful  (Bigelow,  1926,  Fig.  59). 

A.  tonsa — probably  an  indicator  of  coast  water — swarmed  at  three  stations 
near  the  mouth  of  Delaware  Bay,  in  August  1916;  otherwise,  we  have  no  record 


304  memoir:  museum  of  comparative  zoology 

of  it  within  our  limits,  nor  did  Wilson  (1932a)  report  it  from  Chesapeake  Bay, 
though  it  is  a  dominant  species  inshore,  and  in  brackish  situations  near  Woods 
Hole  in  summer  (Fish,  1925;  Sharpe,  1910;  Sumner,  Osborne,  and  Cole,  1913). 

Anomalocera  pattersoni 

The  "blue  copepod,"  widespread,  though  never  very  abundant,  in  the  Gulf 
of  Maine  (Bigelovv,  1926,  Fig.  63)  had  already  been  reported  in  summer  at  various 
localities  along  the  continental  shelf  to  the  offing  of  Chesapeake  Bay,  inshore  and 
offshore  aUke  (Bigelow,  1915,  Fig.  69).  And  the  records  for  the  period  1929- 
1932  corroborate  this  distributional  picture,  being  generally  distributed  across 
the  shelf,  from  the  northern  boundary  of  our  area  to  the  southern  (Fig.  22D). 
It  seems,  however,  that  Anomalocera  is  considerably  less  frequent — though  so 
widespread — to  the  west  and  south  of  Cape  Cod  than  it  is  over  George's  Bank 
and  in  the  Gulf  of  Maine,  for  it  was  detected  at  about  4%  only  of  the  stations  for 
the  series  as  a  whole.  And  being  so  conspicuous  an  object,  even  after  its  beautiful 
blue  color  has  faded  in  the  preservative,  it  is  not  likely  that  any  of  its  adults  were 
overlooked. 

It  is  questionable,  with  so  few  data,  whether  the  monthly  distribution  of 
catches  (2%  of  the  stations  for  February,  7%  for  April,  4-5%  for  May  and  June, 
and  12%  for  July)  indicates  any  regularly  seasonal  cycle  between  late  winter  and 
midsummer.  But  failure  to  take  it  at  all  during  the  one  October  cruise  may  per- 
haps reflect  autumnal  impoverishment. 

We  have  no  reason  to  suppose  that  Anomalocera  is  ever  of  volumetric  im- 
portance in  the  waters  under  study,  except  on  rare  occasions,  for  while  one  catch 
was  at  the  rate  of  32  c.c.  (near  Cape  May,  June  1930),  most  of  the  other  records 
for  it  were  based  on  occasional  specimens  only. 

Calanus  finmarchicus 

Frequency.  In  February,  when  (by  present  indications)  Calanus  is  close  to 
its  lowest  ebb  for  the  year,  it  occurred  generally  throughout  the  area  southward 
to  Latitude  36°,  both  in  1930  and  in  1931;  southward,  in  fact,  beyond  Cape  Hat- 
teras  in  the  latter  year.  But  it  was  recorded  at  38%  only,  of  the  stations  inshore, 
in  the  southern  sector,  in  the  abnormally  warm  February  of  1932,  though  it  was 
universal  then  in  the  north,  and  almost  equally  so  along  the  offshore  belt  in  the 
south  (90%  of  the  stations). 

If  the  data  for  April,  of  1929  and  of  1930,  be  representative  of  the  normal 
mid-spring  state,  it  appears  (as  might  be  expected)  that  Calanus  tends  by  that 


BIGELOW   AND   SEARS:    NORTH   ATLANTIC    ZOOPLANKTON    STUDIES 


305 


month  to  populate  any  restricted  areas  where  it  may  have  been  absent  at  the 
end  of  the  winter.  And  it  has  either  proved  universal  (100%  of  the  stations), 
throughout  the  area  as  a  whole  from  April  or  May  through  June,  as  was  the  case 
in  1930  and  1931,  or  has  at  most  been  lacking  at  scattered  stations,  here  and 
there,  as  in  1929  and  1932: 

Percentage  of  stations  with  Calanus,  for  the  area  as  a  whole 


Month 

1929 

1930 

1931 

1932 

February 

— 

77% 

100% 

56% 

April 

100% 

95 

— 

— 

May 

100 

100 

100 

95 

June 

91 

100 

100 

97 

July 

91 

100 

100 

— • 

Calanus  has  proved  equally  universal  every  year  in  July,  in  the  sector  east- 
ward from  the  offing  of  New  York.  But  wide  differences  in  this  respect  are 
evidently  to  be  expected  from  year  to  year  in  the  southern  sector,  at  that  season, 
for  while  it  was  universal  there  also,  in  the  summers  of  1916"^  and  of  1929,  record 
of  it  south  of  New  York  in  July  1913  was  confined  to  a  few  scattered  stations. 
Clarke  and  Zinn  (1937)  found  it  regularly  through  August  and  September  near 
Martha's  Vineyard.  And  it  continues  universal  over  our  area  as  a  whole  through 
the  autumn  (at  least  in  the  years  when  it  is  widespread  and  abundant  in  sum- 
mer) for  it  was  recorded  at  12  out  of  13  stations  along  the  offshore  belt,  north  and 
south,  in  October  1931,  and  at  every  station  in  November  1916.  Conditions  in 
February  (p.  304)  suggest  that  at  least  a  sparse  population  of  Calanus  persists 
throughout  the  area  as  a  whole,  over  the  winter  in  most  years,  as  is  certainly  the 
case  near  Martha's  Vineyard  in  the  northeast  (Clarke  and  Zinn,  1937).  But  our 
records  do  not  show  how  early,  in  autumn,  Calanus  may  repopulate  the  southern 
sector,  after  a  summer  when  it  is  scarce  or  absent  there,  nor  at  what  season  it  be- 
comes scarce  there,  in  a  year  (e.g.,  1932)  when  winter  cooling  does  not  proceed 
to  the  usual  extreme. 

Abundance.  Clarke  and  Zinn  (1937)  report  Calanus  as  at  its  minimum,  near 
Martha's  Vineyard,  during  the  autumn  and  winter,  and  this  applies  over  our 
area  as  a  whole,  with  an  average  volume  of  only  about  14  c.c.  for  February  1930, 
1931,  and  1932  combined  (Table,  p.  309). 

The  most  interesting  event  in  the  seasonal  cycle  of  this  copepod  is  that  it 

'  Reexamination  of  samples,  from  the  July-August  cruise  of  1916  has  revealed  the  presence  of  at 
least  a  few  Calanus  at  every  station. 


306  memoir:  museum  of  comparative  zoology 

greatly  increases  in  abundance  at  some  time  during  the  spring.  Unfortunately, 
data  for  March  are  confined  to  the  immediate  vicinity  of  Martha's  Vineyard 
(Clarke  and  Zinn,  1937),  where  Calanus  was  still  as  scarce  in  that  month  as  in 
February.  But  the  catches  (for  the  area  as  a  whole)  averaged  18  times  as  great 
in  April  as  in  the  preceding  February  in  the  one  year  (1930),  when  surveys  were 
made  in  both  these  months,  and  29  times,  9  times,  and  10  times  as  great  in  May 
as  in  February  of  1930,  of  1931,  and  of  1932,  respectively  (Table,  p.  309).  Average 
volumes  for  successive  months  suggest  that  following  this  vernal  augmentation, 
Calanus  in  the  north  may  either  decrease  after  April,  as  in  1929,  or  may  con- 
tinue at  a  roughly  constant  level  of  abundance  through  May,  June,  and  July, 
as  in  1930  and  1931'  After  an  unusually  tardy  spring — as  in  1916 — Calanus 
may,  in  fact,  continue  in  great  abundance  through  August  (Bigelow,  1922), 
even  in  the  southern  sector.  But  in  other  years,  illustrated  by  1929,  Calanus 
markedly  decreases  in  the  south,  through  late  spring  and  early  summer;  or  in 
a  warm  summer — by  the  evidence  of  1913 — it  may  practically  disappear  thence 
by  July.  We  should,  however,  caution  the  reader  that  description  of  the  seasonal 
trend  in  terms  as  general  as  the  foregoing  applies  only  if  regional  and  short  term 
irregularities  be  smoothed  out  by  the  process  of  averaging,  and  that  some  such 
succession  of  peaks  and  valleys,  as  appears  in  Clarke  and  Zinn's  (1937,  Fig.  4) 
graph  of  seasonal  abundance  near  Martha's  Vineyard,  would  more  correctly  pic- 
ture the  conditions  to  be  expected  at  any  given  station. 

Data  for  early  autumn  are  confined  to  the  vicinity  of  Martha's  Vineyard, 
where  Clarke  and  Zinn  (1937)  record  a  sharp  decline  in  the  abundance  of  Calanus 
from  July  through  August,  with  only  trifling  numbers  in  September,  or  later  in 
the  autumn.  And  this  probably  applies  over  our  area  generally,  judging  from  the 
facts  that  average  and  maximum  values  had  fallen,  by  October,  about  back  to 
those  existing  in  the  preceding  February,  in  the  one  year  (1931),  when  surveys 
were  made  in  both  these  months,  and  that  Calanus  also  greatly  decreased  in 
abundance  between  July-August  and  November  in  1916  (Bigelow,  1922).  But 
the  data  fail  to  show  whether  it  is  characteristic  for  this  autumnal  impoverish- 
ment to  follow  after  the  spring  or  summer  as  abruptly,  in  other  parts  of  our  area, 
as  it  does  in  the  extreme  northeast,  as  illustrated  by  Clarke  and  Zinn's  station, 
nor  is  any  information  available  for  December  or  January,  elsewhere  within  our 
limits. 

No  rich  centers  for  this  species  yet  exist  in  February,  nor  do  the  average 

'  July  data  for  1930  and  1931  were  limited  to  the  northern  sector. 


BIGELOW   AND   SEARS:    NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  307 

volumes  (Table,  p.  309)  suggest  any  definite  gradient,  either  north-south,  or  in- 
shore-offshore, as  generally  characteristic  at  this  time  of  year,  at  least  down  to 
Latitude,  about  36°  N.,  south  of  which  Calanus  was  negligible  quantitatively  in 
the  one  year  of  record  (1931).  In  normal  years,  the  vernal  augmentation  of 
Calanus  is,  however,  accompanied  by  a  localization  of  the  chief  centers  of 
abundance  within  the  sector  bounded  by  the  Montauk  and  Cape  May  profiles 
— such,  at  least,  was  the  case  in  1929,  and  again  in  1930  (Fig.  23).  And  while 
the  data  for  April  do  not  indicate  any  more  likelihood  of  greatest  abundance  in 
any  one  locality  than  in  another  within  these  boundaries,  the  center  of  abundance 
yearly  becomes  still  more  definitely  concentrated  by  May  within  the  area  out- 
lined in  Fig.  23A.  This  alteration  involves,  on  the  one  hand,  a  contraction 
seaward  of  the  inshore  margin  of  the  productive  area  in  the  south,  and  on  the 
other  hand,  its  expansion  eastward,  at  least  as  far  as  the  Martha's  Vineyard 
profile. 

In  normal  summers  (e.g.,  1929,  1930,  1931),  the  situation  in  this  respect,  is 
much  the  same  in  June,  and  in  July  as  it  is  in  May.  But  after  a  cold  and  tardy 
spring  (e.g.,  1916),  rich  concentrations  of  Calanus  may  also  persist  along  the 
outer  belt  of  the  shelf  as  far  south  as  the  offing  of  Chesapeake  Bay,  until  well  into 
August.  Hence,  (presumably)  in  a  year  of  this  type,  vernal  augmentation  on  a 
large  scale,  is  not  as  definitely  centered  in  the  northern  sector,  as  usual.  Even  in 
1916,  however,  the  largest  catches  of  all'  were  to  the  northward  of  the  Cape  May 
profile,  while  catches  of  more  than  800  c.c.  for  May,  June,  and  July  of  all  years 
combined  (Fig.  23),  have  been  so  definitely  concentrated,  offshore,  in  the  sector 
north  and  east  of  the  Atlantic  City  profile,  that  this  may  well  be  named  the 
"Calanus"  belt.  Here,  the  average  volume  for  these  months  has  been  252  c.c. 
(65  c.c,  in  1932;  339  c.c.  in  1929-1931),  the  maximum  1556  c.c,  the  minimum 
Oc.c 

In  the  easternmost  sector,  east  of  Longitude  about  73°  W.,  the  area  of 
abundance  may  or  may  not  reach  shoreward  within  5-10  miles  of  the  coast,  in 
late  spring  or  summer.  But  the  facts  that  om-  largest  catch  at  any  one  of  the 
three  June-July  stations  near  Block  Island  was  only  583  c.c,  with  10-33  c.c.  at 
the  other  two,  and  that  the  maximum  reported  near  Martha's  Vineyard  by  Clarke 
and  Zinn  (1937)  corresponds  to  less  than  90  c.c.  when  reduced  to  our  present 
standard,  makes  it  unhkely  that  dense  aggregations  of  Calanus  ever  occur  close 
in  to  this  part  of  the  shore  line,  even  at  the  season  of  peak  abundance.  And  this 

'  Bigelow,  1922,  3000  c.c.  or  more  per  haul. 


308 


memoir:  museum  of  comparative  zoology 


Fig.  23.  Locality  records  for  large  volumes  of  Calanus  finmarchicus,  all  cruises  combined:  A,  April  and 
May;  B,  June  and  July. 


BIGELOW   AND    SEARS:    NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


309 


inshore  belt  of  summer  scarcity,  some  20  miles  wide  off  New  York,  expands  sea- 
ward toward  the  south  in  normal  years  until  it  includes  the  entire  breadth  of  the 
shelf  off  Winterquarter,  as  outUned  in  Fig.  23B.  Neither  have  large  volumes  ever 
been  recorded  farther  seaward  than  the  200-meter  contour. 

Data  as  to  chief  centers  of  abundance  are  lacking  for  September.  But  the 
situations  existing  in  October  1931  and  in  November  1916  (Bigelow,  1922), 
make  it  unlikely  that  any  areas,  describable  as  "rich"  by  spring  or  autumn 
standards,  characteristically  exist  anywhere  westward  or  southward  from  the 
offing  of  Martha's  Vineyard  in  normal  years,  at  any  time  from  late  summer, 
through  autumn  or  winter. 

Average  volumes  of  Calanus  Jinmarchicus 


Month 

Year 

Inshore 

Offshore 

North 

South 

Area 
surveyed 

Maxi- 
mum 

Mini- 
mum 

February 

1930 
1931 
1932 

9 
29 

7 

5 
30 

1 

11 
6 

7 

3 

32 

4 

7 

29 

5 

24 

117 

43 

0 

<1 

0 

April 

1929 
1930 

115 
129 

256 
119 

228 
172 

128 
71 

173 
121 

1532 

852 

<1 

0 

May 

1929 
1930 
1931 
1932 

51 
163 
117 

22 

152 
260 
429 
106 

136 

259 

309 

60 

35 
138 
105 

33 

91 
206 
251 

50 

435 
1063 
1514 

407 

<1 

<1 

<1 

0 

June 

1929 
1930 
1931 
1932 

44 
176 
301 

19 

135 
228 

578 
79 

109 

240 

497 

42 

29 

40 

237 

48 

81 
197 
406 

42 

411 
694 
950 
228 

0 
<1 
<1 

0 

July 

1929 
1930 
1931 

81 
281 
402 

68 
236 

589 

99 
254 
473 

18 

75 

254 
473 

377 

856 

1274 

0 
<1 
<1 

October 

1931 

— 

17 

18 

15 

17 

62 

0 

Average  volumes  of  C.  jinmarchicus,  all  years  combined 


Month 

North 

South 

Inshore 

Offshore 

February 

8 

13 

15 

12 

April 

196 

99 

109 

189 

May 

191 

78 

88 

234 

June 

222 

93 

134 

255 

July 

255 

18' 

298 

275 

October^ 

18 

15 

— 

17 

1929  only. 


2 1931  only. 


310 


memoir:  museum  of  comparative  zoology 


Breeding  periods.  It  seems  reasonably  established,  from  the  brevity  of  the 
annual  period  of  abundance,  that  there  is  only  one  major  breeding  period  for 
Calanus  in  the  waters  southward  from  the  New  York  profile.  Clarke  and 
Zinn's  (1937)  analysis  of  relative  percentages  of  different  stages  pointed,  how- 
ever, to  two  shorter-lived  generations  during  the  spring,  followed  by  a  longer 
lived  one  in  summer  in  the  northeastern  sector  of  our  area.  The  offshore  catches 
throw  no  light  on  this  point,  for  the  younger  stages  were  not  adequately  sampled. 


JUNE      1929, 
1931,     1932 


JULY     1929 
1931 


Fig.  24.  Calanus  finmarchicus:  percentage  of  cases,  in  different  months,  where  the  deep  catch  was  twice 
as  large  was  the  shoal  (A) ;  the  shoal  catch  twice  as  large  as  the  deep  (B) ;  and  neither  catch  twice 
as  large  as  the  other  (C). 


But  whether  separate  generations  can  be  distinguished  or  not,  the  volumetric 
succession  from  month  to  month  is  sufficient  evidence  that  effective  reproduction 
is  confined  to  spring  and  early  summer,  north  as  well  as  south. 

Vertical  distribution.  In  February  (1932),  April  (1929),  and  May  (1929, 
1931,  1932)  combined,  the  shoal  catch  was  about  as  large  as  the  deep  (ratio, 
less  than  1  to  2)  at  about  one-fourth  to  one-half  the  stations  where  hauls  were 
made  at  two  or  more  levels,  and  significantly  the  larger  as  often  as  the  reverse 
among  the  remaining  cases  (Fig.  24).   The  deeper  catch  was,  however,  at  least 


BIGELOW   AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


311 


twice  the  more  voluminous  in  about  61  %  of  the  cases  in  June,  and  in  about  88% 
in  July,  with  only  about  6%  of  the  cases  failing  to  show  definite  stratification  of 
the  one  order  or  the  other  in  the  latter  month  (Fig.  24) .  The  volumes  of  Calanus 
for  all  pertinent  stations  combined  also  averaged  about  2-6  times  as  great  at 
20-40  meters  as  at  10-0  meters  in  February,  April,  and  May,  more  than  25  times 
as  large  in  June  and  about  400  times  as  large  in  July,  as  follows: 

Average  volumes  of  C.  finmarchicus  at  deeper 
levels  relative  to  those  at  10-0  meters 


Month 

Year 

20-40  Meters 

No.  of 
Cases 

More  than 
40  Meters 

No.  of 
Cases 

February 

1932 

2.6 

11 

1.3 

7 

April 

1929 

5.8 

8' 

2.3 

12 

May 

1929 
1931 
1932 

4.4 

92 

0.8 

18 

June 

1929 
1931 
1932 

23.5 

80 

8.2 

14 

July 

1929 
1931 

399.2 

28 

105.8 

13 

'  Omitting  one  case,  where  the  deep  catch  was  135  times  as  great  as  the  shoal. 

And  the  vertical  distribution  of  rich  ( >  300  c.c.)  catches  has  shown  a  cor- 
responding transition  from  June  to  July : 

Frequency  of  large  volumes  ( >  300  c.c.)  at  deeper 
levels  relative  to  the  upper  10  meters 


20-40  M. 

<40M. 

February,  1932 

— 

— 

April,  1929 

1.8  to  1 

0.8  to  1 

May  1929,  1931,  1932 

1.8  to  1 

1.4  to  1 

June  1929,  1931,  1932 

6.2  to  1 

4.3  to  1 

July  1929,  1931 

29.0  to  1 

12.5  to  1 

These  depth  relationships  may  be  summarized  as  follows:    In  late  winter 
and  through  the  spring,  the  richest  population  of  Calanus  is  close  to  the  surface 


312 


memoir:  museum  of  comparative  zoology 


about  as  often  as  in  mid-depths,  but  the  stock  averages  somewhat  more  volumi- 
nous in  the  20-40  meter  stratum,  than  in  the  superficial  10  meters  of  water,  even 
at  this  season.  Rather  an  abrupt  transition  then  takes  place  between  May  and 
June,  to  the  summer  state,  in  which  Calanus  is  definitely  concentrated  at  depths 
of  20  meters  or  more,  and  this  vertical  gradient  is  greatly  accentuated  from  June 
to  July.  But  it  is  not  until  the  latter  month  that  the  stock  living  deeper  than 
about  40  meters  shows  any  great  increase  in  abundance  relative  to  that  near  the 
surface. 

Diurnal  migration.  Successive  hauls,  day  and  night,  were  made  at  one 
station  only,  near  Fire  Island  on  May  17,  1929.  And  it  is  questionable  how  these 
should  be  interpreted,  as  regards  Calanus — for  while  a  disappearance  of  Calanus 


:oo  CO 


MO 


Fig.  25.  Vertical  distribution  of  Calanus,  near  Fire  Island,  May  17,  1929,  at  3;00  a.m.,  11:00  a.m.,  6:00 
p.m.  and  11 :00  p.m. 

from  the  immediate  surface,  during  the  interval  between  3  A.M.  and  11  A.M., 
followed  by  reappearance  in  abundance  there  between  11  A.M.  and  6  P.M.  is 
compatible  with  diurnal  migration,  obviously  this  can  not  explain  the  subsequent 
decrease  in  the  ratio  of  surface  catch  to  deep  catch  between  6  P.M.  and  11  P.M., 
nor  the  progressive  increase  in  the  average  abundance  of  this  copepod  for  the 
water  column  as  a  whole  that  took  place  during  the  period  over  which  the  obser- 
vations were  extended  (Fig.  25).  We  are,  therefore,  forced  to  turn  to  a  compari- 
son of  deep  catches  with  shoal,  from  station  to  station,  for  information  as  to  the 
extent  to  which  diurnal  migration  affects  the  mass  distribution  of  Calanus.  Such 
a  comparison  shows  that  the  deeper  catch  was  significantly  the  larger  of  the  pair 
about  1.3  times  as  often  by  day  as  by  night  as  tabulated  below,  but  that  the 


BIGELOW   AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


313 


shoaler  was  the  larger  about  1.4  times  as  often  by  night  as  by  day,  for  the  months 
February-June,  as  a  whole.  And  while  a  contrast  of  the  reverse  order  prevailed 
in  July,  the  number  of  night  hauls  for  that  month  was  so  small  (12)  that  it  may 
not  be  significant. 

Percentage  of  cases  in  which  the  shoal  catch  was  twice  as  great  as  the  deep 
(A),  the  deep  catch  twice  as  great  as  the  shoal  (B),  and  neither  one  twice  as 
great  as  the  other  (C),  by  day  and  by  night. 


Day 

Night 

A 

B 

C 

Cases 

A 

B 

C 

Cases 

February,  1932 

17% 

17% 

67% 

6% 

20% 

30% 

50% 

10% 

April,  1929 

18 

45 

36 

11 

25 

17 

58 

12 

Average  — 
February-April 

17 

31 

52 

17 

23 

24 

54 

22 

May,  1929, 1931, 1932 

26 

37 

37 

38 

38 

21 

40 

47 

June  1929, 1931, 1932 

8 

70 

23 

40 

10 

61 

29 

49 

July  1929, 1931 

10 

80 

10 

20 

8 

92 

0 

12 

The  volumetric  ratio  of  deep  catch  to  shoal,  similarly,  averaged  about  5  times 
as  great  by  day  as  by  night  for  February,  April,  and  May  combined,  about  1.5 
times  as  great  by  day  as  by  night  for  June,  and  about  2.2  times  as  great  by  day  as 
by  night  for  July,  details  for  individual  cruises  being  as  follows: 


Volumetric  ratio,  deep  catch  to  shoal 


8  A.M.-4  P.M. 

8  P.M.-4  A.M. 

Ratio, 

Cases 

Ratio, 

Cases 

all  Stas. 

all  Stas. 

February,  1932 

3.3  to  1 

6 

10.0    to  1 

10 

April,  1929 

30.4  to  1 

11 

1.0    to  1 

13 

May,  1929 

14.5  to  1 

11 

0.9    to  1 

13 

May,  1931 

20.7  to  1 

9 

0.6    to  1 

7 

May,  1932 

4.5  to  1 

17 

2.0    to  1 

27 

June,  1929 

2.4  to  1 

14 

7.0    to  1 

16 

June,  1931 

35.7  to  1 

12 

8.8    to  1 

11 

June,  1932 

35.0  to  1 

14 

34.0    to  1 

22 

July,  1929 

159.0  to  1 

16 

75.7    to  1 

7 

July,  1931 

311.9  to  1 

4 

136.2    to  1 

5 

314 


memoir:  museum  of  comparative  zoology 


If  we  omit,  from  the  calculation  the  one  station  for  each  cruise  where  the 
ratio  of  deep  volume  to  shoal  was  largest,  thus  minimizing  the  effects  of  occa- 
sional cases  of  very  strong  stratification,  we  find  this  ratio  averaging  2-4  times 
as  great  by  day  as  by  night,  and  with  no  outstanding  alteration  in  this  respect 
between  spring  and  summer,  as  follows : 

Volumetric  ratio,  deep  catch  to  shoal 


8  A.M.-4  P.M. 

8  P.M.-4  A.M. 

Ratio 

Cases 

Ratio 

Cases 

February  1932,  April  1929, 
May  1929,  1932,  1932 

3.8  to  1 

50 

0.9  to  1 

65 

June  1929,  1931,  1932 

11.7  to  1 

37 

6.0  to  1 

46 

July  1929,  1931 

124.1  to  1 

18 

35.5  to  1 

10 

A  prevailing  tendency  toward  diurnal  migration  on  the  part  of  adult  Calanus, 
upward  toward  (or  even  to)  the  surface  during  the  hours  of  darkness,  and  down- 
ward again  by  day,  and  on  a  scale  sufficient  to  cause  widespread  alterations  of 
similar  order  in  the  vertical  distribution  of  the  stock  seems  the  only  plausible 
explanation  for  the  relationship  just  brought  out.  And  it  is  especially  interesting 
to  find  the  evidence  of  this  hardly  less  clear  for  July,  when  the  temperature  of  the 
upper  10  meters  has  risen  above  the  optimum  for  Calanus,  than  it  is  for  June  or 
for  May,  when  the  entire  water  column  is  of  a  temperature  suitable  for  this 
copepod. 

It  is  equally  clear,  however,  that  the  effects  on  the  distribution  of  Calanus 
of  this  tendency  toward  vertical  migration  varies  widely  from  time  to  time  and 
from  place  to  place,  for  it  has  frequently  happened  that  the  shoal  catch  has  been 
the  greater  even  by  day,  and  still  more  often  that  neither  haul  was  much  more 
productive  than  the  other,  irrespective  of  the  time  of  day.  But  our  data  are  not 
sufficiently  detailed  to  show  whether  irregularities  of  these  sorts  result  from 
differences  in  the  degree  to  which  different  individuals  respond  to  the  effective 
stimulus  (light),  or  from  mass  movements  of  the  water,  with  its  contained 
plankton. 

Distribution  in  relation  to  temperature.  In  the  summer  of  1916,  the  most 
abundant  stock  of  Calanus  was  li\'ing  at  depths  and  in  the  belt  where  temperature 
was  lower  than  about  7°  (Bigelow,  1922).  And  the  catches  larger  than  about  400 


BIGELOW   AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


315 


c.c,  on  our  recent  cruises  have  all  been  made  at  times  and  localities  where  the 
bottom  stratum  was  at  least  as  cold  as  about  8°,  i.e.,  where  a  reservoir  of  cool 
water  was  not  more  than  about  50  meters  distant,  even  though  the  Calanus  may 
have  been  close  to  the  surface,  and  in  much  higher  temperatures,  at  the  time  of 
capture.  The  boundaries  of  the  "Calanus  zone"  for  June  and  July  (Fig.  23B) 
also  correspond  closely  to  those  of  the  pool  of  cold  bottom  water  characteristic  of 
the  mid-  and  outer  part  of  the  shelf,  to  the  eastward  of  the  Barnegat  profile,  at 


"c 

T 

29   CASES 

' 

80° 

\ 

^32  CASES 

• 

ie° 

- 

\ 

Z\    CASES 

" 

i«° 

- 

V 

^     41    CASES 

" 

14' 

- 

i 

f 

34    CASES 

" 

12° 

- 

—                 ^'^   CASES 

- 

10° 

- 

4  3  cAses^ 

- 

.'• 

- 

40    CASES  ' 

i 

<l° 

. 

. 

^ 

SOO  300 

AVERAGE     CATCH     IN    CC 


Fig.  26.  Average  catch  of  Calanus  in  water  of  different  temperatures,  irrespective  of  depth. 


that  season.  Clarke  and  Zinn  (1937)  found,  it  is  true,  that  Calanus  was  most 
numerous  near  Martha's  Vineyard  during  July  and  August,  when  the  bottom 
water  averaged  about  13°,  but  their  maximum  catches  were  at  the  rate  of  less 
than  90  c.c,  when  reduced  to  our  standards,  contrasted  with  the  many  catches 
larger  than  1000  c.c.  that  were  made  closer  to  cold  water  on  the  "Albatross" 
cruises.  And  the  relationship  between  average  catch  and  temperature,  for  the 
several  cruises  on  which  hauls  were  made  at  two  or  more  levels  (Fig.  26),  suggests 
that  the  lowest  temperatures  persisting  on  the  shelf  in  summer  (usually  6-10°) 
support,  on  the  average,  considerably  the  densest  population  of  Calanus,  and 
that  the  temperature  range  between  12°  and  16°  also  may  be  classed  as  at  least 
moderately  favorable,  with  no  apparent  gradient  within  these  limits.  But  Cala- 


316  memoir:  museum  of  comparative  zoology 

nus  is  much  less  abundant  at  higher  temperatures,  largely  deserting  the  surface 
stratum  whenever  and  wherever  the  latter  warms  above  18°,  and  practically  dis- 
appearing thence  (probably  by  sinking),  whenever  the  temperature  rises  above 
19-20°.  In  fact,  24  out  of  a  total  of  43  hauls  centering  in  water  of  19°  or  warmer, 
proved  absolutely  barren  of  Calanus,  while  each  of  10  others  yielded  not  more 
than  1  c.c,  of  the  sizes  large  enough  to  be  sampled  by  our  nets.  And  while  excep- 
tions occur  to  this  as  to  every  other  "rule"  regarding  the  distribution  of  plank- 
ton,' we  seem  justified  in  concluding  that  the  warming  of  the  water  column  from 
above  is  the  chief  (if  not  the  only)  factor  responsible  for  the  progressive  intensi- 
fication of  the  vertical  volumetric  gradient  for  this  copepod  from  May  through 
June  to  July.  Light,  we  may  note  in  passing,  seems  ruled  out  as  the  control  in 
this  case,  by  the  fact  that  stratification  of  Calanus  is  much  more  pronounced  in 
July  than  in  May,  though  the  intensity  of  illumination  is  about  the  same  in  the 
one  month  as  in  the  other. 

Annual  fluctuations.  The  tabulation  of  average  volumes  (p.  309)  makes  it 
clear  that  the  stock  of  Calanus  varies  widely  in  strength  from  year  to  year,  not 
only  near  the  southern  margin  of  its  range  (where  this  might  be  expected),  but 
equally  in  the  more  northern  sector  where  the  species  usually  has  its  center  of 
abundance.  The  average  catch  was  in  fact  about  4  times  as  great  in  the  most  pro- 
ductive year  (1931)  as  in  the  poorest  (1932)  for  February,  5  times  as  great  for 
May,  9  times  as  great  for  June,  and  at  least  6  times  as  great  for  July.  And  while 
the  comparatively  small  volumes  prevailing  in  1932  perhaps  approximate  the 
lowest  level  to  which  Calanus  is  likely  to  fall  in  our  area,  it  may  not  be  unusual 
for  it  to  exist  in  considerably  greater  volume  than  in  1931,  for  such  seems  to  have 
been  the  case  in  the  summer  of  1916,  when  swarms  were  encountered  (Bigelow, 
1922,  p.  143). 

The  record  also  suggests  that  when  Calanus  is  relatively  abundant  at  the 
end  of  winter  (e.g.,  1931),  it  is  hkely  to  maintain  preeminence  in  this  respect 
through  the  spring  and  summer,  but  that  a  year  that  is  poor  in  Calanas  in  Febru- 
ary may  either  continue  poor  through  spring  and  early  summer,  as  happened  in 
1932,  or  may  experience  an  active  vernal  multipUcation  of  this  copepod  as  in  1930. 

Comparison  of  the  average  catch,  with  temperature,  for  different  years, 
leads  to  the  rather  astonishing  conclusion  that  the  rate  of  vernal  augmentation 
is  affected  only  indirectly  by  the  thermal  factor,  if  at  all,  for  the  year  (1932)  when 

'  The  following  productive  catches  were  made  in  hauls  centering  in  the  upper  10  meters  in  water  warmer 
than  18°:  150  c.c,  20.2°,  Shinnecock  II,  July  1931;  166  c.c,  18.9°,  Martha's  Vineyard  I,  July  1929;  163 
c.c,  18.2°,  Cape  May  IV,  June  1932;  and  113  c.c,  18.1°,  New  York  IV,  June  1932. 


BIGELOW    AND    SEARS :    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES  317 

production  was  poorest  was  not  abnormally  warm  at  the  end  of  the  spring  or  in 
early  summer,  though  it  had  been  so  in  February.  It  is  a  question  for  the  future, 
whether  a  very  warm  winter  in  this  part  of  the  sea  ordinarily  presages  a  poor  pro- 
duction of  Calanus  during  the  ensuing  spring. 

Calanus  hyperboreus 

Calanus  hyperboreus^  was  taken  at  one  station,  only,  in  February,  and  was 
either  absent  altogether  in  April  (1930)  or  was  so  scarce  then  that  it  was  not  de- 
tected. But  it  has  appeared  in  one  part  of  the  area  or  another  in  May  in  each  year 
of  record  with  sufficient  regularity  to  suggest  that  this  is  an  annual  event. 

In  1930,  for  example,  a  year  when  a  considerable  indraft  of  cold  water  took 
place  into  the  most  eastern  sector  sometime  in  February  or  early  March  (Bigelow, 
1933,  p.  30),  C.  hyperboreus  was  generally  distributed  over  the  entire  area  by  the 
middle  of  May  right  down  to  the  offing  of  Chesapeake  Bay  (Fig.  27),  locally  in 
some  abundance  (up  to  40-50  c.c).  It  was  similarly  widespread  east  of  the  New 
York  profile  in  that  month  of  1931,  though  chiefly  confined  to  the  outer  part  of 
the  shelf  farther  south,  which  corresponds  to  the  fact  that  little  cold  water  had 
drifted  past  Nantucket  in  that  year.  And  the  first  half  of  May  again  saw  it  at 
several  stations  in  the  eastern  and  mid-sectors,  in  1932,  and  even  off  Chesapeake 
Bay  where  one  haul  yielded  23  c.c,  seemingly  following  the  indraft  from  the 
east  that  had  taken  place  earlier  (Bigelow,  1933,  p.  30),  though  its  presence 
was  so  short-lived  that  it  was  not  found  at  all  in  the  third  week  of  the  month. 

In  some  years  repeated  invasions  may  also  occur,  as  in  1932,  when  hyper- 
boreus reappeared  in  small  numbers,  at  the  end  of  May,  along  a  tongue  extending 
from  the  outer  part  of  the  shelf  off  Martha's  Vineyard  and  New  York,  westward 
to  the  offing  of  Atlantic  City  (Fig.  27),  following  a  second  invasion  of  cold  water 
that  had  passed  the  offing  of  New  York,  two  weeks  or  so  before  (Bigelow,  1933, 
p.  30).  But  in  other  years  (e.g.,  1930  and  1931),  there  may  be  only  one  invasion, — 
sometime  perhaps  none  at  all.  In  any  case,  we  have  no  evidence  that  such  events 
normally  happen  later  than  the  end  of  May,  for  our  records  for  hyperboreus  in 
summer  and  autumn  are  confined  to  one  station  in  July  1931,  two  in  June  1932, 
and  one  in  August  1916,  off  Martha's  Vineyard,  off  Montauk,  off  New  York, 
and  off  Delaware  Bay  respectively. 

The  evidence  is  thus  conclusive  that  C.  hyperboreus  is  not  an  endemic  in- 
habitant of  our  area,  but  that  it  may  be  expected  to  appear  here  and  there  in  late 
spring,  after  indrafts  of  water  from  the  east,  which  accords  with  previous  knowl- 

'  No  regular  record  was  kept  of  the  presence  or  absence  of  this  species  in  1929. 


318 


memoir:  museum  of  comparative  zoology 


edge  that  it  is  widespread  throughout  the  Gulf  of  Maine  at  that  season,  at  least 
in  some  years,  as  well  as  on  George's  Bank  (Bigelow,  1926,  Fig.  68,  69,  Fish  and 
Johnson,  1937).  It  also  seems  certain — from  the  brevity  of  the  periods  during 
which  C.  hyperboreiis  has  been  present  in  our  waters — that  the  effects  of  each 
wave  of  immigration  endure  only  for  the  lives  of  the  individual  concerned,  i.e., 
that  this  species  does  not  reproduce  successfully  an3n;vhere  west  of  Nantucket. 


40 


Fig.  27.  Areas  of  occurrence  of  Calanxis  hyperboreu^:  A,  May  12-23,  1930;  B,  May  2-6  (dotted),  and 
May  9-16  (hatched),  1932. 

C.  hyperboreus  is  both  so  large  and  so  easily  recognized  that  it  is  perhaps  the 
most  reliable  indicator-species  for  water  from  this  source  in  our  area.  It  is,  how- 
ever, neghgible  from  the  volumetric  standpoint,  except  for  brief  periods,  and  of 
minor  importance  even  then,  witness  a  maximum  catch  of  only  62  c.c.  and  an 
average  of  25  c.c.  within  its  area  of  occurrence  in  the  month  (May  1930)  when 
it  was  taken  most  frequently  and  in  greatest  abundance. 


Candacia  armata 
Previous  records  of  this  widely  distributed  copepod  within  our  area  were 
confined  to  the  extreme  south,  where  it  was  found  in  some  abundance  near  the 
outer  edge  of  the  continental  shelf  and  over  the  continental  slope  in  the  summers 
of  1913  and  1920,  as  well  as  in  Chesapeake  Bay  on  one  occasion  (Bigelow,  1915; 
Wilson,  1932a).  But  it  was  also  to  be  expected  in  the  north,  having  been  found 
in  the  Gulf  of  Maine,  as  well  as  off  the  seaward  slope  of  the  latter  (Bigelow,  1926, 
p.  219;  Fish  and  Johnson,  1937,  Fig.  25).  And  the  records  for  1929-1932  (Fig.  28) 
show  that  it  does  in  fact  occur  about  as  generally  as  does  Euchirella  rostrata, 


BIGELOW   AND   SEARS:    NORTH   ATLANTIC   ZOOPLANKTON   STUDIES  319 

throughout  the  offshore  belt  of  our  area  (40%  of  the  stations)  and  not  rarely  in- 
shore as  well  (13%  of  the  stations).   It  has  also  proved  about  as  frequent  rela- 


Fig.  28.  Locality  records,  all  cruises  combined:  A,  Candacia  armata,  oflfshore  and  inshore;  B,  Centwpages 
hamatus;  C,  Centropages  typicus,  large  volumes,  in  February;  D,  C.  typicus,  large  volumes  in  May. 

tively,  north  (27%  of  the  stations)  as  south  (21%).  The  monthly  percentage  of 
stations  at  which  Candacia  was  taken  in  the  offshore  belt  (16-17%,  February- 
April;  40-54%,  May- July;  60%,  October),  show  it  as  entering  our  boundaries 


320  memoir:  museum  of  comparative  zoology 

much  more  often  from  late  spring  through  the  summer,  than  in  late  winter  or 
early  spring,  and  perhaps  somewhat  more  often  still,  in  the  autumn,  while  the 
maximum  catch  of  113  c.c.  was  also  made  in  October.  But  we  have  no  evidence 
that  this  seasonal  invasion  extends  to  the  inshore  belt,  where  on  the  contrary, 
it  proved  about  as  frequent  in  February  and  April  (19%,  10%  of  the  stations) 
as  in  May,  June,  or  July  (14%,  13%,  10%),  a  regional  difference  associated  no 
doubt  with  prevailing  drifts. 

In  132  out  of  the  154  cases,  the  catches  were  of  stray  specimens.  A  catch  of 
113  c.c.  was,  however,  made  midway  of  the  shelf  off  Cape  May  on  October  23, 
1931,  evidence  that  Candacia  may  occasionally  occur  in  abundance  over  re- 
stricted areas  in  autumn,  suggesting  local  centers  of  reproduction.  But  we  have 
no  evidence  that  this  ever  happens  earlier  in  the  season,  for  five  only  of  the  other 
catches  were  larger  than  10  c.c,  with  16  c.c.  as  the  maximum  for  this  group. 

Centropages  hamatus 

This  copepod,  generally  considered  as  of  more  definitely  boreal  habit  than 
its  relative,  C.  typicus,  and  conamon  in  the  Gulf  of  Maine,  had  previously  been 
reported  as  far  south  as  the  offing  of  Chesapeake  Bay  as  well  as  within  the  latter 
(Wilson,  1932a).  The  records  for  the  years,  1929-1932,  show,  in  fact,  that  its 
range  actually  extends  southward  past  the  offing  of  Chesapeake  Bay  nearly  to 
Cape  Hatteras  (Fig.  28),  in  early  spring.  But  while  Wilson  (1932a)  reports  it 
off  Chesapeake  Bay  in  August,  we  have  no  records  of  it  south  of  Delaware  Bay 
as  late  as  July  either  of  1929  or  of  1913,  suggesting  that  it  tends  to  disappear  from 
the  southernmost  sector  by  midsummer  in  normally  warm  years. 

It  is  also  interesting  to  find  that  the  records  of  occurrence  of  C.  hamatus — 
though  most  frequent  inshore — extend  across  the  whole  breadth  of  the  continen- 
tal shelf,  from  the  northern  end  of  oui-  area  to  the  southern,  in  one  month  or 
another  (Fig.  28B),  whereas  in  the  Gulf  of  Maine,  this  copepod  is  chiefly  confined 
to  the  close  vicinity  of  the  coast  (Bigelow,  1926,  Fig.  70).  The  monthly  distribu- 
tion of  catches  (12-13%  of  the  stations  for  February  and  April,  16%  for  May, 
19%  for  June,  and  10%  for  July),  indicates  a  general  tendency  for  this  species  to 
increase  somewhat  in  frequency  from  late  winter  to  June,  and  then  to  decrease 
somewhat  later  in  the  summer.  And  the  data  for  the  one  October  of  record  point 
to  a  still  farther  decline  in  the  autumn,  for  it  was  not  recorded  at  all  on  that 
cruise,  nor  in  November  1916. 

On  rare  occasions,  a  rich  stock  of  C.  hamatus  may  develop  locally  in  the 
northern  sector,  suggesting  active  reproduction,  cases  in  point  being  189  c.c, 


BIGELOW   AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON    STUDIES 


321 


near  Martha's  Vineyard,  June  1930;  59  c.c.  at  that  same  locality  on  May  19, 
1932;  and  35  c.c,  close  in  to  Chesapeake  Bay,  in  April,  1929.  Other  than  these, 
however,  the  largest  catches  were  10-16  c.c.  (5  cases),  the  average  for  the  whole 
series  being  about  4  c.c.  at  the  stations  where  C.  hamatus  occurred,  and  less  than 
1  c.c,  if  the  stations  where  it  failed  be  included  in  the  calculation.  Neither  did 
C.  hamatus  form  as  much  as  1%  in  any  subdivision  of  the  area  during  any  cruise. 

Centropages  typicus 
Frequency.  Centropages  typicus  is  but  little  less  omnipresent  than  Calanus 
finmarchicus  (p.  304)  throughout  the  whole  area,  from  north  to  south,  and  out  to 
the  200-meter  contour, — recorded,  in  fact,  at  about  90%  of  the  stations,  for  all 
months  and  years  combined — ,  and  it  is  possible  that  young  stages  of  the  species 
may  have  been  actually  present,  even  at  the  few  localities  where  the  catches 
failed  to  show  its  presence. 

Percent  of  stations  with  Centropages  typicus 


Year 

February 

April 

May 

June 

July 

October 

1913 

— 

— 

— 

— 

63% 



1929 

— 

100% 

95% 

— • 

91 

— 

1930 

86% 

100 

93 

61% 

94 

— 

1931 

86 

— 

80 

65 

71 

100% 

1932 

76 

— 

98 

100 

— 

Aver. 

82 

100 

94 

77 

85 

— 

The  percentages  of  the  stations  where  C.  typicus  was  recorded  suggest  some 
increase  as  characteristic  from  February  to  April,  succeeded  in  some  years  by 
some  decline  in  summer,  but  then  by  recovery  by  October.  In  some  years  (e.g., 
1932),  however,  this  copepod  may  be  as  universal  in  June  as  in  April. 

Abundance.  In  the  northern  sector,  the  average  stock  of  C.  typicus  is  at  its 
lowest  ebb  at  the  end  of  winter,  and  continues  relatively  low  through  spring  and 
early  summer,  when  an  average  of  less  than  15  c.c.  is  to  be  expected  in  normal 
years,  but  increases  some  4-30  fold  (to  90-100  c.c.)  by  July.  Clarke  and  Zinn's 
(1937)  records  further  show  that  C.  typicus  may  continue  numerous  through 
August,  in  the  easternmost  sector  of  our  area,  with  abundant  production  of 
young,  and — by  the  evidence  of  the  single  year,  1931 — it  may  be  expected  to 
remain  at  about  the  same  high  level  until  mid-autumn. 

The  seasonal  cycle  is  much  less  regular  in  the  south,  where  the  average 
volume  remained  at  a  comparatively  constant  level  from  early  spring  through 


322 


memoir:  museum  of  comparative  zoology 


midsummer  in  one  year  (1929),  but  where  more  or  less  impoverishment  took 
place  in  May  or  June  in  the  other  years,  either  for  a  brief  period  only,  or  of  longer 
duration  (see  following  Table).  Monthly  averages,  however,  for  the  several 
years  combined  of  83  c.c.  for  February,  31  c.c.  for  April,  21  c.c.  for  May,  25  c.c. 
for  June,  and  28  c.c.  for  July  (this  latter  for  1929  only)  suggest  that  volumes  less 
than  half  as  large  are  the  normal  expectation  through  spring  and  summer  than 
at  the  end  of  the  winter,  but  (by  the  evidence  of  1931)  with  the  peak  of  abundance 
to  be  expected  in  early  autumn,  when  the  average  may  rise  above  100  c.c. 

Average  volumes  of  Centropages  typicus 


Month 

Year 

Inshore 

Offshore 

North 

South 

Area 
Surveyed 

Maxi- 
mum 

Mini- 
mum 

February 

1930 

124 

1 

12 

102 

59 

915 

0 

1931 

1 

12 

1 

11 

8 

76 

0 

1932 

169 

5 

8 

135 

101 

459 

0 

April 

1929 

24 

21 

18 

27 

23 

87 

<1 

1930 

33 

13 

5 

35 

21 

368 

<1 

May 

1929 

14 

16 

14 

16 

15 

58 

0 

1930 

33 

5 

6 

26 

15 

92 

0 

1931 

1 

2 

1 

1 

1 

6 

0 

1932 

17 

31 

9 

42 

25 

276 

0 

June 

1929 

15 

12 

6 

27 

14 

107 

0 

1930 

7 

1 

3 

2 

3 

35 

0 

1931 

11 

7 

9 

12 

10 

120 

0 

1932 

31 

22 

13 

57 

32 

276 

<1 

July 

1929 

59 

31 

35 

28 

46 

256 

0 

1930 

208 

12 

92 

— 

92 

1341 

0 

1931 

146 

11 

95 

— 

95 

1070 

0 

October 

1931 

80 

111 

32 

80 

256 

<1 

In  February,  of  the  two  years  when  Centropages  occurred  in  more  than 
minimal  numbers  in  that  month,  the  largest  catches  were  made  close  to  the  mouth 
of  Delaware  Bay  (1932),  and  from  Chesapeake  Bay  southward  (1930).  But  the 
locations  of  the  centers  of  greatest  abundance  have  varied  widely,  from  year  to 
year,  during  the  other  months  (Fig.  28,  29),  and  also  from  year  to  year.  In  1930, 
for  example,  the  concentration  of  Centropages  that  was  encountered  off  Delaware 
Bay  in  February  had  been  largely  obhterated  by  April.  In  May  of  1929,  the 
richest  concentration  (58  c.c.)  lay  mid-way  out  on  the  shelf,  on  the  Martha's 
Vineyard  profile,  while  in  1932,  the  more  northerly  of  the  two  centers  where 
Centropages  had  been  most  numerous  in  February,  had  broken  down  by  May, 


BIGELOW   AND    SEARS:    NORTH   ATLANTIC    ZOOPLANKTON    STUDIES  323 

whereas  the  more  southerly  (off  Chesapeake  Bay)  had  persisted,  merely  shifting 
some  miles  northward. 

It  seems,  however,  that  the  rich  centers  tend  on  the  whole  to  become  more 
generally  distributed  from  February  and  April  to  May,  out  across  the  shelf  in 


Fig.  29.  Locality  records,  all  cruises  combined:  A,  Centropages  typicus,  large  volumes  for  June;  B,  C. 
typicus,  as  above  for  July;  C,  volumes  of  C.  typicus,  per  standard  haul,  October  19-28,  1931;  D. 
localit}'  records  for  Centropages  inolaceus,  all  cruises  combined. 

the  sector  southward  from  the  offing  of  New  York  (to  which  they  are  so  far  con- 
fined). And  although  they  may  then  contract  again,  in  June,  if  the  population 
be  decreasing  (cf.  Fig.  28  and  Fig.  29),  centers  of  abundance  may  again  appear 
offshore,  from  June  through  July,  to  the  north  as  well  as  to  the  south,  while  the 


324  memoir:  museum  of  comparative  zoology 

distribution  for  October  1931  shows  the  center  of  abundance  (250  c.c),  as 
lying  off  Martha's  Vineyard,  instead  of  off  New  York  and  off  New  Jersey  as  had 
been  the  case  in  the  preceding  July. 

The  composite  picture  is  thus  of  a  progressive  shift  in  the  situation  of  the 
center  of  population  from  the  Delaware  Bay-Chesapsake  Bay  sector  in  February- 
April,  progressively  northward  to  the  offing  of  New  York  in  July,  and  finally  by 
mid-autumn  to  the  eastern  boundary  of  our  area.  The  result  is  that  the  volume 
of  Centropages  typicus  may  average  about  as  large  in  the  north,  at  the  season  of 
peak  abundance  there  (July-October),  as  it  is  in  the  south  in  the  months  when 
largest  there — February-April  or  June,  according  to  the  year.  But  Centropages 
has  been  so  much  more  plentiful  south  than  north  in  most  of  the  individual 
months,  other  than  July  or  October,  that  the  average  for  the  series  as  a  whole  is 
more  than  twice  as  large  for  the  southern  sector  (42  c.c.)  as  for  the  northern  (17 
c.c).  C.  typicus  is  therefore  to  be  classed  as  primarily  a  southern  species  in  our 
area.  And  while  it  occurs  in  considerable  abundance  as  far  north  and  east  as  the 
Gulf  of  Maine,  it  appears  that  Cape  Sable,  Nova  Scotia,  marks  its  approximate 
boundary  in  that  direction  (Bigelow,  1926,  p.  223). 

The  inshore  belt  has  also  averaged  from  1.5  to  7  times  as  productive  of  it  as 
the  offshore  in  each  month,  of  the  several  years  combined,  maximum  catches  of 
400-1000  c.c.  or  more,  not  being  imusual  there,  at  the  season  of  maximum  abun- 
dance. 

Annual  variations.  The  averages  given  in  the  column  headed  "Area  sur- 
veyed" in  the  table  on  page  322,  show  that  while  Centropages  may  vary  widely 
in  abundance  from  year  to  year,  at  the  end  of  the  winter  (when  it  has  averaged 
12-13  times  as  plentiful  volumetrically  in  the  richest  year  as  in  the  poorest)  and 
also  through  the  spring,  it  would  be  a  decidedly  unusual  event  for  a  sununer  to 
be  either  extraordinarily  rich  or  extraordinarily  poor  in  this  copepod. 

Vertical  distribution.  In  the  one  February  of  record,  Centropages  was  dis- 
tributed with  approximate  uniformity  vertically,  at  more  than  half  the  stations 
(Fig.  30)  and  it  was  as  often  most  plentiful  deep  as  shoal  at  the  others.  But  the 
center  of  abundance  was  much  more  often  shoal,  than  deep  in  each  other  month, 
with  more  than  half  the  stations  showing  one  or  the  other  type  of  vertical  strati- 
fication. Sunilarly,  the  relative  frequency  of  cases  in  which  the  catch  was  ten 
times  as  great  fi'om  the  one  level  as  from  the  other,  fails  to  suggest  any  definite 
vertical  gradient  in  February  or  April,  but  enforces  the  conclusion  that  in  May, 
June,  and  July,  the  zone  of  chief  abundance  for  Centropages  was  much  more 
often  in  the  upper  10  meters  or  so,  than  at  20  meters  or  deeper. 


BIGELOW  AND    SEARS!   NORTH   ATLANTIC   ZOOPLANKTON   STUDIES 


325 


On  the  other  hand,  the  volumetric  ratios  of  deep  catch  to  shoal  (listed  below) 
suggest  a  seasonal  reversal,  for  the  deep  averaged  considerably  the  more  volumi- 
nous of  the  pair  on  four  cruises  in  February,  April,  and  May,  but  the  shoal  the 
more  voluminous  on  five  cruises  in  May,  June,  and  July. 

Average  ratio  of  volume  at  10-0  meters  to  that  at  20-40  meters:  February, 
1932,  1  to  2.5;  April,  1929,  1  to  4.8;  May,  1929,  1  to  2.3;  May,  1931,  1  to  1.1; 


JUNE     1929, 
1931,    1932 


JULY       1929, 
1931 


Fig.  30.  Centropages  lypicus:  percentage  of  cases,  in  different  months,  when  the  deep  catch  was  twice 
as  large  as  the  shoal  (A);  in  which  the  shoal  catch  twice  as  large  as  the  deep  (B);  and  in  which 
neither  catdi  was  twice  as  large  as  the  other  (C). 


May,  1932,  1  to  0.6;  June,  1929,  1  to  0.6;  June,  1931,  1  to  0.7;  June,  1932,  1  to 
0.6;  July,  1929,  1  to  0.5;  July,  1931,  1  to  1.0. 

The  danger  of  confusing  regional  and  temporal  gradients  with  vertical  is  so 
great,  when  the  data  are  so  few,  that  any  attempt  to  harmonize  such  conflicting 
pictures,  on  the  basis  of  present  information,  seems  idle.  The  most  we  dare  say 
is  that  the  center  of  abundance  for  Centropages  through  spring  and  early  summer 
lies  much  more  often  in  the  upper  10  meters  than  at  any  considerable  depth, 
while  its  relative  volumetric  distribution  through  the  water  column  is  highly 
variable  from  time  to  time  and  from  place  to  place. 


326  memoir:  museum  of  comparative  zoology 

The  fact  that  a  considerable  proportion  of  the  stock  is  Uvang  in  the  upper 
part  of  the  vertical  range  of  temperature  from  June  on,  shows  that  values  higher 
than  18-20°  are  not  unfavorable  for  Centropages,  which  added  to  its  considerable 
abundance  in  February  1932  in  temperatures  of  9-13°,  is  evidence  that  the 
vertical  gradient  of  temperature  is  not  an  important  factor  in  determining  the 
vertical  distribution  of  this  particular  copepod. 

Centropages  violaceus 

The  records  for  this  species  have  been  confined  (with  three  exceptions)  to 
the  outermost  belt  of  the  shelf,  within  15-16  miles  of  the  200-meter  contour 
(Fig.  29D).  And  the  latitudinal  distribution  suggests  that  C.  violaceus  is  as  apt 
to  stray  in  across  the  edge  of  the  continent  about  as  often  in  the  one  sector  (north 
or  south)  as  in  the  other.  With  so  few  data  (20  locaUties),  we  can  only  say  of  its 
seasonal  incidence  that  we  have  record  of  it  within  our  hmits  in  February,  May, 
June,  and  October;  based  in  every  case,  on  scattered  specimens. 

Corycaeus  sp. 

Wilson  (1932a,  p.  42)  has  already  reported  four  species  of  Corycaeus  off 
Chesapeake  Bay.  And  our  records  again  yielded  a  scattering  of  these  tiny  cope- 
pods  in  the  months  of  February,  April,  May,  June,  July,  and  October,  not  only 
in  that  same  general  region,  but  also  northward  along  the  edge  of  the  continent 
to  the  offing  of  Delaware  Bay  (Fig.  31C). 

EucALANUs  sp. 

Two  species  of  this  genus  have  been  identified  in  the  catches,  atteriuatus  and 
elongatus,  their  chief  interest,  in  the  present  connection  being  that  they  are  sure 
indicators  of  a  warm  water  source  (see  Wilson,  1932,  for  summaries  of  their 
known  distribution).  In  the  case  of  attenuatus  the  records  are  concentrated  in 
■the  southernmost  sector  (Fig.  31A),  where  they  extend  close  inshore,  both  off 
Chesapeake  Bay  and  off  Delaware  Bay.  To  the  northward,  they  are  confined 
to  the  extreme  outer  edge  of  the  shelf,  though  E.  attenuatus  again  approaches  the 
land  in  the  Gulf  of  Maine  with  the  peripheral  drift  of  water  from  offshore  (Bige- 
low,  1926,  Fig.  71).  The  three  records  for  E.  elongatus  were  all  well  out  on  the 
shelf.  And  juveniles  and  damaged  specimens,  of  the  one  species  or  the  other 
were  also  detected  at  the  additional  stations  marked  on  Figure  31  A.  All  but  one 
of  the  records  for  attenatus  were  for  the  month  of  April,  as  were  two  of  the  three 


BIGELOW   AND    SEARS :    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES  327 


Fig.  31.  Locality  records,  all  cruises  combined:  A,  Eucalanus  attenualus,  E.  elongatus  and  Eucalanus  sp.; 
B,  Ew-hirella  rostrata  and  Euchirella  sp.  offshore  and  inshore;  C,  Corycaeus  and  Oncaea;  D, 
Volumes  of  Melridia  lucens  greater  than  100  c.c.  (solid  circles),  boundary  separating  this  area 
from  that  where  the  volume  was  usually  less  than  5  c.c. 


328  memoir:  museum  of  comparative  zoology 

records  for  elongatus,  the  others  for  May,  whereas  Eucalanus  of  doubtful  identity 
were  taken  at  all  seasons. 

It  is  interesting  that  while  E.  atlenuatus  occurs  but  so  seldom  in  our  waters 
and  while  the  largest  catch  was  only  32  c.c,  11  out  of  the  16  catches  were  of  1  c.c. 
or  greater,  averaging  nearly  6  c.c,  which  suggests  that  a  considerable  population 
of  it  exists  along  the  continental  slope.  But  the  records  for  elongatus  were  for 
strays  only. 

Euchirella  rostrata 

Previous  records  of  this  copepod,  west  and  south  of  Cape  Cod,  were  con- 
fined to  the  extreme  outer  edge  of  the  shelf  and  continental  slope  (Bigelow,  1915; 
1922).  The  data  for  1929-1932  (Fig.  31B)  show,  however,  that  while  this  is  dis- 
tinctly an  offshore  species  within  our  hmits  (recorded  at  5%  only  of  the  inshore 
stations),  it  occurs  much  more  generally  in  the  offshore  belt  there  (38%  of  the 
stations)  than  it  does  in  the  Gulf  of  Maine,  where  it  appears  to  be  sharply  con- 
fined to  the  chief  drift-tracks  (Bigelow,  1926,  Fig.  71;  Fish  and  Johnson,  1937, 
Fig.  25).  It  has  also  averaged  considerably  more  frequent  in  the  northern  sector 
(26%  of  the  stations)  than  in  the  southern  (12%)  for  all  cruises  combined.  And 
the  fact  that  it  was  recorded  about  as  frequently,  relatively,  in  one  month  as  an- 
other from  April  to  July  (17-32%  of  the  stations),  but  not  at  all  in  February,  on 
the  one  hand,  or  in  October  on  the  other,  equally  identifies  it  as  a  spring  and 
summer  species  in  our  waters. 

Only  two  of  the  catches  were  larger  than  1  c.c,  with  6  c.c.  as  a  maximum,  all 
other  records  being  of  stray  individuals  only.  Thus,  we  have  no  reason  to  sup- 
pose that  it  ever  succeeds  in  breeding  in  significant  amounts  inside  the  conti- 
nental edge,  even  at  the  warmest  season,  but  rather  that  its  status  there  is 
strictly  that  of  a  warm  oceanic  immigrant,  which  makes  it  a  useful  indicator- 
species. 

Mecynocera  clausi 

It  appears  that  M.  clausi,  like  Centropages  violaceus  and  Pleuromamma 
gracilis,  strays  in  over  the  shelf  about  as  frequently  in  the  north  as  in  the  south. 
And  the  records  show  that  it  is  as  liable  to  drift  farther  in  than  do  either  of  these, 
as  is  also  the  case  in  the  Gulf  of  Maine,  where  it  has  been  taken  near  land,  on 
three  occasions  (Bigelow,  1926,  p.  245).  Even  for  it,  however,  the  great  majority 
of  the  captures  were  more  than  30  miles  from  the  coast  (Fig.  33A).  It  also  oc- 
curs, at  times,  in  moderate  numbers,  for  catches  of  1-4  c.c.  were  recorded  on  6 
occasions,  11  c.c,  once.    The  percentage  of  stations  at  which  it  was  recorded 


BIGELOW   AND    SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


329 


(30%  for  February,  0%  for  April,  6-14%  for  May-July,  but  87%  for  October) 
may  indicate  a  greater  tendency  for  it  to  drift  shoreward  at  the  end  of  winter 
and  in  autumn  than  during  the  intervening  period.  We  suspect,  however,  that 
these  seasonal  differences  are  evidence  of  irregular  fluctuations,  rather  than  of 
any  definitely  seasonal  succession. 

Metridia  lucens 

Frequency.  The  records  for  M.  lucens  show  wide  differences  on  individual 
cruises  in  the  frequency  and  generality  of  its  distribution,  the  one  extreme  being 
illustrated  by  April,  1929,  when  it  occurred  at  only  27%  of  the  stations,  the  other 
by  February  1932,  when  it  was  at  88%.  Neither  does  any  definitely  seasonal 
cycle  appear  in  this  respect  for  the  area  as  a  whole,  monthly  averages  being; 
65%  for  February;  53%  for  April;  59%  for  May;  57%  for  June;  55%  for  July; 
and  70%  for  October. 

But  Metridia  was  not  only  more  frequent  ofTshore  than  inshore  in  every 
month  and  year,  as  listed  below,  but  was  lacking  altogether  over  larger  or  smaller 
areas  next  to  the  land  on  most  of  the  cruises.  The  variations  in  the  inshore 
boundary  to  its  occurrence  have  been  so  considerable  from  cruise  to  cruise  and 
from  year  to  year  that  it  is  difficult  to  reduce  them  to  any  regular  order.  In 
three  years,  however,  out  of  the  four,  the  barren  belt  next  the  land  was  most  ex- 
tensive in  February  (1930,  1931)  or  April  (1929,  Fig.  32).  But  Metridia  was 
found  close  in  to  the  land  at  one  locality  or  another  by  May  in  1929  and  1931, 
while  in  1930,  it  was  at  most  of  the  inshore  as  well  as  the  offshore  stations  by  the 
last  part  of  April  (Fig.  32).  In  these  same  years,  this  shoreward  encroachment 
was  then  followed  by  a  retreat  from  the  land,  from  May  to  June,  most  pro- 
nounced in  1929  and  1930,  but  suggested  also  in  1931.  In  the  fourth  year  of 
the  series  (1932),  Metridia  was  much  more  general  inshore  at  the  end  of  the 
winter  than  in  the  other  years,  but  was  absent  from  belts  of  fluctuating  extent 
next  the  land  through  May  and  June. 


Frequency  ratio  of  Metridia  lucens  offshore  relative  to  inshore 

Month 

1929 

1930 

1931 

1932 

Average 

February 

April «. 

May 

June 

July 

Average 

1  toO 
1  to  0.5 
1  to  0.4 
1  to  0.2 
1  to  0.3 

1  to  0.3 
1  to  0.7 
1  to  0.7 
1  to  0.2 
1  to  0.6 
1  to  0.5 

1  to  0.2 

1  to  0.2 
1  to  0.6 
1  to  0.6 
1  to  0.5 

1  to  0.8 

1  to  0.7 
1  to  0.7 

1  to  0.7 

1  to  0.4 
1  to  0.5 
1  to  0.5 
1  to  0.4 
1  to  0.5 

330 


memoir:  museum  of  comparative  zoology 


40 


Fig.  32.  Areas  of  occurrence  of  Metridia  lucens:  A,  April  19-24  (hatched)  and  May  10-18  (dotted),  1929; 
B,  May  28- June  5  (dotted)  and  July  11-August  1  (hatched),  1929;  C,  February  5-13  (dotted) 
and  April  3-May  1  (hatched),  1930;  D,  May  12-23  (hatched)  and  June  7-18  (dotted),  1930;  E, 
February  13-March  5  (hatched)  and  May  16-22  (dotted),  1931;  F,  June  12-19  (hatched)  and 
July  10-16  (dotted),  1931. 


BIGELOW   AND   SEARS :   NORTH    ATLANTIC    ZOOPLANKTON   STUDIES 


331 


These  alternate  states  suggest  that  Mctridia  is  to  be  expected  near  land  only 
locally,  if  at  all,  at  the  end  of  a  normally  cold  winter,  but  that  it  tends  to  spread 
close  in  to  the  coast  line  through  considerable  sectors  at  some  time  in  the  spring, 
to  disappear  again  progressively,  from  parts  of  the  inshore  belt,  during  the  sum- 
mer. After  a  warm  winter,  Metridia  may,  however,  be  widespread  inshore  as 
early  as  February,  and  the  tendency  toward  local  disappearance  from  the  coast- 
wise belt  may  become  effective  as  early  as  the  middle  of  May.  Minor  or  short- 
period  oscillations  in  the  inshore  boundary  to  the  occurrence  of  this  copepod, 
no  doubt  reflect  the  appearances  and  disappearances  of  successive  broods,  or 
incursions  from  the  offshore  reservoir  where  the  species  is  constantly  present. 

Metridia  also  averaged  consistently  more  frequent  in  the  north  than  in  the 
south  in  spring  and  summer,  in  the  following  ratios : 


Frequency  ratio  of  Metridia  lucens,  north  relative  to  south 


Month 

Year 

Month 

Year 

February 

1930 

1  to  1.0 

June 

1929 

1  to  0.4 

(t 

1931 

1  to  2.7 

n 

1930 

— 

tt 

1932 

1  to  0.8 

t( 

1931 

1  to  0.1 

Average 

1  to  1.5 

tt 

1932 

1  to  0.7 

April 

1929 

1  to  0.2 

Average 

1  to  0.5 

It 

1930 

1  to  0.9 

July 

1929 

1  to  0.8 

Average 

1  to  0.5 

It 

1930 

— ■ 

May 

1929 

1  to  0.7 

tt 

1931 

— 

ti 

1930 

1  to  0.7 

October 

1931 

1  to  0.9 

II 

1931 

1  to  0.1 

(t 

1932 

1  to  0.9 

Average 

1  to  0.6 

Its  frequency  indeed,  averaged  as  high  at  this  season  (70-100%)  as  that 
of  Calanus  (p.  305)  in  the  northern  offshore  sector.  But  the  fact  that  it 
averaged  rather  more  frequent  in  the  south  than  in  the  north  in  February  (3 
years)  suggests  a  seasonal  reversal  in  this  respect. 

Abundance.  Metridia  averaged  considerably  more  abundant  (volumetri- 
cally)  offshore  than  inshore,  on  15  of  the  16  surveys  (Table,  p.  332),  as  well  as 
more  frequent,  the  sole  exception  being  in  February  1932.  And  the  offshore-in- 
shore ratio  of  about  5  to  1  that  results  for  the  series  as  a  whole  is  perhaps  a  rea- 
sonable expectation  for  the  vernal  six  months  combined,  taking  one  year  with 
another.  The  tabulation  suggests,  however,  that  there  may  be  some  tendency 
for  this  regional  contrast  to  reach  a  maximum  in  April  or  May,  in  which  months 
the  ratios  of  volume  ofTshore  to  inshore  averaged,  respectively,  about  12  to  1 


332 


memoir:  museum  of  comparative  zoology 


and  about  8  to  1,  contrasted  with  average  ratios  of  about  5  or  6  to  1  in  June  and 
July,  and  about  1.2  to  1  in  February. 

Average  volumes  of  Metridia  lucens 


Month 

Year 

Inshore 

Offshore 

North 

South 

Area 

Maxi- 

Mini- 

surveyed 

mum 

mum 

February 

1930 

<1 

6 

5 

2 

6 

42 

0 

1931 

<1 

23 

2 

20 

13 

78 

0 

19.32 

34 

13 

7 

33 

26 

165 

0 

April 

1929 

0 

19 

13 

3 

8 

106 

0 

1930 

16 

78 

71 

26 

47 

272 

0 

May 

1929 

2 

6 

5 

2 

4 

37 

0 

1930 

10 

68 

57 

9 

36 

168 

0 

1931 

2 

59 

31 

14 

26 

193 

0 

1932 

8 

47 

28 

17 

25 

220 

0 

Juno 

1929 

11 

27 

11 

9 

11 

210 

0 

1930 

<1 

24 

11 

2 

9 

83 

0 

1931 

6 

28 

17 

9 

14 

159 

0 

1932 

5 

35 

13 

13 

15 

189 

0 

July 

1929 

<1 

9 

6 

<1 

3 

71 

0 

19.30 

<1 

23 

13 

— 

13 

102 

0 

1931 

3 

6 

4 

— 

4 

20 

0 

October 

1931 

17 

26 

4 

17 

104 

0 

It  is  not  astonishing  in  view  of  the  foregoing  that  nearly  all  the  large  catches 
(  >  100  c.c.)  of  Metridia  were  made  more  than  35  miles  out  from  land,  and  north 
of  Latitude  38°  (Fig.  3 ID),  the  only  exception  being  that  two  large  catches  were 
made  close  inshore,  off  Currituck  and  Bodie  Island  in  February  1932.  Equally 
demonstrative  of  the  offshore  nature  of  Metridia  is  the  contrasting  fact  that  only 
18  out  of  204  stations,  within  20  miles  of  land  yielded  volumes  as  large  as  5  c.c, 
whatever  the  season  of  the  year,  most  of  these  being  northward  from  the  offing 
of  Delaware  Bay.  Indeed,  no  one  of  the  32  stations  near  Chesapeake  Bay,  or 
thence  northward  along  the  coast  of  Virginia  yielded  as  much  as  20  c.c,  while 
Metridia  failed  altogether  at  twenty-five  of  them,  marking  this  as  the  most  barren 
region  for  this  species.  Unfortunately,  no  information  is  available  in  this  respect 
for  autumn,  the  one  October  cruise  having  been  confined  to  the  offshore  belt. 
The  volume  of  Metridia  has  also  averaged  greater — usually  more  than  twice  as 
great — in  the  north  than  in  the  south  in  every  individual  month,  April  to  July, 
excepting  in  June  1932,  when  it  averaged  about  the  same  in  the  one  sector  as  in  the 
other.  But  the  fact  that  an  even  stronger  contrast  of  the  reverse  sense  obtained  in 


BIGELOW   AND    SEARS:   NORTH    ATLANTIC    ZOOPLANKTON   STUDIES 


333 


February,  in  two  ycar.s  out  of  (lie  thnn",  .suggests  a  regularly  seasonal  reversal  in 
this  respect.  Monthly  changes  in  the  volume  of  Metridia  have  been  so  irregular 
and  so  small  inshore,  that  the  ostensible  succession  there  may  not  be  seasonally 
significant.  But  the  volume  of  this  copepod  has  averaged  2-11  times  as  plentiful 
(by  volume)  in  the  offshore  belt  in  April,  May,  or  June,  as  in  February,  in  at  least 
three  out  of  the  four  years,  and  maximum  catches  have  shown  a  corresponding 
monthly  contrast,  suggesting  that  more  or  less  vernal  augmentation  is  characteris- 
tic for  it  in  the  zone  of  chief  abundance. 

Maximum  volumes  of  M.  lucens 


Month 

1929 

1930 

1931 

1932 

February 

— 

42 

78 

13 

April 

106 

272 

— 

— 

May 

21 

168 

193 

47 

June 

210 

83 

1.59 

35 

July 

71 

102 

18 

— 

Present  indications  are  that  this  offshore  augmentation  may  culminate  as 
early  as  April  (e.g.,  1930),  or  by  June,  at  latest  (e.g.,  1929),  and  that  it  is  then 
usually  followed  by  some  impoverishment.  In  some  years  (1931,  for  example), 
partial  recovery  then  follows  in  the  offshore  belt  by  October.  But  our  failure  to 
find  Metridia,  at  all,  in  the  November  catches  of  1916  suggests  that  in  other 
years,  it  may  continue  extremely  scarce  right  through  the  autumn.  And  we  lack 
quantitative  data  for  December  or  for  January. 

Annual  variations.  The  differences  that  have  been  recorded  from  year  to 
year,  in  the  in.shore  boundary  to  the  regular  occurrence  of  Metridia  in  different 
months  are  discussed  above  (p.  329).  We  should  also  point  out  that  the  volume 
of  Metridia  averaged  rather  more  than  three  times  as  large  for  the  area  as  a 
whole  in  the  richest  years  (average,  about  22  c.c.  in  1930  and  1931,  all  cruises 
combined)  as  in  the  poorest  (1929,  average,  about  6  c.c). 


OlTHONA  sp. 

The  copepods  of  this  genus  are  so  small  (about  0.75-1.5  mm.  long  when  adult) 
that  consequently  even  its  adults  may  have  passed  through  the  nets.  And  because 
Oithona  never  formed  as  much  as  1%  of  the  catch  in  any  subdivision,  we  have 
not  attempted  the  time-consuming  task  of  estimating  the  relative  abundance  of 
the  two  species,  similis  and  brevicornis,  that  have  long  been  known  to  occur  in 
the  area. 


334  memoir:  museum  of  comparative  zoology 

Oithona,  of  one  species  or  other,  was  recorded  in  the  catches  at  92%  of  the 
stations  in  February,  59%  in  April,  51%  in  May,  36%  in  June,  53%  in  July,  and 
57%  in  October,  proving  that  it  is  as  regular,  and  probably  as  frequent  an  in- 
habitant of  our  area  as  it  is  of  the  Gulf  of  Maine  and  Bay  of  Fundy  (Fish,  1936b), 
also  about  as  frequent,  north  (49%  of  the  stations)  as  south  (54%),  and  offshore 
(48%)  as  inshore  (53%).  Oithona  must,  in  fact,  be  extremely  numerous  at  times, 
for  on  one  occasion  (off  Bodie  Island,  in  February  1932),  no  less  than  30  c.c,  of 
these  tiny  copepods  were  entangled  in  our  coarse-meshed  net.  But  most  of  the 
other  catches  were  less  than  1  c.c. — often  odd  specimens  only.  And  the  very 
strong  probabihty  that,  in  many  cases,  a  good  part  of  the  local  stock  passed 
through  the  nets,  prevents  us  from  drawing  any  conclusions  as  to  seasonal  varia- 
tions in  abundance,  from  the  monthly  variations  in  frequency  of  occurrence. 
As  a  case  in  point,  we  may  quote  a  frequency  of  86%  on  the  second  June  cruise 
of  1932,  contrasting  with  0%  on  the  third  cruise  that  same  month,  evidence,  per- 
haps, of  the  presence  of  a  strong  stock  of  adults  on  the  one  occasion,  but  of  their 
replacement  by  juveniles,  by  the  time  of  the  second. 

Oithona,  being  so  small,  is  not  likely  ever  to  form  any  considerable  per- 
centage of  the  volume  of  the  total  plankton  present  in  our  waters.  But  from 
experience  elsewhere,  and  from  its  frequency  of  occurrence,  it  may  well  be  a 
major  item  in  the  local  diets  of  larval  fishes. 

Oncaea  sp. 

The  grouping  of  the  localities  of  capture  of  the  scattering  specimens  of 
Oncaea  (Fig.  31C)  suggests  that  most  of  them,  at  least,  belong  to  the  widely  dis- 
tributed pelagic  species,  0.  venusta,  which  Wilson  (1932a)  has  already  reported 
in  some  abundance  over  the  outer  edge  of  the  shelf,  off  Chesapeake  Bay,  but  not 
within  the  latter.  Apparently,  this  is  a  warm-water  stray  within  our  limits. 
But  it  may  be  of  greater  faunistic  importance  there,  on  occasion,  than  might  be 
suggested  by  the  fact  that  our  records  are  all  based  on  stray  individuals,  for  our 
nets  did  not  adequately  sample  animals  so  small. 

Pleuromamma 

Two  species  of  Pleuromamma  were  detected  in  the  catches,  robusta  and 
gracilis,  the  only  previous  reports  of  which,  inside  the  200-meter  contour,  within 
the  limits  of  the  present  survey,  were  near  Martha's  Vineyard  (Wilson,  1932), 
though  both  of  them  enter  the  Gulf  of  Maine  not  infrequently,  as  strays  from  the 


BIGELOW   AND    SEARS:    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES  335 

continental  slope  (Bigelow,  1926;  Fish  and  Johnson,  1937).  P.  gracilis  was  taken 
not  uncommonly  on  the  outer  edge  of  the  shelf  (Fig.  33),  but  only  five  of  the  40 


!6'  ry  74-  ?}•  ?!• 

Fig.  33.  Locality  records,  all  cruises  combined:  A,  Mecynocera  dausi;  B,  Pleuromamma  gracilis  and  P. 
robusta;  C,  Paraeuchaela  norvegica;  D,  large  volumes  of  Pseudocalanus. 

records  of  it  were  more  than  10  miles  in  from  the  200-meter  contour,  nor  have  we 
any  evidence  that  it  ever  strays  shoreward  farther  than  the  mid-belt  of  the  shelf. 


336  memoir:  museum  of  comparative  zoology 

P.  robusta,  on  the  other  hand,  was  recorded  about  as  often  in  the  inshore  belt 
(4  stations)  as  offshore  (5  stations),  once  close  in  to  land  on  the  Montauk  profile 
(Fig.33B). 

Since  it  seems  certain  that  both  these  species  enter  our  waters  from  offshore, 
this  distributional  contrast  suggests  that  gracilis  is  the  more  sensitive  of  the  two, 
to  the  conditions  it  encounters  in  over  the  shelf. 

The  monthly  distribution  of  the  captures  suggests  that  P.  gracilis  crosses 
the  offshore  boundary  of  the  shelf  much  more  often  in  late  winter  and  early 
spring,  and  again  in  mid-autumn  than  during  the  intervening  season,  for  it  was 
taken  at  30-37%  of  the  stations  in  the  offshore  belt  in  February,  April,  and 
October,  but  at  17%  only  in  May,  and  3%  in  June  and  July.  Catches  of  1  c.c. 
or  more  (15  in  number)  were  correspondingly  confined  to  the  months  of  February, 
April,  May,  and  October.  And  the  fact  that  the  maximum  volume  of  P.  gracilis 
was  only  10  c.c.  (off  Montauk,  May  21,  1931)  is  good  evidence  that  it  would  be 
a  most  unusual  event  for  this  species  ever  to  develop  a  rich  center  within  our 
limits.  In  the  case  of  P.  robusta,  the  captures  were  confined  to  the  months  of 
February-June.  But  the  number  of  captures  was  not  large  enough  to  warrant 
any  conclusion  as  to  seasonal  gradient  within  this  period.  And  the  records  are 
based  in  each  case  on  stray  specimens. 

Paracalanus  parvus 

This  copepod,  hke  Pseudocalanus,  is  so  small  that  our  nets  could  be  expected 
adequately  to  sample  the  adults  alone.  Even  with  this  reservation,  however,  the 
record  points  to  very  wide  fluctuations  from  month  to  month  and  year  to  year 
both  in  its  frequency  of  occurrence,  and  in  its  abundance.  In  1929,  for  example, 
it  was  not  detected  at  all  in  April,  May,  or  June,  but  was  found  at  100%  of  the 
stations  inshore  and  02%  offshore  in  July,  when  it  averaged  2  c.c.  in  volume  over 
the  area  as  a  whole.  But  it  seems  to  have  vanished  entirely  from  our  area  at  some 
time  during  the  subsequent  autumn  or  winter,  for  it  was  not  found  at  all  in  1930, 
and  at  one  station  only,  dui'ing  the  late  winter,  spring  or  summer  of  1931.  But 
it  was  at  every  station  visited  that  October,  averaging  62  c.c.  in  volume.  And  it 
may  have  persisted  through  the  following  winter  for  it  was  at  86%  of  the  stations 
in  the  next  February  (1932),  averaging  26  c.c,  and  about  as  frequent  in  one  sub- 
division of  the  area  as  in  another.  However,  it — or  its  adults,  at  any  rate — had 
entirely  vanished,  by  the  following  May  and  June. 

In  the  absence  of  any  knowledge  as  to  the  abundance  of  its  young  stages,  it 
is  an  open  question  whether  these  fluctuations  in  its  status  are  connected  with 


BIGELOW   AND    SEARS :   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


337 


the  incidence  of  successive  broods,  or  wlietlier  they  are  the  visible  evidence  of 
extreme  susceptibihty,  on  the  part  of  this  particular  species,  to  the  favorability, 
or  the  reverse,  of  its  external  surroundings.  In  either  case,  it  is  clear  that  the  pro- 
duction of  a  large  crop  of  adults  is  a  decidedly  unusual  event  within  our  limits. 
But  when  this  does  occur,  which  may  be  either  in  late  winter,  in  high  summer,  or 
in  autumn,  the  resultant  volumes  may  be  so  large  as  to  make  Paracalanus  an 


WLg^   "" 

BW^^^^        ■13'' 

■M                 .B6    1^7      'V 

w 

f    / 

t 

89,..-' 

/ 

9.7  . 

-'' 

130-/ 

-1 

4  0 


Fig.  34.  Volumes,  per  standard  haul,  of  Paracalanus,  all  cruises  combined. 

important  item  in  the  total  community,  for  the  time  being;  and  perhaps  as 
important  in  one  subdivision  as  in  another,  for  the  concentration  of  rich  catches 
offshore  shown  in  Fig.  34  may  reflect  simply  the  failure  to  survey  the  inshore 
belt  in  the  month  (October  1931)  when  this  copepod  was  the  most  abundant 
(p.  336).  The  largest  catches  for  February  were  in  the  south,  those  for  October  in 
the  north,  the  maximum  so  far  recorded  being  134  c.c.  (Fig.  34). 


Paraeuchaeta  norvegica 

This  large  copepod — a  familiar  inhabitant  of  mid-depths  in  boreal  seas — is 
one  of  the  most  characteristic  members  of  the  plankton  in  the  deep  basin  of  the 
Gulf  of  Maine,  where  females  with  their  blue  egg  clusters  are  famiUar  objects, 
and  larvae  are  plentiful  (Campbell,  1934,  p.  4,  34).  It  also  occurs  with  consider- 
able frequency  from  the  offing  of  Martha's  Vineyard  westward  and  southward 
(Fig.  33C)  to  that  of  Chesapeake  Bay,  outside  the  100-meter  contour,  where 
Wilson  (1932a)  also  reports  it.    But  it  so  seldom  drifts  farther  in  on  the  shelf, 


338  memoir:  museum  of  comparative  zoology 

that  while  taken  at  23%  of  the  stations  offshore,  it  was  at  only  2%  inshore  within 
our  limits.  When  this  does  happen  it  is  about  as  apt  to  be  off  one  part  of  the 
coast  as  off  another;  stray  specimens,  indeed,  may  come  close  in  to  the  land,  and 
consequently  into  very  shoal  water  on  rare  occasions,  as  may  any  other  oceanic 
waif. 

The  percentages  of  the  stations  at  which  P.  norvcgica  was  taken  in  the  off- 
shore belt  in  different  months  point  to  a  rather  regular  increase  in  the  frequency 
of  its  occurrence  from  February  (23%)  through  April  (32%)  to  May  (49%.),  fol- 
lowed by  a  decrease  in  June  (14%)  and  July  (12%),  leading — by  present  data — 
to  a  complete  disappearance  from  this  sector  of  the  shelf  in  the  autumn,  for  it 
was  not  found  anywhere  inside  the  200-meter  contour  at  this  season  either  in 
1916  or  in  1931.  And  fluctuations  of  this  sort  are  of  special  interest  in  the  case 
of  this  particular  species,  because  of  its  rehability  as  an  indicator  of  continental 
slope  water  within-our  limits. 

Paraeuchaeta  has  proved  entirely  negligible  from  the  volumetric  standpoint, 
throughout  the  entire  series  of  observations,  for  the  records  have  been  of  stray 
specimens  only,  irrespective  of  the  time  of  year. 

PSEUDOCALANUS  MINUTUS 

Frequency.  Pseudocalanus  was  detected  at  about  82%  of  our  stations, 
evidence  that  it  is  one  of  the  most  regularly  distributed  species  over  the  area  as 
a  whole,  and  one  of  the  most  nearly  universal  there.  But  this  copepod  is  so  small 
(adults  average  only  about  1-1.6  mm.  in  length)  that  most  of  the  younger  indi- 
viduals and  even  a  considerable  percentage  of  adults  may  have  passed  through 
the  nets.  It  is  therefore  possible  that  an  increase  in  recorded  frequency  from  an  av- 
erage of  79%  of  the  stations  in  February,  to  84%  in  April,  and  92%  in  May,  fol- 
lowed by  a  decrease  to  74%  in  June  and  60%,  in  July,  but  by  a  subsequent  increase 
to  73%  in  October  may  have  been  due  to  seasonal  alterations  in  the  relative 
abundance  of  the  older  stages  (that  were  caught)  and  of  the  juveniles  (that  were 
not),  rather  than  to  corresponding  fluctuations  in  the  actual  frequency  of  the 
species. 

The  average  frequency  was  about  the  same  inshore  (80%)  as  offshore  (77%) ; 
about  the  same  also  for  the  southern  sector  as  a  whole  (75%)  as  for  the  northern 
(81%),  right  down  to  the  offing  of  Chesapeake  Bay,  where  Pseudocalanus  was 
found  at  77%  of  the  stations.  But  the  fact  that  it  proved  somewhat  less  frequent 
farther  south,  and  was  lacking  at  four  out  of  the  five  stations  near  Cape  Hatteras 


BIGELOW   AND   SEARS :    NORTH   ATLANTIC   ZOOPLANKTON   STUDIES  339 

(February  1931),  suggests  that  the  southern  boundary  of  regular  occurrence  for 
it  lies  not  many  miles  to  the  south  of  the  Chesapeake  Bay  profile,  as  also  for 
sundry  other  boreal  animals. 

Abundance.  Pseudocalanus,  though  so  nearly  universal,  was  usually  pres- 
ent in  only  small  volumes,  as  emphasized  above  (Table,  p.  229),  in  the  discussion 
of  the  relative  importance  of  different  species.  In  fact,  the  catch  was  at  the  rate 
of  not  more  than  1  c.c.  at  about  J^  (109)  of  the  410  stations  where  this  copepod 
was  recorded,  while  only  eight  of  the  catches  were  greater  than  100  c.c,  three 
greater  than  200  c.c,  with  560  c.c.  as  the  maximum.  But  for  the  reason  just 
stated,  it  is  hkely  that  this  understates  the  actual  abundance  of  this  species  (see 
also,  Fish,  1936a). 

The  average  catches  for  the  area  as  a  whole  parallel  the  monthly  frequencies 
(p.  338),  in  showing  a  slight  increase  from  February  (average,  10  c.c.)  to  April 
(14  c.c.)  and  May  (15  c.c.)  followed  by  a  slight  decrease  through  June  (12  c.c) 
to  July  (10  c.c).  And  while  the  record  for  October  is  confined  to  a  single  year 
(1931),  the  low  average  for  that  cruise  (5  c.c),  as  contrasted  with  the  preceding 
July  (14  c.c),  probably  represents  the  normal  seasonal  cycle,  for  that  year  was 
the  most  productive  of  the  series  for  this  particular  copepod.  Adults  of  Pseudo- 
calanus thus  appear  to  average  most  numerous  over  the  area  late  in  spring,  much 
less  so  in  autumn,  though  with  some  recovery  through  the  winter.  And  while  so 
few  juveniles  were  taken  that  they  do  not  figure  to  any  significant  degree  in  this 
summary,  they  are  individually  so  small  that  their  omission  would  not  greatly 
affect  the  volumetric  picture. 

Average  catches  for  February-June  (1929-1932)  combined,  of  14  c.c.  in  the 
inshore  belt,  9  c.c.  offshore,  11  c.c  in  the  northern  sector,  and  9  c.c.  in  the  southern 
do  not  difTer,  one  from  the  other  more  widely  than  might  be  expected  from  the 
roughness  of  the  method.  Neither  do  the  small  catches  made  in  July  of  1929  or 
1930  show  any  definite  regional  gradient,  though  the  waters  in  the  north  aver- 
aged some  7  times  as  productive  inshore  (21  c.c.)  as  ofTshore  in  that  month  of 
1931.  And  it  is  doubtful  whether  the  contrast,  between  greater  abundance  in  the 
south  (10  c.c)  than  in  the  north  (3  c.c)  in  the  offshore  belt  in  the  following 
October,  can  be  taken  as  characteristic  for  the  time  of  year.  In  short,  we  have  no 
definite  evidence  that  any  one  part  of  the  area  averages  significantly  more  pro- 
ductive of  Pseudocalanus  than  another.  But  the  distribution  of  the  richer  catches 
and  especially  of  the  few  that  were  more  productive  than  100  c.c.  (Fig.  33)  sug- 
gests that  it  is  rather  more  common  for  localized  centers  of  abundance  to  develop 
within  30-35  miles  of  the  land  than  farther  out  on  the  shelf. 


340 


memoir:  museum  of  comparative  zoology 


Average  volumes  of  Pseudocalanus  for  area  surveyed 


Month 

1929 

1930 

1931 

1932 

February 
April 

May- June 
July 

4 
2 
1 

10 

24 

1 

1 

7 

31 
14 

13 
25 

Annual  variations.  The  volume  of  Pseudocalanus  averaged  about  twice  as 
large,  for  the  area  as  a  whole  in  the  richest  year  (1932,  13  c.c.)  as  in  the  poorest, 
at  the  end  of  winter,  while  May  and  June,  combined,  show  an  average  some  25-30 
times  as  great  in  two  of  the  years  as  in  the  other  two,  a  range  much  wider  than 
any  seasonal  or  regional  contrast  that  can  be  deduced  from  our  data.  This  marks 
this  species  as  one  of  the  most  variable  from  year  to  year,  of  those  that  figure 
regularly  in  the  catches. 

It  is  also  interesting  that  the  relative  ranking  of  a  given  year  as  to  the 
abundance  of  this  copepod  may  either  continue  low  or  high  right  through  the 
spring  and  early  summer  as  happened  in  1929  and  1932,  or  it  may  be  abruptly 
reversed  as  happened  in  1930,  when  Pseudocalanus  was  relatively  abundant  in 
February  and  April,  but  very  scarce  from  May  to  July. 


Rhincalanus  nasutus 

Frequency.  This  offshore  copepod  has  been  taken  widespread  throughout 
the  area  at  one  time  or  another  (Fig.  35A)  except  in  the  coastal  belt  between  the 
New  York  and  Martha's  Vineyard  profiles,  where  we  have  no  record  of  it.  But 
the  localities  of  record  have  been  chiefly  concentrated  in  the  offshore  belt,  as  was 
indeed  to  be  expected,  some  61%  of  the  catches  on  the  shelf  north  of  Latitude 
36°  N.,  having  been  made  within  30  miles  of  the  200-meter  contour.  It  is  no 
doubt  a  corollary  to  the  narrowness  of  the  shelf  to  the  southward,  that  Rhin- 
calanus appears  much  more  frequently  near  land,  south  of  Delaware  Bay,  than 
to  the  northward. 

Rhincalanus  has  averaged  about  equally  frequent  in  February,  April,  and 
May,  but  progressively  less  so  in  June  and  July,  and  this  seasonal  decline  in  the 
regularity  of  occurrence  takes  place  both  earlier  inshore,  i.e.,  between  February 
and  April,  and  is  more  abrupt  there  than  offshore,  with  no  apparent  difference 
in  this  respect  between  the  northern  and  southern  sectors,  as  follows: 


BIGELOW   AND    SEARS!    NORTH    ATLANTIC    ZOOPLANKTON   STUDIES  341 


Fig.  35.  Locality  records,  all  cruises  combined:  A,  Rhincalanus  nasulus;  B,  Temora  longicornis  inshore 
and  offshore;  C,  Temora  stylifera  and  Scoledthrix  danae;  D,  Evadne  and  Podon. 


342 


memoir:  museum  of  comparative  zoology 


Percentage  of  stations  with  Rhincalanus 

Month 

Offshore 

Inshore 

North 

South 

February 

57% 

53% 

49% 

55% 

April 

52 

4 

22 

32 

May 

51 

15 

25 

41 

June 

37 

5 

11 

31 

July 

18 

0 

— 

— 

October 

66 

— 

— 

— 

The  evidence  from  1931  is  that  the  frequency  of  Rhincalanus  rises  again 
during  the  early  autumn  in  some  years — at  least  in  the  offshore  belt  to  which  the 
data  for  that  October  were  confined.  But  it  was  not  detected  at  all  within  our 
limits  in  November  in  1916,  showing  that  conditions  may  vary  widely  in  this 
respect  from  year  to  year.  We  have  no  information  as  to  the  status  of  Rhin- 
calanus in  any  part  of  our  area  during  the  first  two  months  of  winter. 

Abundance.  The  great  majority  of  the  records  for  Rhincalanus  inside  the 
200-meter  contour  have  been  based  on  occasional  individuals,  and  this  large 
copepod  is  so  easily  recognized  that  the  distributional  picture  is  no  doubt  more 
nearly  complete  for  it,  than  for  other  less  conspicuous  strays.  In  fact,  Rhinca- 
lanus never  averaged  as  much  as  1%  of  the  catch  for  any  cruise,  even  in  the  off- 
shore belt.  A  catch  of  74  c.c.  was  made  on  one  occasion,  however,  with  an  aver- 
age of  40  c.c.  at  three  stations  near  the  200-meter  contour  from  the  offing  of 
Delaware  Bay  to  that  of  Chesapeake  Bay  (May  2-6,  1932),  evidence  that  it  may 
occasionally  be  of  considerable  volumetric  importance,  within  short  sectors  along 
the  continental  edge — doubtless  over  the  slope  as  well.  And  when  this  does 
happen,  Rhincalanus  may  be  expected  to  form  an  important  item  in  the  diet  of 
any  local  pelagic  fishes,  thanks  to  its  large  size.  Catches  of  5  c.c.  or  more  have 
also  been  recorded  more  frequently  in  the  southern  sector  (8  cases)  than  in  the 
northern  (2  cases). 

It  appears  that  Rhincalanus  may  be  expected  to  invade  the  shelf  in  appre- 
ciable quantity  more  often  in  late  spring,  than  either  in  winter,  on  the  one  hand, 
or  in  summer  or  autumn,  on  the  other,  3  catches  larger  than  5  c.c.  having  been 
made  inside  the  200-meter  Une  in  April,  and  seven  in  May,  but  only  one  in  July 
and  none  in  October,  November,  or  February. 

The  distributional  picture,  as  outlined  above,  seems  sufficient  ground  for 
concluding  that  Rhincalanus  enters  our  area  only  as  a  stray  from  the  waters  of 
the  continental  slope.  There  is  no  evidence  that  it  can  maintain  itself,  anywhere 
over  the  continental  shelf  north  of  Cape  Hatteras. 


BIGELOW   AND    SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  343 

SCOLECITHRIX  DANAE 

The  only  previous  records  of  this  oceanic  copepod  off  the  east  coast  of  North 
America,  with  which  we  are  acquainted,  are  for  George's  Bank  and  for  the  offings 
of  Nantucket  and  Martha's  Vineyard  (Wilson,  1932,  p.  82).  The  present  records — 
nine  in  number  and  scattered  along  the  offshore  belt  from  the  offing  of  Currituck 
to  that  of  Montauk — show,  however,  that  it  is  to  be  expected  anywhere  on  the 
outer  part  of  the  shelf,  within  our  limits  (Fig.  35C).  Four  out  of  the  nine  records 
are  for  October,  though  one  cruise  (1931)  only  was  made  in  that  month  during 
the  four  years.  The  two  catches  of  more  than  minimal  numbers  (3  c.c.  in  each 
case)  that  were  made  inside  the  200-meter  contour  were  also  in  that  October, 
cumulative  evidence  that  this  oceanic  visitor  crosses  our  boundaries  most  often 
in  the  autumn,  but  never  in  any  great  frequency  or  abundance. 

Temora  longicornis 

In  the  winter — by  the  evidence  of  1931 — the  range  of  this  little  brown 
copepod  extends  southward  to  Cape  Hatteras  (Fig.  35B).  And  it  is  possible  that 
this  is  also  true  through  the  spring,  for  it  was  found  on  the  Bodie  Island  profile  on 
each  April  cruise  and  off  Currituck  on  the  one  May  cruise  (1929)  that  extended 
so  far  south.  In  June,  however,  it  was  taken  at  but  one  out  of  eight  stations  on 
the  Chesapeake  Bay  profile,  while  in  July,  it  was  recorded  southward  only  as  far 
as  Winterquarter  in  1929,  as  far  as  Fire  Island  in  1913,  and  as  far  as  New 
York  in  1916.  While  not  conclusive,  this  evidence  suggests  that  the  southern 
boundary  to  its  frequent  occurrence  tends  to  recede  northward,  during  the  sum- 
mer, though  to  different  degrees  in  different  years,  corresponding  to  which,  it 
was  recorded  at  22%  of  the  stations  in  the  northern  sector  (all  cruises  combined), 
but  at  14%  only  in  the  southern.  In  the  one  year  (1916),  however,  when  the 
inshore  waters  west  and  south  from  Cape  Cod  were  visited  in  autumn,  its  recorded 
range  extended  to  the  offings  of  Delaware  Bay  and  of  Chesapeake  Bay  in  Novem- 
ber (Bigelow,  1922,  p.  146). 

T.  longicornis  occurs  with  some  regularity  right  out  to  the  edge  of  the  conti- 
nent. But  it  is  much  more  frequent  inshore  (26%  of  the  stations)  than  offshore 
(11%),  which  agrees  with  its  status  in  the  Gulf  of  Maine,  where  it  has  its  center 
of  distribution  inside  the  100-meter  contour,  including  in  that  case  the  extensive 
shoal  ground  of  George's  Bank  (Bigelow,  1926,  Fig.  85).  It  may,  in  fact,  occur  as 
frequently  close  in  to  the  land  as  farther  out,  witness  the  frequency  of  records  at 
our  innermost  stations  off  Delaware  Bay  and  thence  north  and  east  (Fig.  35B), 


344 


memoir:  museum  of  comparative  zoology 


likewise  at  Woods  Hole  (Fish,  1925).  But  available  evidence  suggests  that  it 
does  not  regularly  penetrate  estuarine  situations  along  our  coasts,  unless  the 
renewal  of  water,  from  outside,  be  active,  for  while  it  occurs  abundantly  in 
Passamaquoddy  Bay,  tributary  to  the  Bay  of  Fundy  (McMurrich,  1917;  Bigelow, 
1926;  Fish  and  Johnson,  1937),  Cowles  (1930)  and  Wilson  (1932a)  report  it  in 
Chesapeake  Bay  only  at  stations  near  the  mouth. 

The  percentages  of  the  stations  at  which  Temora  was  recorded  north  and 
south,  in  different  months  (1929-1932),  tabulated  below,  show  it  as  averaging 
the  most  frequent  in  both  sectors  in  May,  followed  by  impoverishment  through 
the  summer,  much  more  extreme  in  the  south — as  just  outUned — than  in  the 
north.  The  evidence  of  1929  and  1930  combined,  also  is  that  Temora,  in  the 
north,  suffers  a  marked  impoverishment  for  a  time  in  early  spring,  of  which  no 
evidence  appears  in  the  south,  which  can  hardly  be  charged  to  invasions  by 
eastern  water,  because  Temora  is  relatively  high  in  frequency  in  the  Gulf  of 
Maine  at  that  same  season. 


Percentag 

;e  of  stations 

with  T.  long 

icornis 

February 

April 

May 

June 

July 

North 
South 

17% 
13 

3% 
12 

38% 
22 

19% 
12 

20% 
8' 

» 1929  only. 

During  the  cruises  of  1929-1930,  only  14  out  of  the  604  stations  yielded 
catches  of  Temora  as  large  as  1  c.c,  only  one  as  large  as  10  c.c,  with  13  c.c.  as 
the  maximum. 

The  most  interesting  feature  of  these  records  is  that  Temora  should  have 
invariably  been  insignificant  in  the  general  community,  throughout  our  area, 
during  every  cruise,  for  swarms  of  it  have  been  encountered  close  to  Martha's 
Vineyard,  July  26,  1916,  over  Nantucket  Shoals,  on  July  9,  1913,  and  near 
Gloucester,  on  October  31,  1916,  while  Fish  (192.5,  p.  143)  describes  the  winter 
samples  at  Woods  Hole  as  "literally  filled  with  them"  in  some  years.  Nor  does 
it  seem  likely  that  our  nets  can  have  consistently  missed  the  richer  aggregations 
of  Temora,  characteristic  though  it  be  of  the  latter  to  form  "swarms  of  great 
density  but  of  limited  extent"  (Farran,  1910,  p.  72),  when  one  considers  that 
hauls  were  made  at  so  many  stations  in  different  months  and  years. 

We  can  only  conclude  that  if  Temora  ever  does  occur  in  sufficient  volume 
within  our  limits  to  be  an  important  item  there,  this  happens  but  rarely.   And 


BIGELOW    AND    SEARS :    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES 


345 


this  prevailing  scarcity,  in  the  southern  part  of  its  range,  raises  the  question  of 
the  origin  of  the  local  stock,  for  while  we  have  encountered  no  centers  of  active 
reproduction,  the  distributional  picture  is  not  of  a  sort  to  suggest  regular  immi- 
gration, whether  out  from  the  coast  line,  or  from  the  more  abundant  stocks 
that  exist  in  winter  in  the  Woods  Hole  region  (Fish,  1925)  and  in  late  spring 
and  early  summer  in  the  Gulf  of  Maine. 


Temoba  stylifera 
The  only  previous  reports  of  this  wide-ranging  tropical  copepod  off  the  east 
coast  of  North  America  were  near  Cape  Sable,  Nova  Scotia  (Bigelow,  1926, 
p.  307),  and  at  six  "Albatross"  stations  in  the  general  vicinity  of  Nantucket  and 
Martha's  Vineyard  (Wilson,  1932).  The  records  for  1929-1932  now  show  that  it 
not  rarely  enters  our  area,  and  about  equally  frequent  inshore  and  offshore  (Fig. 
35C).  But  it  is  so  definitely  a  southern  species  there  that  only  two  out  of  the  22 
captures  of  it  were  north  of  Delaware  Bay — both  in  May — although  a  consider- 
ably greater  number  of  stations  were  occupied  in  the  northern  sector  than  in  the 
southern.  The  total  number  of  records  for  this  species  was  so  small  (22)  that  we 
doubt  whether  its  capture  at  50%  of  the  southern  stations  in  July  and  19%  in 
April,  but  at  only  2-6%  in  February,  May,  or  June,  and  not  at  all  in  October, 
represents  any  regularly  seasonal  cycle.  The  maximum  catch,  near  Winter- 
quarter,  July  1929  was  at  the  rate  of  26  c.c,  with  6  c.c.  and  9  c.c.  at  two  stations 
off  Chesapeake  Bay  in  that  same  month,  evidence  that  T.  stylifera  may  be  of 
faunistic  importance  locally  in  the  south  in  midsummer.  But  the  catches  in  other 
months  were  all  minimal. 

Other  copepods 
The  regional  distribution  of  records  for  other  copepods,  stray  specimens  of 
which  were  recorded  here  and  there,  is  summarized  in  the  following  table: 


Species 

Inshore 

Offshore 

Inshore 

Offshore 

North 

North 

South 

South 

Aetidius  armalus 

0 

0 

0 

3 

Calanus  minor 

0 

0 

0 

1 

Undinula  vulgaris 

0 

0 

0 

1 

Calocalanus  pavo 

1 

1 

0 

1 

Cenlropages  f  areata 

0 

0 

0 

1 

Euchaeta  marina 

0 

0 

0 

1 

Eurytemora  sp. 

1 

0 

1 

0 

Eurytemora  herdmani 

1 

0 

0 

0 

Pseudophaenna  typica 

0 

0 

0 

1 

Sapphirina  sp. 

0 

2 

0 

1 

346  memoir:  museum  of  comparative  zoology 

CLADOCERANS 

PODON  AND  EvADNE 

The  captures  of  Podon  and  Evadne  do  not  adequately  represent  the  volu- 
metric status  of  these  little  cladocerans  of  neritic  affinity,  for  they  are  so  small  that 
the  greater  part  of  the  existent  stocks  may  well  have  passed  through  the  nets. 
Nevertheless  the  records  for  Evadne,  between  the  offings  of  Martha's  Vineyard, 
and  of  Chesapeake  Bay,  are  generally  distributed  across  the  whole  breadth  of 
the  continental  shelf,  while  those  for  Podon  also  extend  some  40  miles  out  from 
the  coast  (Fig.  35D ) .  This  distribution  is  in  striking  contrast  to  the  situation  as 
existing  in  the  Gulf  of  Maine,  where  they  have  been  seldom  found  more  than 
15-16  miles  out  from  land  (Bigelow,  1926;  Fish  and  Johnson,  1937),  suggesting 
that  coastal  water  is  much  more  generally  dispersed  offshore  across  the  shelf 
west  and  south  of  Cape  Cod  than  is  the  case  to  the  east  of  the  latter. 

So  far  as  frequency  is  concerned,  the  monthly  averages  suggest  that  the 
winter  spores  of  Evadne  do  not  hatch  until  well  into  the  spring,  but  that  the 
peak  of  abundance  for  it  is  reached  very  soon  thereafter,  to  be  followed  by  a 
decline  in  frequency  through  the  summer,  for  the  genus  was  not  recorded  at  all 
in  February  and  April,  was  at  21%  of  the  stations  in  May,  but  at  only  13%  for 
June,  and  7%  for  July,  whereas  in  the  Gulf  of  Maine  the  peak  is  not  reached  until 
late  summer  and  early  autumn  (Bigelow,  1926,  p.  307).  The  data  for  October 
1931,  when  Evadne  was  detected  at  3  out  of  15  stations,  would  point  to  a  sec- 
ondary peak  in  autumn,  if  accepted  at  face  value.  But  we  hesitate  to  draw  the 
apparently  ob\dous  conclusion,  from  so  small  a  number  of  hauls,  all  from  the 
outer  half  of  the  shelf. 

The  records  for  Podon  are  scattered  through  May,  June,  July,  and  October. 
But  they  are  not  sufficiently  numerous  to  warrant  deduction  as  to  the  seasonal 
cycle  within  this  period. 

The  largest  catches  were  at  the  rates  of  6  c.c.  and  9  o.c.  for  Evadne  (both  in 
May),  of  1  c.c.  for  Podon,  in  October,  all  other  records  being  based  on  small 
numbers  of  individuals. 

Penilia 
Penilia  schmackeri  Richard,  was  so  abundant  midway  out  on  the  shelf  off 
Cape  May  on  October  23,  1931  (121  c.c.)  that  it  formed  21%  of  the  total  catch 
at  one  station  there,  though  detected  at  only  one  other  station  off  Winter  quarter 
on  that  cruise,  and  not  at  all  on  any  other.  Indeed,  the  only  previous  record  for 
it  on  the  Atlantic  coast  of  North  America  is  at  Beaufort,  North  Carolina,  where 


BIGELOW   AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  347 

according  to  Sudler  (1899,  p.  109),  it  sudik'nly  appeared  in  swarms  during  June 
1898,  to  disappear  as  suddenly.  IJut  the  fact  that  it  has  been  reported — always 
near  land — in  the  Straits  of  Gibraltar  and  the  Mediterranean,  along  the  east 
coast  of  South  America,  off  the  west  coast  of  Africa,  off  the  Cape  of  Good  Hope, 
off  India,  off  Austraha,  off  New  Zealand,  and  off  southern  China  (Gibitz,  1922; 
Ramner,  1933)  shows  that  it  is  to  be  expected  anywhere  on  the  continental 
shelves  in  warm  latitudes,  and  even  in  the  temperate  belt  in  the  warm  half  of 
the  year. 

CHAETOGNATHS 

EUKROHNIA  HAMATA 

Eukrohnia  hamata,  an  indicator  of  water  from  mid-depths  along  the  conti- 
nental slope  (Bigelow,  1922,  Fig.  50)  within  our  limits,  though  a  regular  in- 
habitant of  the  deep  water  of  the  Gulf  of  Maine  (Bigelow,  1926,  p.  328),  was 
recorded  at  22  stations  scattered  along  from  the  offing  of  Martha's  Vineyard  to 
that  of  Currituck,  all  but  one  of  them,  however,  within  some  20  miles  of  the 
continental  edge  (Fig.  36A).  The  seasonal  distribution  of  these  records  (Febru- 
ary, 0;  April,  16; May,  4;  June,  1;  and  July,  1)  suggests  that  this  species  is  much 
more  apt  to  invade  our  area  in  early  spring  than  at  any  other  time  of  year. 

In  each  case,  the  catch  was  of  stray  individuals  only. 

Sagitta  elegans 

Frequency.  Sagitta  elegans  rivals  Calanus  finmarchicus  in  its  frequency  of 
occurrence  throughout  our  area,  but  it  is  doubtful  whether  its  range  extends 
farther  south  than  Latitude  about  36°,  for  it  was  not  taken  near  Cape  Hatteras 
on  the  winter  cruise  of  1931,  though  present  at  every  station  to  the  northward  on 
that  occasion.  It  also  corresponds  to  Calanus  in  being  least  frequent  on  the  aver- 
age in  February  (67%  of  the  stations,  all  cruises  combined).  And  while  it  may  be 
widespread  southward  past  the  offing  of  Chesapeake  Bay,  at  the  end  of  winter,  in 
some  years  (e.g.,  1931),  the  offing  of  Delaware  Bay  may  be  the  southern  boundary 
to  its  range  at  that  season  in  others,  as  was  the  case  in  1932.  And  the  fact  that  it 
is  the  southern  margin  of  its  range  that  most  clearly  shows  these  variations  from 
winter  to  winter,  accords  with  the  general  rule  that  it  is  in  the  north  that  it  is 
most  abundant,  even  at  times  when  its  distribution  is  universal  to  the  south- 
ward, as  well. 

In  the  years  when  S.  elegans  is  scarce  or  absent  in  the  southern  sector  at  the 
end  of  winter,  its  range  expands  southward  as  the  season  advances,  as  happened 


348 


memoir:  museum  of  comparative  zoology 


Fig.  36.  Locality  records,   1929-1932;  A,  Eukrohnia  hamala;  B,  Sagilta  enflata;  C,  Krohnila  subiilis, 
Plcrosagilta  draco,  Sagitta  maxima  and  S.  hcxaplera;  D,  large  volumes  of  S.  elegans. 


BIGELOW    AND    SEARS!    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES  349 

in  1929,  when  its  boundary  shifted  about  thirty  miles  in  that  direction  between 
the  third  week  of  April  and  the  third  week  of  May;  and  again  in  1932,  when  it 
was  lacking  south  of  Delaware  Bay  in  February,  but  was  taken  at  scattered  sta- 
tions to  the  offing  of  Chesapeake  Bay,  in  May.  And  its  presence  at  74%  of  our 
stations,  for  April,  and  at  89%  for  May  (all  cruises  combined)  is  perhaps  a  fair 
index  to  the  normal  expectation,  taking  one  year  with  another.  S.  elegans  was 
taken  at  every  station  on  the  continental  shelf,  in  June,  down  to  the  offing  of 
Chesapeake  Bay  in  1929,  1930,  and  1931;  at  97%  of  the  stations  in  1932— as  close 
an  approach  to  universal  distribution  as  is  ever  likely  to  obtain  for  any  plank- 
tonic  animal,  over  any  considerable  extent  of  sea. 

In  the  northern  sector,  it  continues  equally  frequent  into  July  in  some  years 
(1930,  100%;  1931,  95%),  nearly  as  frequent  in  others  (1929,  7Q%).  But  the 
facts  that  it  had  disappeared  from  the  inshore  stations  off  Chesapeake  Bay 
between  June  and  July  of  1929,  that  it  was  lacking  both  there  and  near  Delaware 
Bay  in  the  latter  month  of  1916,  that  it  was  not  found  at  all  in  the  southern  sector 
in  July  of  1913,  and  that  it  was  lacking  at  three  out  of  seven  stations  south  from 
the  New  York  profile  in  October  1931,  show  that  it  tends  to  become  less  regular 
in  the  southern  part  of  its  range  from  mid-summer  to  mid-autumn,  or  even  to 
disappear  over  considerable  areas  there  at  this  season.  And  this  accords  with 
Cowles'  (1930)  report  of  it  as  common  inside  Chesapeake  Bay  in  winter  and  early 
spring,  but  very  scarce  or  even  absent  there  in  summer. 

In  some  years,  as  in  1916,  S.  elegans  becomes  universally  distributed,  once 
more,  south  to  the  offing  of  Chesapeake  Bay  by  November,  but  its  February 
status  (p.  347),  indicates  that  in  other  years  its  range  may  not  expand  southward 
again  until  spring. 

The  present  data  fail  to  show  how  far  S.  elegans  may  penetrate  the  estuaries, 
or  other  indentations  of  the  coast.  Cowles  (1930),  however,  has  already  reported 
that  it  regularly  enters  Chesapeake  Bay  in  considerable  numbers,  through  the 
winter  and  spring  with  the  drift  of  saUne  water  near  the  bottom,  and  that  while 
its  numbers  decrease  going  up  the  Bay,  it  may  survive,  in  abundance,  in  saUnities 
as  low,  even,  as  13%o.  Offshore,  the  boundary  to  its  regular  occurrence  lies  near 
the  200-meter  contour,  which  accords  with  its  neritic  nature  in  boreal  seas 
generally. 

Abundance.  In  a  year  when  S.  elegans  is  relatively  abundant  in  February 
(e.g.,  1931),  it  may  remain  at  about  the  same  level  through  the  spring  and  early 
summer,  in  the  more  productive  inshore  belt,  increasing  sfightly  meantime  in 
the  less  productive  offshore  belt.  But  in  years  when  it  averages  low  in  abundance 


350  memoir:  museum  of  comparative  zoology 

in  February  or  April  (e.g.,  1929,  1930,  1932),  a  decided  augmentation  may  take 
place  through  the  late  spring,  both  inshore  and  offshore,  to  culminate  either 
in  June  (1930,  1932),  or  in  July  (1929),  though  with  wide  irregularity  from 
month  to  month,  and  with  the  richest  centers  often  very  small  in  extent.  The  fact 
that  the  volume  of  S.  elegans  averaged  30  times  as  great  over  the  region  as  a 
whole,  at  the  peak  season,  as  in  the  preceding  winter  in  1932,  and  46  times  as 
great  in  the  offshore  belt  in  July  as  in  April,  in  1929,  gives  some  measure  of  the 
magnitude  of  vernal  augmentation  in  years  of  this  type,  while  the  rapidity  with 
which  the  volume  of  .S.  elegans  may  increase  at  a  given  locality,  with  the  advance 
of  the  season,  may  be  illustrated  by  the  following  examples: 

Station,  Martha's  Vineyard  I,  April  3,  1930,  99  c.c;  April  29,  1930,  418  c.c. 
New  York  II,  April  10,  1930,  20  c.c;  April  28,  1930,  203  c.c. 
Cape  May  II,  April  5,  1930,  1  c.c;  April  24,  1930,  166  c.c. 
Martha's  Vineyard  I,  May  19,  1932,  11  c.c;  May  28,  1932,  208  c.c. 

At  the  seasonal  peak,  the  volume  of  this  chaetognath  has  averaged  about 
90  c.c.  inshore  and  40  c.c.  offshore,  for  all  years  combined. 

The  e\ndence  of  1931  suggests  that  an  average  decrease  by  about  J^  in  the 
average  volume  of  S.  elegans,  is  to  be  expected  in  the  offshore  belt,  between  mid- 
summer and  mid-autumn  (October).  But  nothing  is  known  of  its  quantitative 
status  then,  or  later  in  the  season,  in  the  inshore  belt,  except  that  it  was  reported 
in  relatively  high  abundance,  in  the  extreme  northeastern  sector,  in  January  and 
February  of  1936,  by  Clarke  and  Zinn  (1937). 

The  only  significant  exceptions  to  the  general  rule  that  »S.  elegans  has  aver- 
aged somewhat  more  abundant  volumetrically,  in  the  inshore  belt  than  the  off- 
shore, was  in  1929,  when  the  reverse  was  true  during  May,  but  with  the  more 
usual  relationship  reestabhshed  in  June.  Evidently,  then,  this  can  be  accepted 
as  the  normal  state,  interrupted  only  for  brief  periods,  in  some  years,  but  not  at 
all  in  others  (Table,  p.  351).  For  the  series  as  a  whole,  the  volume  have  averaged 
about  three  times  as  great  inshore  (57  c.c.)  as  offshore  (22  c.c),  and  large  catches 
( >  100  c.c.)  have  for  the  most  part,  been  within  40  miles  of  land.  But  the 
strength  of  this  inshore-offshore  contrast  has  varied  so  widely  from  cruise  to 
cruise  that  calculation  of  seasonal  ratios  would  not  be  significant. 

The  order  of  north-south  contrast  has  been  much  less  regular,  for  while 
S.  elegans  averaged  from  10-23  times  as  abundant  in  the  north  as  in  the  south  in 
one  year  (1929),  it  was  most  abundant  in  the  south  (though  the  spread  was  not 
so  wide)  in  a  second  (1931),  while  a  shift  in  the  center  of  population  from  the 
northern  sector  to  the  southern  took  place  in  the  other  two  years,  between  Febru- 


BIGELOW   AND    SEARS:   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


351 


ary  and  June  in  the  one  case  (1930),  but  between  the  first  and  fourth  weeks  of 
May  in  the  other  (1932). 

And  the  catches  of  100  c.c.  or  more  (whatever  the  month  or  year)  have  been 
distributed  indifferently,  north  and  south. 


Average, 

maximum,  and  minimum 

volumes  of  Sagitta  elegans 

Month 

Year 

Inshore 

Offshore 

North 

South 

Area 
surveyed 

Maxi- 
mum 

Mini- 
mum 

February 

1930 
1931 
1932 

17 
121 
<1 

2  • 

16 
<1 

19 

29 

<1 

1 

73 

<1 

9 

58 
<1 

87 
431 
<1 

0 
0 
0 

April 

1929 
1930 

10 
37 

1 

10 

10 
44 

0 

8 

6 

24 

82 
418 

0 
0 

May 

1929 
1930 
1931 
1932 

22 

60 

132 

28 

35 
37 
19 
12 

46 
39 
54 
16 

2 

58 

158 

9 

25 

49 
84 
18 

229 
230 
406 
203 

0 
0 
0 
0 

June 

1929 
1930 
1931 
1932 

40 

78 

136 

37 

33 
11 

88 
15 

55 
34 
61 
39 

3 

129 

225 

32 

39 

51 

118 

31 

217 

1296 

448 

326 

0 

0 

<1 

0 

July 

1929 
1930 
1931 

105 
44 
45 

46 
16 
23 

81 
27 
37 

73 

78 
27 
37 

520 
129 
242 

0 
0 
0 

October 

1931 

— 

11 

8 

16 

11 

69 

0 

Annual  differences.  The  fact  that  1932  was  somewhat  the  least  productive 
year  of  the  series  for  S.  elegans  (Table  above),  1929  and  1931,  on  the  whole,  the 
most  so,  suggests  that  conditions  are  less  favorable  for  this  chaetognath  in  our 
area  after  a  warm  winter  than  after  a  cool.  With  S.  elegans  averaging  nearly  4 
times  as  abundant  in  some  years  (1929,  1931)  as  in  another  (1932),  at  the  peak 
season,  and  more  than  58  times  as  abundant  at  the  end  of  winter,  within  a  term 
of  four  years  only,  a  longer  observational  series  might  reveal  extremes  still  further 
apart. 

Sources  of  the  local  stock.  On  the  occasions  when  the  boundary  of  occurrence 
for  S.  elegans  shifts  southward  from  April  to  June,  as  happened  in  the  years  1929 
and  1932  (p.  349),  we  may  assume  that  dispersal  from  the  populated  waters 
farther  north  has  been  responsible,  it  being  unlikely  (in  the  case  of  so  large  an 
animal)  that  enough  specimens  were  actually  present  in  the  south,  early  in  the 
season  (but  passed  through  our  nets)  for  the  local  increase  to  have  been  the  result 


352 


memoir:  museum  of  comparative  zoology 


of  their  progeny.  Cowles  (1930),  however,  has  already  offered  strong  evidence 
that  in  others  years,  elegans  may  breed  continuously  from  late  winter  through  the 
spring  as  far  south  as  the  immediate  offing  of  Chesapeake  Bay,  though  perhaps 
never  within  the  latter.  And  it  seems  sufficiently  established  that  S.  elegans  is 
regularly  endemic  farther  north,  by  the  facts  that  the  winter  and  early  spring 
catches  have  contained  a  considerable  proportion  of  very  small  individuals,  and 
that  the  geographic  locations  of  the  richest  catches  have  afforded  no  evidence  of 
renewals  from  the  waters  to  the  east  of  our  area. 


APRIL 
1929 


MAY      1929, 
1931.     1932 


JUNE      1929, 
1931,      1932 


JULY     1931. 
1932 


Fig.  37.  Sagilla  elegans:  percentage  of  cases  in  which  the  deej)  catcli  was  twice  as  great  as  the  shoal  (A), 
the  shoal  catch  twice  as  great  as  the  deep  (B),  anil  neither  twice  as  great  as  the  other  (C). 

Vertical  distribtdion.  The  deeper  catch  of  S.  elegans  was  significantly  the 
larger  of  the  pair  much  more  often  than  the  reverse  (Fig.  37)  in  every  month 
when  pertinent  data  were  obtained,  except  for  February  (1932),  when  the  num- 
bers were  not  large  enough  to  be  significant  in  this  respect.  The  extreme  case 
was  in  April  1929,  when  S.  elegans  was  twice  as  abundant  deep  as  shoal  at  80% 
of  the  stations,  whereas  May  1932  showed  the  closest  approach  to  vertical  uni- 
formity with  the  deep  catch  significantly  the  larger  at  35%  of  the  stations,  the 


BIGELOW   AND   SEARS:    NORTH   ATLANTIC    ZOOPLANKTON    STUDIES 


353 


shoal  catch  the  larger  at  21%,  and  with  the  one  catch  about  as  productive  as 
the  other  at  the  remaining  44%. 

The  volumetric  ratio  of  deep  (20-40  meters)  catch  to  shoal  (less  than  10 
meters)  has  similarly  averaged  greater  than  1  to  1  on  every  cruise  (Table,  below), 
tending  to  increase  from  the  period  April-June  (average  about  10  to  1)  to  July, 
when  it  averaged  131  to  1  for  1929  and  1931.  Catches  of  100  c.c.  or  greater  were 
also  made  about  twice  as  frequently,  relatively,  at  10-20  meters  (16%  of  the 
cases)  and  at  20-40  meters  (16%)  as  in  hauls  centering  at  depths  smaller  than  10 
meters  (7%).  Thus,  it  seems  sufficiently  established  that  S.  elegans  has  its  center 
of  abundance  in  the  deeper  strata  of  water  below  10  meters.  The  combined  data — 
if  taken  at  face  value — would,  in  fact,  suggest  that  S.  elegans  averages  somewhat 
the  most  abundant  of  all,  at  depths  greater  than  40  meters,  at  least  in  April,  May, 
and  June.  But  the  situation  was  so  variable  in  this  respect  from  cruise  to  cruise 
that  the  ostensible  richness  of  the  deepest  strata  may  perhaps  have  been  caused 
by  the  chance  that  the  net  encountered  rich  concentrations  of  this  species  at  one 
particular  level,  but  missed  them  at  another. 

Average  volumetric  ratios  of  S.  elegans  at  deeper  levels 
relative  to  volumes  at  10-0  meters 


Month 

Year 

20-40  Meter.s 

No.  of  Cases 

More  than 
40  Meters 

No.  of  Cases 

April 

1929 

12.3    to  1 

6 

53.2    to  1 

5 

May 

1929 
1931 
1932 

10.9    to  1 

5.6  to  1 

4.7  to  1 

8 
16 
57 

48.0    to  1 
1.2    to  1 

12 
6 

June 

1929 
1931 
1932 

10.1    to  1 

32.3    to  1 

6.2    to  1 

12 
24 
44 

31.3    to  1 
1.2    to  1 

11 
3 

July 

1929 
1931 

168.6    to  1 
83.0    to  1 

10 

14 

10.2    to  1 
24.9    to  1 

8 
4 

Average,  April,  May,  June 

11.7    to  1 

27.0    to  1 

Average,  July 

125.8    to  1 

17.5    to  1 

The  picture  as  regards  diurnal  migration  is  not  as  clear  for  S.  elegans,  as  it 
is  for  Calanus  finmarchicus,  because  of  wide  variability  from  station  to  station,  in 


354 


memoib:  museum  of  comparative  zoology 


the  volumetric  ratio  of  the  deeper  catch  to  the  shoaler,  this  having  ranged  be- 
tween 540  to  1  and  0.1  to  1,  by  day,  and  between  348  to  1  and  0.01  to  1,  by 
night,  on  one  cruise  or  another.  An  average  ratio,  however,  of  about  37.3  to  1, 
by  day,  for  April,  May,  and  June'  (all  pertinent  stations  combined),  but  of  only 
5.7  to  1  by  night,  is  evidence  of  a  prevailing  tendency  for  S.  elegans  to  sink  from 
the  surface  by  day,  to  rise  again  by  night,  at  this  season  of  the  year.  But  the 
great  variabihty,  just  emphasized,  shows  that  this  tendency  is  frequently  counter- 
balanced by  other  factors,  probably  by  movements  of  the  water.    Catches  made 


M,  0 


100  0  50    0    50   0 


100  CC 


10 


20 


25 


30 


35 


3  A.M. 


1 1  P.  M. 


"\ 

u 

o  1 

"i 

o 

"1 

4 

lO 

^1 

I 

1 

Fig.  38.  Vertical  distribution  of  Sagitla  elegans,  near  Fire  Island,  May  17,  1929,  at  3:00  a.m.,  11 :00  a.m., 
6:00  p.m.,  and   11:00  p.m. 

at  3  A.M.,  11  A.M.,  6  P.M.,  and  11  P.M.,  off  Fire  Island,  May  17,  1929,  do,  in 
fact,  provide  as  good  an  example  of  this  for  S.  elegans  (Fig.  38)  as  they  do  for 
Calanus  (p.  312),  for  while  diurnal  migration  may  explain  the  impoverishment  of 
the  surface  recorded  between  3  A.M.  and  11  A.M.,  an  increase  in  the  ratio  of 
the  deep  catch  to  surface  catch  between  6  P.M.  and  midnight  is  not  explicable 
on  this  basis,  nor  is  the  increase  that  took  place  between  11  A.M.  and  6  P.  M.  in 
the  total  volume  of  plankton  present  in  the  water  column  as  a  whole.  And  the 
fact  that  the  ratio  of  deep  catch  to  shoal  in  July  has  averaged  about  the  same 


'  5.  elegans  was  so  sparsely  represented  in  February  1932,  that  this  cruise  is  omitted  from  the  calcu- 
lation. 


BIGELOW  AND   SEARS:    NORTH   ATLANTIC   ZOOPLANKTON   STUDIES 


355 


by  day  (65.4  to  1),  as  by  night  (61.9  to  1),  is  sufficient  evidence  that  vertical 
migrations,  of  a  diurnal  sort,  are  almost  entirely  prohibited  in  mid-summer,  prob- 
ably by  high  temperature. 

The  vertical  thermal  gradient  in  our  area  is,  for  the  most  part  so  small  from 
February  through  May  that  it  offers  no  apparent  explanation  for  the  prevailing 
concentration  of  S.  elegans  at  depths  greater  than  20  meters,  at  this  season.  But 
the  average  catches  in  hauls  centering  at  different  temperatures,  irrespective  of 
depth,  show  the  following  decrease  with  rising  temperature  for  the  several  June 
and  July  cruises  for  1929,  1931,  and  1932,  combined: 

Average  volumes  of  S.  elegans  at  different  temperatures 


Temperature  at 
mid-level  of  haul 

6-8° 

8.1-10° 

10.1-12° 

12.1-14° 

14.1-16° 

16.1-18° 

18.1-20° 

>20° 

Average  in  CO. 

52 

174 

84 

59 

50 

12 

4 

1 

No.  cases 

37 

31 

43 

39 

37 

21 

30 

30 

On  only  four  occasions,  in  fact,  was  the  shoalest  catch  larger  than  the  next 
deeper,  at  a  station  where  the  upper  10  meters  were  warmer  than  16°,  namely: 

Station,  New  York  IV,  July  22,  1929,  Surface,  21°,  12  c.c; 

58  meters,  8.1°  less  than  1  c.c. 
New  York  III,  June  7,  1932,  8  meters,  16.2°,  21  c.c; 

27  meters,  8.6°,  less  than  1  c.c. 
Cape  May  III,  June  16,  1932,  8  meters,  19°,  38  c.c; 

27  meters,  9.4°,  24  c.c 

This  relationship  between  catch  and  temperature  obviously  suggests  that  the 
lower  values  are  the  more  favorable  for  this  species  with  18-20°  perhaps  the  upper 
limit  for  its  continued  existence.  The  warming  of  the  surface  thus  appears  to  be 
the  factor  chiefly  responsible  for  the  rather  abrupt  rise  between  June  and  July,  in 
the  volumetric  ratio  of  deep  catches  of  S.  elegans  to  shoal  (p.  352),  just  as  for 
Calanus,  and  likewise  for  the  coincident  break-down  of  effective  diurnal  migration. 
We  hesitate,  however,  to  assert  that  8-10°  is  the  optimum  for  S.  elegans,  as 
the  tabulation  would  suggest  if  accepted  without  reservation,  for  the  chance 
that  particular  hauls  hit  or  missed  rich  aggregations  may  have  been  partly 
responsible.  And  this  is  made  the  more  likely  by  the  following  wide  range  of 
volumes  from  station  to  station  at  each  temperature  interval  (irrespective  of 
depth),  in  the  years  when  hauls  were  made  at  two  levels  or  more: 


Temp. 

6-8° 

8.1-10° 

10.1-12° 

12.1-14° 

14.1-16° 

16.1-18° 

18.1-20° 

>20° 

C.C. 

< 1-540 

< 1-1258 

< 1-1040 

1-823 

0-679 

0-75 

0-38 

0-13 

356  memoir:  museum  op  comparative  zoology 

Sagitta  enflata 

This  warm-oceanic  chaetognatli,  already  recorded  not  only  along  the  conti- 
nental slope,  but  coastwise  south  of  Delaware  Bay  (Bigelow,  1915,  Fig.  71;  1922, 
Fig.  50),  and  even  as  a  stray  in  Chesapeake  Bay  (Cowles,  1930,  p.  334),  was 
found  widespread  from  the  one  end  of  our  area  to  the  other,  for  about  30  miles 
in  from  the  edge  of  the  continent  (Fig.  36B),  which  brings  its  area  of  most  fre- 
quent occurrence  close  in  to  the  land  to  the  south  of  Chesapeake  Bay.  North- 
ward, however,  from  Delaware  Bay,  the  coastal  belt  has  usually  been  bare  of  it, 
with  the  one  notable  exception  that  a  large  catch  of  juveniles  was  made  close  in 
to  Martha's  Vineyard  in  1935.  In  this  respect,  the  shelf  west  of  Cape  Cod  con- 
trasts strongly  with  the  sector  next  to  the  east — George's  Bank  and  the  Gulf  of 
Maine — where  S.  enflata  has  not  yet  been  found  inside  the  200-meter  contour. 

The  distribution  of  stations  does  not  suggest  any  greater  tendency  for  this 
species  to  enter  our  area  more  frequently  in  the  one  sector  (north  or  south)  than 
in  the  other.  Its  presence,  however,  at  6%  of  the  stations  in  the  offshore  belt  in 
February,  at  24%  in  April,  at  31  %  in  May,  at  22%  in  June,  and  at  15%  in  July, 
but  at  80%  in  October,  suggests  that  it  invades  the  shelf  least  frequently  in 
winter  (as  was  to  be  expected)  and  most  frequently  in  autumn,  with  some  tend- 
ency toward  a  second  (but  minor)  peak  of  frequency  in  the  early  spring. 

The  maximum  catch  was  66  c.c.  off  Winterquarter  in  October  1931,  and 
four  other  catches  of  3-7  c.c.  were  recorded  in  that  same  month.  Other  than 
this,  the  record  of  it  in  our  waters  is  based  on  odd  individuals  only. 

Sagitta  serratodentata 

Frequency.  This  warm-water  chaetognath  ranks  among  the  half  dozen  most 
generally  distributed  members  of  the  plankton  in  the  offshore  belt,  north  and 
south,  where  it  was  recorded  at  92%  of  the  total  stations  and  at  every  station  on 
16  out  of  the  24  cruises.  It  was,  in  fact,  at  every  station  throughout  the  area,  as 
a  whole,  for  February  1930  and  1932.  And  it  has  averaged  but  little  less  frequent 
in  the  inshore  belt  in  the  south,  than  offshore.  It  has  even  been  recorded  occa- 
sionally in  Chesapeake  Bay  (Cowles,  1930).  But  it  has  been  much  less  frequent 
close  to  the  land  to  the  northward  of  Chesapeake  Bay,  and  especially  so  to  the 
eastward  of  New  York  (Fig.  39),  where  it  failed  altogether  at  54  out  of  77  stations. 

In  the  face  of  this  regional  contrast,  it  is  interesting  and  somewhat  astonish- 
ing that  segregation  of  the  data  by  months  fails  to  suggest  any  definitely  seasonal 


BIGELOW   AND   SEARS:   NORTH   ATLANTIC    ZOOPLANKTON    STUDIES  357 

gradient,  in  frequency  of  occurrence,  for  S.  serratodentata,  whether  inshore,  where 
the  species  is  least  frequent  or  offshore,  where  it  is  most  so.    At  the  most,  some 


Fig.  39.  A,  relative  frequency  of  occurrence  of  Sagitta  serralodentata  in  different  areas;  B,  locality  records 
for  volumes  of  S.  serralodentata  greater  than  10  ec;  C,  volumes  of  S.  serratodentata  greater  than 
50  cc;  D,  locahty  records  for  Tomopteris  catherina  and  for  Tomopteris  sp.? 


decrease  may  be  indicated  from  February-April  to  June-July  for  the  inshore  belt 
from  New  York  eastward,  but  data  here  are  perhaps  not  numerous  enough  to 
warrant  definite  conclusion. 


358 


memoir:  museum  of  comparative  zoology 


Percentage  of  stations  with  S.  serratodentata 


Month 

Year 

Inshore 

Offshore 

North 

South 

Area 
surveyed 

February 

1930 
1931 
1932 

100% 
57 
100 

100% 
77 
100 

100% 
20 
100 

100% 
91 
100 

100% 
69 
100 

April 

1929 
1930 

69 
86 

84 
100 

46 

87 

100 
97 

76 
93 

May 

1929 
1930 
1931 
1931 

50 

86 
30 
84 

64 

100 

86 

92 

33 

93 
58 
79 

79 
91 
20 
93 

52 

92 
54 
84 

June 

1929 
1930 
1931 
1931 

30 
36 
47 

58 

64 
100 
100 

93 

25 
65 
62 
61 

75 
50 

77 
100 

43 
62 
67 

72 

July 

1929 
1930 
1931 

47 
59 
84 

81 
100 
100 

69 
81 
90 

50 

63 
81 
90 

October 

1931 

— 

100 

100 

100 

100 

Abundance.  S.  serratodentata,  while  ranking  so  high  in  our  area  in  frequency 
of  occurrence,  ranks  far  below  its  relative,  S.  elegans  (Table,  p.  351)  in  volumetric 
abundance,  for  the  highest  average  for  it  in  any  month  was  only  53  c.c,  even  in 
the  subdivision  that  was  richest  in  it  at  the  time,  with  a  general  average,  for  all 
regions  and  months  combined  of  only  12  c.c,  contrasted  with  225  c.c.  and  39  c.c, 
respectively,  for  S.  elegans;  589  c.c.  and  144  c.c.  for  Calanus finmarchicus ;  and 
208  c.c.  and  37  c.c.  for  Centropages  typicus.  The  maximum  catch  of  Sagitta  ser- 
ratodentata— 335  c.c. — likewise  falls  far  below  that  for  S.  elegans  (1296  c.c),  for 
Calanus  (1532  c.c),  for  Limacina  (2666  c.c),  or  for  Centropages  (1341  c.c). 

It  is  doubtful  whether  any  definite  inshore-offshore  gradient,  in  average 
abundance  could  be  deduced  for  Sagitta  serratodentata  from  our  data,  for  volumes 
have  averaged  about  as  large  in  the  one  belt  as  in  the  other,  both  in  the  south 
(average,  35  c.c,  offshore;  25  c.c,  inshore)  and  in  the  north  (average,  6  c.c, 
inshore;  5  c.c,  offshore),  while  one  very  large  catch  (335  c.c),  close  to  Atlantic 
City,  on  February  8,  1930,  was  responsible  for  the  only  case  when  any  great  pre- 
ponderance was  indicated  for  either  sector — inshore,  on  this  occasion.  But  S. 
serratodentata  has  on  the  whole  averaged  considerably  more  abundant  in  the 
southern  sector  than  in  the  northern,  not  only  for  the  .series  as  a  whole,  but  in 


BIGELOW   AND   SEARS :   NOUTH   ATLANTIC    ZOOPLANKTON   STUDIES  359 

most  of  the  individual  months  as  well,  while  in  no  months  was  there  a  strong 
contrast  of  the  reverse  order. 

The  contrast  in  this  respect  between  the  two  sectors  may,  in  fact,  average 
as  high  as  13  to  1  for  a  given  year  as  a  whole  (1931),  or  35  to  1  for  an  individual 
cruise  (February  1931),  while  the  north-south  ratio  averaged  4  to  1  even  in  the 
year  (1930)  when  it  was  smallest.  And  the  southerly  nature  of  this  species  is 
further  illustrated  by  the  fact  that  the  rich  catches  (50  c.c.  or  more)  have  been 
very  definitely  concentrated  from  the  offing  of  Delaware  Bay  southward  (Fig. 
39B,  C).  It  appears,  furthermore,  that  the  northern  boundary  to  common  occur- 
rence in  abundance  is  a  surprisingly  sharp  one,  for  while  catches  as  great  as 
50  c.c.  were  made  repeatedly  on  the  Cape  May  profile  this  seldom  happened 
north  of  the  latter,  though  an  occasional  concentration  (17-31  c.c.)  of  serrato- 
dentata  may  develop  as  far  east  as  the  Martha's  Vineyard  profile  (e.g.,  July,  1929), 
or  even  in  the  Gulf  of  Maine,  for  that  matter. 

The  volume  of  S.  serratodentata  in  the  southern  sector,  inshore  and  offshore, 
has  averaged  largest  either  in  February  or  in  April,  and  considerably  smaller  in 
May,  but  with  little  evidence  of  any  further  seasonal  alteration  through  mid- 
summer, while  the  only  high  average  for  the  north  was  also  recorded  in  February. 
On  the  other  hand,  the  majority  of  occasions  when  Sagilta  serratodentata  has 
averaged  nearly  or  quite  as  abundant  in  the  north  as  in  the  south  have  fallen 
within  the  period  mid-May  to  October,  whereas  most  of  the  occasions  when 
there  was  a  notable  preponderance  of  this  species  in  the  south  have  fallen  as  early 
as  mid-May,  or  earlier.  Averages,  for  all  cruises  combined,  similarly  show  the 
north-south  ratio  as  decreasing  from  24  to  1  in  February  and  18-19  to  1  in  April, 
to  11  to  1  in  INIay,  7  to  1  in  June,  1  to  1  in  July  (one  cruise  only),  and  5  to  1  in 
October. 

The  coastal  belt  from  Delaware  Bay  northward,  thus  appears  more  and 
more  nearly  to  equal  the  more  southerly  and  offshore  waters  in  suitabihty  as  an 
environment  through  the  late  spring  and  early  summer,  as  the  water  warms. 
But  it  is  doubtful  whether  the  small  recorded  contrast  in  this  respect  between 
July  and  October  is  seasonally  significant,  for  the  one  set  of  observations  was 
made  in  one  year,  the  other  in  another.  Neither  is  information  on  this  point 
available  for  the  late  autumn  or  early  winter. 

The  regularity  of  occurrence  of  this  chaetognath,  combined  with  the  nar- 
rowness of  its  fluctuations  from  year  to  year  (p.  360)  points  to  local  reproduction 
rather  than  to  immigration  as  the  chief  source  of  the  stock  in  our  waters.  And 
the  fact  that  in  1932  the  center  of  abundance  continued  in  the  same  region  off 


360 


memoir:  museum  of  comparative  zoology 


Chesapeake  Bay  from  February  through  the  first  three  weeks  in  May  is  further 
evidence  that  the  coincident  decrease  that  took  place  in  its  volumetric  abun- 
dance during  that  period  was  primarily  the  result  of  a  predominance  of  death  rate 
over  production,  not  of  a  mass  drift  of  population  away  from  the  locality  where 
it  had  previously  been  relatively  plentiful. 

Average  and  maximum  volumes  of  S.  serratodentata 


Month 

Year 

Inshore 

Offshore 

North 

South 

Area 
surveyed 

Maxi- 
mum 

February 

1930 
1931 
1932 

53 
55 
19 

10 

2 

15 

36 

1 
2 

25 
35 
24 

30 
23 
18 

335 

170 

70 

April 

1929 
1930 

26 

7 

14 
10 

2 
5 

37 

12 

21 

8 

129 
122 

May 

1929 
1930 
1931 
1932 

4 

1 
<1 

1 

10 
1 

1 

7 

<1 

9 
<1 

1 

14 

5 

<1 

11 

7 

5 

<1 

5 

64 

230 

12 

98 

June 

1929 
1930 
1931 
1932 

2 

<1 

1 

1 

4 

<1 
14 

7 

1 
<1 
<1 

3 

3 

<1 

15 

13 

3 

<1 
6 
6 

35 
3 
133 
70 

July 

1929 
1930 
1931 

2 
<1 

7 

7 
6 
1 

4 
3 
5 

5 

4 
3 
5 

31 
31 

64 

October 

1931 

— 

12 

5 

22 

12 

69 

Annual  Variations.  The  average  volumes  of  S.  serratodentata,  for  the  area 
as  a  whole  (including  the  poorer  areas  with  the  richer)  and  for  all  cruises  for  dif- 
ferent months  combined  were  9.5  c.c.  for  1929,  9.4  c.c.  for  1930,  8.2  c.c.  for  1931, 
and  9.9  c.c.  for  1932,  a  varietal  range  much  smaller  than  the  probable  error  of 
observations  as  rough  as  ours.  And  the  frequency  for  any  year  as  a  whole  was 
only  about  1.4  times  as  great  at  the  maximum  (85%,  1932)  as  at  the  minimum 
(58%,  for  1929).  Such  evidence  marks  this  species  as  varying  much  less  widely 
in  its  status  in  our  area  from  year  to  year  than  do  most  of  the  members  of  the 
dominant  community  that  are  volumetrically  more  abundant  there. 

Other  chaetognaths 
Other  species  of  chaetognaths  (Sagitta  hexaptera,  Sagitta  maxima,  Krohnita 
subtilis,  Pterosagitta  draco),  recorded  over  the  outer  edge  of  the  shelf,  at  the 


BIGELOW   AND    SEARS :   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  361 

localities  shown  on  Fig.  36C,  D  are  no  doubt  to  be  classed  as  strays  from  offshore. 
In  every  case,  the  records  are  based  on  occasional  individuals. 

ANNELIDS 

TOMOPTERIS 

Tomopteris  catharina  was  taken  at  localities  widely  scattered,  inshore  and 
offshore  alike,  from  the  Martha's  Vineyard  profile  to  the  Chesapeake  profile. 
And  inclusion  of  the  additional  locality  records  for  tomopterids  probably  belong- 
ing to  catharina,  but  not  positively  identifiable  because  of  their  poor  condition 
(Fig.  39D),  would  show  this  species  as  generally  distributed  throughout  our  area 
between  these  limits,  as  it  is  also  in  the  Gulf  of  Maine  (Bigelow,  1926,  Fig.  94), 
and  about  as  frequent  in  one  subdivision  as  in  another.  We  have  only  one  record, 
however,  of  Tomopteris  of  any  species  on  the  continental  shelf,  south  of  Chesa- 
peake Bay,  an  additional  reason  for  referring  the  doubtful  specimens  to  T. 
catharina. 

Although  so  general,  T.  caiharina  appears  to  be  much  less  frequent,  in  the 
waters  as  a  whole  to  the  west  of  Cape  Cod,  than  it  is  in  the  Gulf  of  Maine  to  the 
east,  for  while  it  was  taken  at  38-50%  of  the  stations  there,  February-May, 
August,  and  December-January  (Bigelow,  1926),  its  maximum  frequency  in  our 
area  in  any  month  was  only  27%,  the  average  about  14%  for  all  months  com- 
bined. Monthly  frequencies  of  20%  in  February,  4%  in  April,  21-27%  in  May- 
June,  and  7%  in  July  and  October,  also  mark  it  as  more  definitely  seasonal  in 
the  southwestern  part  of  its  range  (with  peaks  in  late  winter  and  in  May-June) 
than  it  is  in  the  more  typically  boreal  regions  to  the  east  and  north,  where  the 
only  indication  of  any  seasonal  cycle,  yet  reported,  is  an  apparent  scarcity  in 
late  autumn  and  early  winter. 

It  is  interesting  that  a  species  so  constantly  present,  and  so  generally  dis- 
tributed, should  never,  in  all  our  experience  have  developed  a  population  abun- 
dant enough  to  yield  even  one  catch  as  large  as  1  c.c,  for  in  the  case  of  so  large  an 
animal  that  would  have  meant  only  a  few  hundred  individuals.  And  the  case  is 
similar  in  the  Gulf  of  Maine,  where  the  catches  were  "usually  from  one  to  half 
a  dozen  individuals  per  haul"  (Bigelow,  1926,  p.  335). 

Other  annelids 
Annelid  worms  (not  yet  identified)  were  recorded  at  one  station  only  (off 
Bodie  Island,  April  8,  1930).   But  on  that  occasion  they  were  in  such  large  vol- 
ume (47  c.c.  or  about  24%  of  the  total  catch)  as  to  show  that  they,  rarely,  may 


362 


memoir:  museum  of  comparative  zoology 


be  an  item  of  considerable  local  importance  in  the  plankton  though  never  (by 
present  evidence)  over  any  considerable  area  within  our  limits. 

MEDUSAE 
Aglantha  digitale' 
Regional.  At  the  end  of  winter  the  records  for  Aglantha — scattered,  wide- 
spread from  the  offing  of  Martha's  Vineyard,  southward  to  Cape  Hatteras — 
have  (with  two  exceptions)  been  within  40  miles  of  the  coast.  With  the  advance 
of  the  season,  it  tends  to  become  more  frequent  inshore,  having  been  recorded  at 
80-100%  of  the  stations  in  that  belt  on  each  of  the  May  cruises  of  1930  and  1932. 


Fig.  40.  Areas  of  occurrence  of  Aglantha  digitale  in  Feliruary  (dotted)  and  in  April  (hatched)  1930. 

It  appears  also  characteristic  for  it  to  disperse  offshore  during  the  last  half  of 
the  spring,  judging  from  its  presence  at  57-90%  of  the  offshore  stations  on  those 
same  cruises,  and  at  about  the  same  percentage  offshore  (46%)  as  inshore  (45%) 
in  that  month  of  1931.  That  Aglantha  is  to  be  classed  as  an  inshore  form  in  our 
area  (notwithstanding  its  holoplanktonic  nature  and  wide  distribution),  also 
appears  in  the  fact  that  catches  greater  than  10  c.c.  have  (with  one  exception) 
been  confined  to  the  inshore  belt  in  February,  April,  and  May,  though  with  the 
offshore  boundary  for  this  level  of  abundance  lying  some  20  miles  farther  out  to 
sea  for  June  (Fig.  40, 41).  It  appears,  however,  to  be  about  equally  frequent  north 

'  For  a  recent  discussion  of  varietal  relationships  within  this  species,  see  Ranson  (1936). 


BIGELOW   AND    SEARS:    NORTH    ATLANTIC    ZOOPLANKTON    STUDIES 


363 


as  south,  and  equally  abundant,  for  the  catches,  for  May,  June,  and  July,  com- 
bined have  averaged  about  the  same  in  the  one  sector  as  in  the  other  (39  c.c, 
north;  43  c.c,  south). 


•  s   AGAL  rilO    fRAGM^NTS 
yi    AGALMA    OKENI 


Fig.  41.  A,  Aglanlha  digitale,  regional  abundance,  February-May,  all  cruises  combined;  B,  Aglantha, 
regional  abundance  for  June;  C,  locality  records  for  Pleurobrachia  and  Mnemiopsis  and  for  large 
and  small  catches  of  Beroe,  1929-1932;  D,  locality  records  for  agalmid  fragments  and  Agalma 
okeni. 

Seasonal  and  annual  variations.  The  fact  that  i^glantha  may  appear  only 
sporadically  in  some  years,  as  in  1929,  when  it  was  recognized  at  two  stations 
only  (both  in  July),  but  widespread  and  in  considerable  volume  in  others  (e.g., 
1932),  shows  that  annual  variation  in  its  status  is  wide.    More  or  less  vernal 


364 


memoir:  museum  of  comparative  zoology 


augmentation  seems,  however,  to  be  characteristic  for  it  in  its  years  of  abun- 
dance. In  1930,  for  example,  it  increased  in  average  volume  from  less  than  1  c.c. 
in  April  to  about  8  c.c.  in  May-June,  while  in  1932,  the  minimal  population 
(average,  less  than  1  c.c.)  existing  in  early  May,  was  succeeded  by  average  vol- 
umes of  56  c.c.  inshore,  17  c.c.  offshore,  37  c.c.  in  the  north,  and  33  c.c.  in  the 
south  in  the  succeeding  month.  Unfortunately,  we  lack  data  later  in  the  season, 
for  that  particular  year.  But  the  facts  that  Aglantha  averaged  less  than  1  c.c, 
in  any  subdivision  in  July  of  1930  and  1931,  that  it  was  found  only  twice  west  of 
Cape  Cod  in  the  summer  of  1913  (Bigelow,  1915),  and  that  the  richest  catch  in 
October  1931  was  less  than  1  c.c,  added  to  its  apparent  absence  in  1916,  whether 
in  August  or  in  November,  marks  it  as  definitely  a  spring  and  early  summer 
species,  as  far  as  occurrence  in  significant  numbers  is  concerned. 

The  very  wide  annual  differences  recorded  within  so  short  a  series  of  observa- 
tions, between  years  (1932),  when  Aglantha  is  in  high  frequency  with  a  very  con- 
siderable vernal  augmentation,  as  described  above,  and  tho.se  (1929),  when 
it  is  represented  wuthin  our  boundaries  by  stray  individuals  only,  are  most 
reasonably  explained  on  the  assumption  that  while  it  may  be  generally  and 
effectively  endemic  in  some  years,  it  may  disappear  altogether  in  others,  with 
repopulation  depending  on  immigration  from  the  east  and  north. 

Average  and  maximum  volumes  of  Aglantha  digitale 


Month 

Year 

Inshore 

Offshore 

North 

South 

Area 
surveyed 

Maxi- 
mum 

February 

1930 
1931 
1932 

2 
<1 
<1 

0 

0 

<1 

1 
<1 
<1 

1 
<1 
<1 

1 
<1 
<1 

10 
<1 
<1 

April 

1929 
1930 

<1 

<1 

<1 

<1 

<1 

10 

May 

1929 
1930 
1931 
1932 

7 

<1 

17 

1 
<1 

8 

3 
<1 

9 

6 

<1 

12 

4 

<1 

11 

32 
<1 
180 

June 

1929 
1930 
1931 
1932 

7 

<1 

56 

1 
<1 

17 

5 

<1 

37 

1 
<1 
33 

4 

<1 

40 

62 

2 

191 

July 

1929 
19.30 
1931 

<1 

0 

<1 

<1 

1 
<1 

<1 
<1 

<1 

0 

<1 
<1 
<1 

<1 

7 

<1 

October 

1931 

— 

<1 

<1 

<1 

<1 

<1 

BIGELOW   AND   SEARS:    NORTH   ATLANTIC    ZOOPLANKTON   STUDIES 


365 


Leptomedusae 

A  mixed  population  of  small  leptomedusae,  belonging  for  the  most  part  to 
the  genus  Obclia,  call  for  notice  here,  since  they  formed  about  1%  of  the  catch 
in  May  and  June  1932,  as  well  as  in  July  1929,  and  4%  in  May  1931  (Table, 
p.  229).  In  frequency  of  occurrence  these  have  shown  an  unmistakable  seasonal 
cycle,  for  while  wholly  lacking  in  February  of  either  year,  and  at  only  3%  of  the 
stations  in  April,  they  were  at  34%  in  May,  and  at  18%  in  June,  but  at  only  10% 
in  July,  while  they  were  not  taken  at  all  on  the  one  October  cruise  (1931) — at 
least  in  the  offshore  belt,  to  which  the  latter  survey  was  confined. 

These  medusae  have  proved  much  more  frequent  inshore  (24%  of  the  sta- 
tions, April-July)  than  offshore  (3%),  as  was  to  be  expected;  also  more  frequent 
in  the  north  (13%)  than  in  the  south  (3%). 

The  average  catches,  for  the  months  when  they  occurred  in  more  than 
minimal  numbers,  show  the  same  contrast  between  larger  inshore  and  smaller 
ofTshore  as  do  the  average  frequencies  of  occurrence;  all  thirteen  of  the  catches 
larger  than  50  c.c.  were,  in  fact,  made  inshore.  But  it  is  doubtful  whether  any 
strong  north-south  contrast  in  their  abundance,  is  characteristic,  for  while  catches 
larger  than  50  c.c.  were  made  more  often  south  (10)  than  north  (3),  the  average 
volumes  were  not  only  somewhat  larger  south  than  north  as  tabulated  below, 
but  the  largest  average  volume  for  either  subdivision,  in  any  individual  month 
(May  1931,  32  c.c.)  was  also  in  the  south. 


Average 

and  maximum  volumes  of  Leptomedusae 

Month 

Inshore 

Offshore 

North 

South 

Area 
surveyed 

Maxima 

May 
June 
July 
Average 

11 
3 

6 

7 

<1 

1 

<1 

<1 

5 
3 
3 
4 

11 
2 
5' 
6 

6 
2 
4 
4 

188 
120 
185 

In  the  month  of  greatest  abundance,  the  average  volume  of  these  small 
leptolines  was  more  than  19  times  as  great  for  the  area  as  a  whole  in  the  year 
when  they  were  most  abundant  (1931,  average,  19  c.c),  than  in  the  year  (1930) 
when  they  were  least  so;  when  in  fact  they  were  not  detected  at  all  though 
minimal  numbers  may  have  actually  been  present  in  the  water. 


1929  only. 


366  memoir:  museum  of  comparative  zoology 

Other  medusae 
Other  medusae  were  negligible  from  the  volumetric  standpoint,  none  other 
than  those  mentioned  above  having  formed  as  much  as  1%  of  any  individual 
catch :  nor  has  time  allowed  complete  identification  of  the  scattered  specimens  of 
various  species  that  were  included  in  the  catches.  We  refer  the  reader  to  earlier 
papers  (Bigelow,  1915;  1922)  for  lists  of  the  hydro-  and  scyphomedusae  taken 
during  the  summers  of  1913  and  1916.  Captures  of  Laodicea  cruciata  off  Chesa- 
peake Bay  and  off  Hog  Island,  in  July  1929,  added  to  the  locality  records  for  it  in 
the  summer  of  1913  (Bigelow,  1915,  Fig.  79)  afford  cumulative  evidence  that 
this  species  is  widespread  in  summer  well  out  on  the  shelf  in  the  southern  sector, 
though  apparently  it  is  closely  confined  to  the  vicinity  of  the  shore  line  in  the 
northern,  for  it  was  not  represented  there  in  any  of  our  July  collections,  though 
plentiful  at  that  season  along  the  coasts  of  southern  New  England  (Mayer,  1910, 
p.  203).  The  genus  Liriope  also  proves  to  occur  over  the  shelf  in  the  southernmost 
sector  in  February,  as  well  as  in  summer,  when  the  inshore  species  (or  race?) 
scutigera  is  common  in  and  off  the  southern  harbors  and  bays,  while  the  genus 
Aequorea — already  known  to  be  widespread  throughout  the  area  in  summer 
(Bigelow,  1915,  p.  319,  Fig.  79) — also  occurs  sparingly  in  May  and  June,  as  well, 
at  least  in  the  southern  sector. 

SIPHONOPHORES 

Agalmidae 

Recent  towings  have  shown  that  one  member  of  this  group,  Stephanomia 
cara,  occasionally  swarms  in  the  Gulf  of  Maine.  But  we  have  no  evidence  that 
any  agalmid  is  ever  of  volumetric  importance  in  the  planktonic  community  of  the 
shelf  waters  west  or  south  of  Cape  Cod,  for  while  agalmid  fragments  were  recorded 
at  some  93  stations  (Fig.  41D),  they  did  not  form  as  much  as  1%  of  the  total 
volume  in  any  subdivision  in  any  individual  month.  Agalmids  appear  to  be  about 
equally  frequent  north  and  south.  But  the  distributional  picture  shows  them  to 
be  much  more  frequent  offshore  than  inshore,  in  our  area,  for  about  76%  of  the 
stations  of  record,  lay  in  the  former  belt  contrasted  with  about  24%  only  in  the 
latter.  The  seasonal  distribution  of  the  records  (present  at  11  %  of  the  stations  in 
February,  at  0%  in  April,  at  13-23%  in  May  and  June,  at  5%  in  July,  and  at 
93%  in  October)  suggests  a  decided  peak  of  frequency  in  autumn,  alternating 
with  a  period  of  great  impoverishment  in  the  early  spring.  Previous  experience 
(Bigelow,  1915,  Fig.  81)  would  suggest  that  in  the  northern  sector,  we  were  deal- 


BIGELOW   AND    SEARS:   NORTH    ATLANTIC   ZOOPLANKTON   STUDIES  367 

ing  with  both  Stephanomia  cara  and  with  Agalma  elegans,  and  with  the  latter 
alone  in  the  soutliern  sector.  But  the  specimens  were  all  in  condition  so  frag- 
mentary (bare  stems,  or  mere  remnants  of  nectophores  and  bracts),  as  to  prevent 
identification. 

MUGGIAEA  KOCHII 

The  records  for  this  calycophore  are  confined  to  the  south,  one  for  June,  and 
eight  for  July,  all  in  the  year  1929.  In  the  latter  month,  the  average  catch  of 
M.  kochii,  "south,"  was,  indeed,  5  c.c,  equivalent  to  about  2%  of  the  total  plank- 
ton for  that  sector  at  the  time  (Table,  p.  229)  with  a  maximum  of  19  c.c.  But  the 
fact  that  it  was  not  recorded  at  all  within  our  hmits,  except  in  the  summer  of  that 
one  year,  shows  that  such  invasions  occur  less  frequently  off  our  coasts  than  they 
do  in  the  English  Channel,  where  Russell  (1934)  found  it  quite  regularly,  in  sum- 
mer, from  1925  to  1931.  We  have  yet  to  learn  whether  M.  kochii  is  a  reUable 
indicator — other  than  of  warm  water  in  general — within  our  limits.  It  is  worth 
comment,  however,  that  the  record  (so  far  as  it  goes)  suggests  that  M.  kochii  and 
M.  atlantica  are  as  mutually  exclusive  in  our  area  as  Russell  (1934)  has  found 
them  to  be  in  English  waters,  for  all  our  records  for  the  former  were  in  one  year 
(1929),  those  for  the  latter  in  another  (1930). 

Other  Siphonophores 

The  seasonal  incidence  of  the  only  other  siphonophores  that  were  taken  at 
more  than  two  stations  each,  was  as  follows : 

Abylopsis  tetragona,  February,  5  stations;  May,  2  stations;  July,  1  station;  October, 
1  station. 

Bassia  bassensis,  February,  1  station;  May,  2  stations;  June,  3  stations. 

Lensia  conoidea,  May,  6  stations;  June,  4  stations;  July,  3  stations;  October,  2  sta- 
tions. 

Muggiaea  atlantica,  February,  3  stations;  April,  5  stations;  all  in  1930. 

Most  of  the  locahties  of  capture  for  each  of  these  lie  close  to  the  200-meter 
contour,  and  this  applies  equally  to  the  few  other  members  of  the  group  that 
were  recorded  within  our  Umits  (Fig.  42).  In  most  cases,  the  records  of  sipho- 
nophores other  than  agalmids  and  Muggiaea  kochii  have  been  based  on  odd 
specimens  only. 

The  Portugese  man-of-war  (PhysaUa)  drifts  in  to  the  coast,  near  Woods 
Hole,  in  considerable  numbers  in  some  years,  in  summer  or  early  autumn  after 


308  memoir:  museum  op  comparative  zoology 

strong  southerly  winds.  And  we  have  heard  reports  of  them  at  various  localities 
in  on  the  shelf  to  the  southward.  Velella  and  Porpita  have  also  been  recorded  at 
Woods  Hole  and  Newport,  Rhode  Island  (Sumner,  Osburn,  and  Cole,  1904, 


1 

It      ^' 

'^-^ 

^ 

"*^ 

1 

w^ 

W^ 

^1 

■ 

• 

o 

• 

1 

1 

P 

•        ^ 

-«t 

m 

•   ^^ 

"'<a  -... 

y 

/ 

D 

\ 

f 

B 

V 

/     X" 

/                                      1 

BASSIA     BASSENSIS           1 
'CERATOCYMBA                     1 

) 

a 

i 

1 

xx. 

r~0 

'                    D'DIPHYIO     EUOOXIDS 

■  cEUDOXOlOES 

SPIRALIS 

O'CHELOPHYES 

APPENOICULATA 

f 

/ 

•  •  LENSIA 

CONOCOEA 

L 

f 

/ 

/ 
/ 

V. 

/ 

• 

•ABYLOPSIS                         "1 
TETRAGONA             \ 

/         A  --  LtNSIA     FOWLER)                            M 

4K^ 

/ 
/ 

X  =   LENSIA     SP 

C3  i  MUGGlAEA     ATLANTi 

CA 

f: 

/ 

V 

/ 
/ 

5' 

\ 

1 

*=    VOGTIA 

PENTACANl 

A 

HA 

•    • 

/ 
I 

c 

tL 

a  W 

>^ 

Fig.  42.  Locality  records,  all  cruises  coml)ined:  A,  Lensia  conoidia,  L.  fowleri,  Mitygiaia  allanlica, 
Pliysophora  hyihosliilica,  and  Voglia  penlacantlta;  B,  Bassia  bassensis,  Ceratocyniba  sagittata, 
dyphiitl  eudoxids,  EiiduxoiJes  spiralis,  Chelophyes  appendiculata,  Abylopsis  tetragona;  C, 
Muggiaea  kochii. 

p.  574).  None  of  these  genera  have  any  real  place,  however,  in  the  plankton  of 
our  area,  except  as  waifs  from  tropical  waters;  neither  were  any  of  them  repre- 
sented in  the  present  collections. 


CTENOPHORES 

Beroe  sp. 

Our  records  for  Beroe  are  based  chiefly  on  field  notes  in  the  station  log  books, 

and  on  fragments  battered  beyond  specific  recognition.    Pre\'ious  experience 

justifies  us,  however,  in  assuming  that  the  two  species,  cucumis  and  forskalii, 

were  both  represented  in  the  catches  (Bigelow,  1915,  p.  316;  1926,  p.  372). 

The  earUer  record  of  Beroe  within  our  limits  was  confined  chiefly  to  the 
offing  of  Chesapeake  Bay,  where  B.  forskalii  was  abundant  in  July  in  1913  and 


BIGELOW   AND    SEARS :   NORTH   ATLANTIC    ZOOPLANKTON   STUDIES  369 

1916.  Since  that  time,  Beroe  has  been  reported  at  a  number  of  stations  thence 
northward  to  New  York  and  again  in  the  ofRng  of  Martha's  Vineyard,  though 
once  only  in  the  intervening  sector.  The  records  include  February,  May,  June, 
and  July — there  are  none  for  April  or  for  October — but  large  catches  were  noted 
in  the  log  only  in  June  (3  instances)  and  in  July  (1  instance).  And  they  were 
likewise  concentrated  regionally,  inshore,  between  the  Atlantic  City  and  Winter- 
quarter  profiles  (Fig.  41C). 

The  most  we  dare  to  hazard  from  the  foregoing  is  that  Beroe  may  occa- 
sionally multiply  to  great  abundance  in  the  inshore  belt,  between  the  offings  of 
Chesapeake  Bay  and  of  Atlantic  City,  most  frequently  toward  the  south,  per- 
haps due  to  the  influence  of  the  outflow  from  Chesapeake  Bay,  nor  have  we  any 
evidence  that  Beroe  is  ever  an  important  factor  in  the  plankton  of  the  north- 
eastern sector  of  our  area,  or  of  the  offshore  belt,  south  or  north.  Rich  aggrega- 
tions of  Beroe  seem,  also,  to  be  confined  to  the  summer  months,  suggesting  that 
forskalii  is  the  species  chiefly  responsible. 

Pleurobrachia  PILEtrS 

During  the  summer  of  1913,  this  familiar  ctenophore  was  taken  at  all  but 
two  of  the  stations  over  the  shelf,  and  in  such  abundance  in  the  inshore  belt 
between  the  New  York  and  Cape  May  profiles  that  it  practically  monopoUzed 
the  deeper  water  layers  there.  In  1916,  however,  the  "Grampus"  had  it  at  but 
four  stations  within  our  hmits  at  this  same  season,  once  only  in  any  abundance 
(off  Chesapeake  Bay),  and  not  at  all  in  that  November,  while  it  was  detected  at 
only  16  of  the  604  stations  for  the  period  1929-1932  (Fig.  41C).  Definite  record 
of  it  within  our  limits  has  also  been  confined  so  far  to  May,  June,  and  July- 
August,  whatever  the  year. 

As  already  remarked  Pleurobrachia  may  have  been  "more  widespread  in 
small  numbers  than  these  captures  suggest,  such  a  fragile  organism  being  easily 
destroyed  in  the  mass  of  unsorted  plankton"  (Bigelow,  1922,  p.  158).  Neverthe- 
less, the  evidence  seems  sufficiently  convincing  that  while  this  ctenophore  is  to 
be  expected  anywhere  within  our  limits — ofTshore  and  inshore  alike — and  while 
it  sometimes  multiplies  enormously,  locally,  near  land,  such  events  are  unusual, 
and  perhaps  confined  to  summers  of  such  years  as  1913,  when  the  surface  waters 
warm  to  a  temperature  somewhat  higher  than  usual.  For  a  further  discussion  of 
the  local  status  of  this  ctenophore,  see  Bigelow,  1915,  p.  321. 


370  memoir:  museum  of  comparative  zoology 

Other  Ctenophores 

In  the  summer  of  1913,  the  large  lobate  ctenophore,  Mnemiopsis  leidyi,  was 
not  only  distributed  generally  over  the  inshore  belt  southward  from  Barnegat, 
but  swarmed  in  the  surface  waters  near  the  coast  between  that  point  and  Cape 
May  to  the  practical  exclusion  of  everything  else,  as  described  elsewhere  (Bige- 
low,  1915,  p.  323).  It  was  again  encountered  in  abundance  locally,  in  that  same 
general  region  in  August  1916,  though  its  observed  range  was  then  restricted  to 
the  vicinity  and  offing  of  Delaware  Bay  (Bigelow,  1922,  p.  158).  And  the  fact 
that  it  has  been  reported  near  Woods  Hole  in  every  month  in  the  year  (Sumner, 
Cole,  and  Osborne,  1904,  p.  579)  points  to  its  constant  presence  in  greater  or 
lesser  number  and  at  one  stage  of  development  or  another,  along  the  inshore 
belt  in  general. 

Unfortunately,  the  catches  for  1929-1932,  add  nothing  to  the  foregoing,  for 
if  they  did  originally  contain  any  representation  of  Mnemiopsis,  the  latter  had 
been  battered  beyond  recognition  before  the  collections  were  examined,  either  in 
the  nets,  or  among  the  other  more  resistant  animals  after  capture.  And  while 
"Mnemiopsis"  is  occasionally  named  in  the  station  log  book,  the  specimens  in 
question  may  actually  have  been  some  other  lobate  genus. 

The  positive  record  for  Mnemiopsis  is,  however,  sufficient  to  show  that  it 
is  likely  to  play  a  very  important  role  in  the  general  planktonic  community  over 
considerable  areas  in  the  inshore  belt  at  least  near  the  surface,  in  any  year  when 
summer  temperatures  are  relatively  high,  and  locally  even  in  years  when  temper- 
atures are  relatively  low.  But  we  have  no  definite  record,  as  yet,  of  such  a  hap- 
pening to  the  eastward  of  New  York,  except  close  in  to  the  coast,  as  at  Woods 
Hole,  where  Mnemiopsis  swarms  in  some  summers.  Neither  have  we  any  reason 
to  suppose  that  the  adult  Mnemiopsis  ever  occurs  beyond  the  Martha's  Vineyard 
profile,  unless  as  a  stray  destined  to  perish  in  the  colder  waters  of  George's  Bank, 
of  Nantucket  Shoals,  or  of  the  Gulf  of  Maine,  to  which  a  drift  in  that  direction 
would  carry  it,  but  where  it  has  never  been  recorded.  If,  however,  low  tempera- 
ture be  actually  the  barrier  to  its  dispersal  in  that  direction,  we  must  assume  that 
the  young  stages  of  this  ctenophore  are  much  more  resistent  to  low  temperature 
than  is  the  adult,  else  this  species  could  not  survive  the  winter  chilling  to  which 
the  waters  of  the  continental  shelf  are  yearly  subjected,  southward  to  Chesapeake 
Bay.  Mnemiopsis  appears  also  to  be  definitely  neritic  in  habit,  for  while  it  has 
been  recorded  well  out  on  the  shelf  (Bigelow,  1915,  Fig.  80),  the  largest  catches  of 
it  were  made  near  shore.    During  the  summer  of  1913,  when  Mnemiopsis  was 


BIGELOW   AND    SEARS :   NORTH   ATLANTIC   ZOOPLANKTON   STUDIES  371 

moro  abundant  than  at  any  other  time  during  our  observational  scries,  it  was 
taken  only  close  to  the  surface,  which  accords  with  its  frequent  abundance, 
right  in  to  the  tide  line  around  Woods  Hole.  Consequently,  "the  swarms  of 
Mnemiopsis  and  of  Pleurobrachia  were  mutually  exclusive"  (Bigelow,  1915,  p. 
324). 

The  evidence  at  hand  that  adult  Mnemiopsis  is  not  abundant — if  it  occurs 
at  all — in  waters  colder  than  about  20°,  or  more  than  a  few  miles  out  from  the 
coast  inclines  us  to  believe  that  lobate  ctenophores  of  large  size,  six  liters  of  which 
were  reported  in  the  log  book  (under  the  name  "Mnemiopsis"),  as  taken  near 
the  outer  edge  of  the  shelf  off  Cape  May  on  May  15,  1929,  and  as  clogging  the 
nets  midway  out,  off  Chesapeake  Bay,  two  days  later,  actually  referred  to  the 
northern  species,  Bolinopsis  infundihulum ,  which  has  long  been  known  to  abound 
from  Arctic  Seas  southward  to  the  Gulf  of  Maine  (Bigelow,  1926,  p.  372)  on  our 
side  of  the  Atlantic. 

PROTOZOANS 

Our  nets  were  too  coarse  of  mesh  for  sampling  any  but  the  largest  protozoa. 
But  the  catches  made  off  Bodie  Island  and  off  Currituck  in  May  1929  yielded  a 
few  Noctiluca,  which  had  already  been  found  in  great  abundance  at  the  mouth 
of  Chesapeake  Bay  in  November  1916  (Bigelow,  1922,  p.  163);  cumulative  evi- 
dence that  it  is  of  widespread  occurrence  in  the  southernmost  sector,  in  late 
spring  and  autumn.  This  agrees  with  its  status  in  Chesapeake  Bay,  where  it  is 
most  plentiful  at  this  season,  and  least  so  in  January  and  March  (Cowles,  1930, 
p.  328). 


372  memoir:  museum  of  comparative  zoology 


BIBLIOGRAPHY 
Apstein,  C. 

1906.  Plankton  in  Nord-  uiid  O.stsec  auf  den  deutschen  Terminfahrten.  Wiss. 
Meeresuntersuch.  Komm.  wiss.  Untersuch.  deutsch.  Meere,  Abt.  Kiel,  N.  F., 
Vol.  IX,  pp.  1-25,  14  figs.,  10  tables. 

Beebe,  William 

1929.  Deep  Sea  Fish  of  the  Hudson  Gorge.  Zoologica.  Sci.  Contr.  N.  Y.  Zool.  Soc, 
Vol.  XII,  No.  1,  19  pp.,  1  textfig. 

BiGELOW,  H.  B. 

1914.  Oceanography  and  Plankton  of  Massachusetts  Bay  and  Adjacent  Waters, 
November,  1912-May,  1913.  Bull.  Mus.  Comp.  Zool.,  Vol.  LVIII,  No.  10, 
pp.  385-419,  7  textfigs.,  1  pi. 

1915.  Exploration  of  the  Coast  Water  between  Nova  Scotia  and  Chesapeake  Bay, 
July  and  August,  1913,  by  the  U.  S.  Fisheries  Schooner  Grampus.  Ocean- 
ography and  Plankton.  Bull.  Mus.  Comp.  Zool.,  Vol.  LIX,  No.  4,  pp.  151-359, 
82  textfigs.,  2  pis. 

1922.  lOxploration  of  the  Coastal  Water  off  the  Northeastern  United  States  in  1916 

by  the  U.  S.  Fisheries  Schooner  Grampus.  Bull.  Mus.  Comp.  Zool.,  Vol.  LXV, 

No.  5,  pp.  87-188,  53  textfigs. 
1926.  Plankton  of  the  OiTshore  Waters  of  the  Gulf  of  Maine.   Bull.  U.  S.  Bureau  of 

Fisheries,  Vol.  XL,  1924  (1926),  Part  2,  pp.  1-509,  134  figs. 
1933.  Studies  of  the  Waters  on  the  Continental  Shelf,  Cape  Cod  to  Chesapeake 

Bay.    I.  The  Cycle  of  Temperature.    Papers  in  Physical  Oceanogr.  and 

Meteorol.,  Vol.  II,  No.  4,  135  pp.,  66  textfigs. 

BiGELOw,  Henry  B.,  and  Schroeder,  William  C. 

1939.  Notes  on  the  Fauna  above  Mud  Bottom  in  Deep  Water  in  the  Gulf  of  Maine. 
Biol.  Bull.,  Vol.  LXXI,  No.  3,  p.  303-324,  8  textfigs. 

Bigelow,  Henry  B.,  and  Sears,  Mary 

1935.  Studies  of  the  Waters  on  the  Continental  Shelf,  Cape  Cod  to  Chesapeake 
Bay.  II.  Salinity.  Papers  in  Physical  Oceanogr.  and  Meteorol.,  Vol.  IV, 
No.  1,  94  pp.,  55  textfigs. 

1937.  Siphonophorae.  Rep.  Dan.  Ocean.  Exp.  1908-1910  to  the  Mediterranean  and 
Adjacent  Seas,  Vol.  II,  Biology,  H.  2,  144  pp.,  83  textfigs. 

Bigelow,  Henry  B.,  and  Welsh,  William  W. 

1925.  Fishes  of  the  Gulf  of  Maine.  Bull.  U.  S.  Bureau  of  Fisheries,  Vol.  XL,  1924 
(1925),  Part  I,  567  pp.,  278  figs. 

Boschma,  II. 

1936.  Biological  data.  Sncllius-Exped.  in  the  Eastern  Part  of  the  Netherlands  East- 
Indies  1929-1930,  Vol.  VI,  29  pp.,  11  textfigs.,  1  chart. 

BusE,  Theodor 

1915.  Quantitative  tinter.suchungen  von  Planktonfiingen  des  Feuerschiffcs  "Feh- 
marnbelt"  voni  April  1910  bis  Marz,  1911.  Wiss.  Meeresuntersuch.  Komm. 
wiss.  Untersuch.  deutsch.  Meere,  Abt.  Kiel,  N.  F.,  Vol.  XVII,  pp.  229-280, 
3  textfigs.,  7  tables. 


bigelow  and  sears :  north  atlantic  zooplankton  studies        373 

Brandt,  K. 

1898.  Beitrage  zur  Konntiii.ss  dcr  oliomischcn  Zusammensetzung  der  Planktons. 
Wiss.  Meeresunteisuch.  Konim.  wiss.  Untersneh.  dcutsch.  Meere,  Abt.  Kiel, 
N.  F.,  Vol.  3,  pp.  43-90,  Tables  A-G. 

Campbell,  M.  H. 

1934.  The  Life  History  and  Post  Embryonic  Development  of  the  Copepods,  Calanus 
tonsus  Brady  and  Euchaela  japonica  Marukawa.  Jour.  Biol.  Board  Canada, 
Vol.  1,  No.  1,  pp.  1-65,  18  textfigs. 

Clarke,  George  L.,  and  Zinn,  Donald 

1937.  Sea.sonal  Production  of  Zoiiplankton  off  Woods  Hole  with  Special  Reference  to 
Calanus  finmarchicus.  Biol.  Bull.,  Vol.  LXXIII,  No.  3,  pp.  464-487,  7 
textfigs. 

CoNSEiL  Permanent  International  pour  l'Exploration  de  la  Mer 

1904.  Bulletin  dcs  Resultats  acquis  pendant  les  courses  periodiques.   Annee  1903- 

1904,  230  pp. 

1905.  Bulletin  des  Resultats  acquis  pendant  les  courses  periodiques.   Annee  1904- 

1905,  195  pp. 

1907.  Bulletin  Trimestriel  des  Resultats  acquis  pendant  les  Croisieres  periodiques 
et  dans  les  Periodes  interm^diares.  Annee  1906-1907,  No.  1,  Juillet-Septembre 

1906,  Part  D,  38  pp. 

Cowles,  R.  p. 

1930.  A  Biological  Study  of  the  Offshore  Waters  of  Chesapeake  Bay.  Bull.  U.  S 
Bureau  of  Fisheries,  Vol.  XLVI,  1930,  pp.  277-381,  16  textfigs. 

Damas,  D. 

1905.  Notes  biologiques  sur  les  Copepodes  de  la  Mer  Norvegienne.  Publ.  Circ, 
No.  22,  Cons.  Perm.  Int.  Expl.  de  la  Mer,  23  pp.,  PI.  1. 

Delff,  Christian 

1912.  Beitrage  zur  Kenntniss  der  chemischen  Zusammensetzung  wirltelloser  Meer- 
estiere.  Wiss.  Meeresuntcrsuch.  Komm.  wiss.  Untersuch.  deutsch.  Meere, 
Abt.  Kiel.,  Vol.  XIV,  pp.  51-81. 

Farran,  G.  P. 

1910.  Copepoda  (Part).  Resume  .sur  le  Plankton  des  Mers.  Bull  trimestr.,  Cons. 
Perm.  Int.  Expl.  de  la  Mer,  Part  1,  1910  pp.  60-80,  Pis.  VIII-X. 

Fish,  Charles  J. 

1925.  Seasonal  Distribution  of  the  Plankton  of  the  Woods  Hole  Region.  Bull.  U.  S. 
Bureau  of  Fisheries,  Vol.  XLI,  1925,  pp.  91-179,  81  textfigs. 

1936.  The  Biology  of  Calanus  finmarchicus  in  the  Gulf  of  Maine  and  Bay  of  Fundy. 
Biol.  Bull.,  Vol.  LXX,  No.  1,  pp.  118-141,  11  textfigs. 

1936a.  The  Biology  of  Pseudocalanus  minutus  in  the  Gulf  of  Maine  and  Bay  of 
Fundy.  Biol.  Bull.,  Vol.  LXX,  No.  2,  pp.  193-216,  9  textfigs. 

1936b.  The  Biology  of  Oithona  similis  in  the  Gulf  of  Maine  and  Bay  of  Fundy. 
Biol.  Bull.,  Vol.  LXXI,  No.  1,  pp.  168-187,  9  textfigs. 

Fish,  Charles  J.,  and  Johnson,  Martin  W. 

1937.  The  Biology  of  the  Zooplankton  Population  in  the  Bay  of  Fundy  and  Gulf  of 
Maine  with  Special  Reference  to  Production  and  Distribution.  Jour.  Biol. 
Board  Canada,  Vol.  3,  Ser.  3,  pp.  189-322,  45  textfigs.,  29  tables. 


374  memoir:  museum  of  comparative  zoology 

Fowler,  H.  W. 

1912.  The  Crustacea  of  New  Jersey.  Report,  New  Jersey  State  Museum,  1911 
(1912),  pp.  29-650,  Pis.  1-150. 

Fraser,  C.  McL. 

1915.  Pelagic  Hydroids,  in  Bigelow,  1915,  pp.  306-315,  textfigs.  73-78. 

FuRRESTiN,  Jean 

1938.  Influence  de  la  salinite  sur  la  repartition  du  genre  Sagitta  dans  I'Atlantique 
nord-est.  Revue  des  Travaux  de  I'office  des  peches  maritimes,  Vol.  XI,  No. 
3,  pp.  425-437,  6  textfigs. 

GiBiTz,  Anton 

1922.  Verbreitung  und  Abstammung  mariner  Cladoceren.  Verhandl.  zool.-botan. 
Ge.sell.schaft  Wicn.  Jahrgang  1921,  Vol.  LXXI,  pp.  85-105,  6  charts. 

Hargitt,  Charles  W. 

1905.  The  Medusae  of  the  Woods  Hole  Region.  Bull.  U.  S.  Bureau  of  Fisheries, 
Vol.  XXIV,  1904  (1905),  pp.  21-79,  pis.  I-VII,  textfigs. 

Hensen,  Victor 

1890.  Einige  Ergebnisse  der  Plankton-Expedition  der  Humboldt-Stiftung.  Math. 

Naturwiss.   Mittheil.   Sitzungsbericht.   Konig  Preuss.   Akad.   Wiss.   Berlin. 

Jahrgang  1890,  pp.  167-177  (243-253). 
1895.  Mcthodik    der    Unter.suchungen    bei    der    Plankton    Expedition.     Ergebn. 

Plankton-Exped.  des  Humboldt-Stiftung,  Vol.  1,  B,  200  pp.,  14  textfigs.,  11 

tables,  1  chart. 

Hjort,  Johan  and  Ruud,  Johan  T. 

1938.  Deep  Sea  Prawn  Fisheries  and  Their  Problems.  Hvalr&dets  Skrifter,  Norske 
Videnskaps-Akad.  Oslo,  No.  17,  Pt.  1,  pp.  1-144,  21  textfigs. 

Holmes,  S.  J. 

1905.  The  Amphipoda  of  Southern  New  England.  Bull.  U.  S.  Bureau  of  Fisheries, 
Vol.  XXIV,  1904  (1905),  pp.  459-529,  pis.  I-XIII,  textfigs. 

Huntsman,  A.  G. 

1919.  Biology  of  Atlantic  Waters  of  Canada.  Some  Quantitative  and  Qualitative 
Plankton  Studies  of  the  Eastern  Canadian  Plankton.  2.  On  the  Quantity  of 
Plankton  obtained  at  one  hundred  and  seventy-nine  stations  during  the 
Canadian  Fisheries  Expedition,  1915.  Canadian  Fish.  Exped.  1914-1915, 
Depart.  Naval  Sci.,  pp.  407-420. 

1919a.  Biology  of  Atlantic  Waters  of  Canada.  Some  Quantitative  and  Qualitative 
Studies  of  the  Eastern  Canadian  Plankton.  3.  A  Special  Study  of  the  Canadian 
Chaetognaths,  their  Distribution,  etc.,  in  the  Waters  of  the  Eastern  Coast. 
Canadian  Fish.  Exped.  1914-1915,  Depart  Naval  Sci.,  pp.  421-485,  12 
textfigs. 

Jespersen,  p. 

1923.  On  the  quantity  of  Macroplankton  in  the  Mediterranean  and  the  Atlantic. 
Rep.  Dan.  Ocean.  Exped.  1908-1910  to  the  Mediterranean  and  Adjacent 
Seas,  Vol.  Ill,  No.  7,  Misc.  Papers,  Pt.  3,  pp.  1-17,  8  textfigs. 

1924.  On  the  quantity  of  Macroplankton  in  the  Mediterranean  and  the  Atlantic. 
Internat.  Rev.  Gesamt.  Hydrobiol.  Hydrogr.,  Vol.  XII,  pp.  102-115. 


BIGELOW  AND   SEARS:    NORTH   ATLANTIC   ZOOPLANKTON   STUDIES  375 

1935.  Quantitative  Investigations  on  the  Distribution  of  Macroplankton  in  Different 
Oceanic  Regions.   Dana  Report,  No.  7,  44  pp.,  28  textfigs. 

Johnstone,  James 

1908.  Conditions  of  Life  in  tlie  Sea.  332  pp.,  29  figs.  Cambridge,  England. 

Kemp,  Stanley 

1938.  Oceanography  and  the  Fluctuations  in  the  Abundance  of  Marine  Animals. 

Brit.  A.SS0C.  Advance.  Sci.  1938  Meet.,  Sect.  D.,  Zool.,  pp.  85-101. 
1938a.  Oceanography  and  the  Fluctuations  in  the  Abundance  of  Marine  Animals. 

Nature,  Vol.  142,  No.  3600,  pp.  777-779. 

Kraefft,  Fritz 

1910.  tJber  das  Plankton  in  Ost-  und  Nordsee  und  den  Verbindungsgebieten,  mit 
besonderer  Beriicksichtigung  der  Copepoden.  Wiss.  Meeresuntersuch.  Komm. 
wiss.  Untersuch.  deutsch.  Meere,  Abt.  Kiel,  N.  F.,  Vol.  XI,  pp.  29-108,  9 
textfigs.,  3  tables. 

LOHMAN,  H. 

1901.  Die  Appendicularien.  Nordisches  Plankton,  Zool.  Tell,  Vol.  II,  3,  pp.  11-21, 
12  textfigs. 

1911.  Die  Appendicularien.  Nachtrag.  Nordisches  Plankton,  Zool.  Teil,  Vol.  II, 
3,  pp.  23-29,  3  textfigs. 

LtJCKE,  Fr. 

1912.  Quantitative  Untersuchungen  an  dem  Plankton  bei  dem  Feuerschiff  "Borkum- 
riff"  in  Jahre  1910.  Wiss.  Meeresuntersuch.  Komm.  wiss.  Untersuch.  deutsch. 
Meere,  Abt.  ffiel,  N.  F.,  Vol.  XIV,  pp.  101-128,  3  textfigs.,  2  tables. 

Mayer,  A.  G. 

1910.  Medusae  of  the  World.  Hydromedusae.  Carnegie  Inst.  Washington,  Publ. 
No.  109,  Vol.  1,  230  pp.,  119  textfigs.,  29  pis. 

1912.  Ctenophores  of  the  Atlantic  Coast  of  North  America.  Carnegie  Inst.  Wash- 
uigton,  Publ.  No.  162,  58  pp.,  17  pis.,  12  textfigs. 

Mielck,  W. 

1911.  Quantitative  Untersuchungen  an  dem  Plankton  der  deutschen  Nordsee- 
Terminfahrten  im  Februar  und  Mai  1906.  Wiss.  Meeresuntersuch.  Komm. 
wiss.  Untersuch.  deutsch.  Meere,  Abt.  Kiel,  N.  F.,  Vol.  XIII,  pp.  313-357. 

Mielck,  W.,  and  KtJNNE,  C. 

1935.  Fischbrut-  und  Plankton-Untersuchungen  auf  dem  Reichsforschungsdamfer 
"Poseidon"  in  der  Ostsee,  Mai-Juni  1931.  Wiss.  Meeresuntersuch.  Komm. 
wiss.  Untersuch.  deutsch.  Meere,  Abt.  Helgoland,  N.  F.,  Vol.  XIX,  120  pp.,  12 
textfigs.,  8  text-tables,  39  tables. 

Orr,  a.  p. 

1934.  On  the  Biology  of  Calanus  finmarchicus.  Pt.  IV.  Seasonal  Changes  in  the 
Weight  and  Chemical  Composition  in  Loch  Fyne.  Jour.  Mar.  Biol.  Assoc, 
U.  K.,  N.  S.,  Vol.  XIX,  pp.  613-632,  5  textfigs. 

OsTENFELD,  C.  H.,  and  Jespersen,  p. 

1924.  Standard  Net  for  Plankton  Collections.  Publ.  Circ,  No.  84,  Cons.  Perm.  Int. 
Expl.  de  la  Mer,  16  pp.,  11  textfigs. 


376  memoir:  museum  of  comparative  zoology 

Parr,  A.  E. 

1933.  A  geographic-ecological  analysis  of  the  seasonal  changes  in  temperature  condi- 
tions in  shallow  water  along  the  Atlantic  coast  of  the  United  States.  Bull. 
Bingham  Oceanogr.  Collection,  Vol.  4,  pp.  1-90,  28  textfigs. 

Ramner,  Walter 

1933.  Die  Cladocereu  der  "Meteor"  Expedition.  Wiss.  Ergebn.  Deutsch.  Atlantisch. 

Exped "Meteor"  1925-1927,  Vol.  XII,  Lief.  2,  pp.  111-121,  textfigs. 

51-52. 

Ranson,  Gilbert 

1936.  Meduses  provenant  des  campagnes  du  Prince  Albert  I  de  Monaco.  Res. 
camp.  sci.  Monaco,  Vol.  XCII,  248  pp.,  2  pis. 

Rathbun,  Richard 

1889.  Notice  of  the  small  surface  organisms  taken  in  the  tow-nets,  and  of  the  con- 
tents of  mackerel  stomachs.  Bull.  U.  S.  Fish  Comm.,  Vol.  VII,  1887  (1889), 
pp.  259-267. 

Redfield,  Alfred  C. 

1939.  The  history  of  a  population  of  Limacina  retroversa  during  its  drift  across  the 
Gulf  of  Maine.   Biol.  Bull.,  Vol.  LXXVI,  No.  1,  pp.  26-47,  10  textfigs. 

ROSENFELD,  GeORG 

1904.  Studien  iiber  das  Fett  der  Meeresorganismen.  Wiss.  Meeresuntersuch.  Komm. 
wiss.  Untersuch.  deutsch.  Meere,  Abt.  Helgoland,  N.  F.,  Vol.  V,  pp.  57-83. 

Russell,  F.  S. 

1925.  The  Vertical  Distribution  of  Marine  Macroplankton.    An  Observation  on 

Diurnal  Changes.  Jour.  Mar.  Biol.  Assoc,  U.  K.,  N.  S.,  Vol.  XIII,  pp.  769- 

809,  6  textfigs. 
1927.  The  Vertical  Distribution  of  Marine  Macroplankton.    V.  The  Distribution  of 

Aiiimals  Caught  in  the  Ring-trawl  in  the  Daytime  in  the  Plymouth  Area. 

Jour.  Mar.  Biol.  Assoc,  U.  K.,  N.  S.,  Vol.  XIV,  pp.  557-608,  11  textfigs., 

4  tables. 

1934.  On  the  Occurrence  of  the  Siphonophores  Muggiaea  atlantica  Cunningham  and 
Muggiaea  kochii  (Will)  in  the  English  Channel.  Jour.  Mar.  Biol.  Assoc, 
U.  K.,  N.  S.,  Vol.  XIX,  pp.  555-558. 

1935.  On  the  Value  of  Certain  Plankton  Animals  as  Indicators  of  Water  Movements 
in  the  English  Channel  and  North  Sea.  Jour.  Mar.  Biol.  Assoc,  U.  K.,  N.  S., 
Vol.  XX,  pp.  309-332,  7  textfigs. 

RuD^,  G.  T. 

1938.  New  methods  of  marine  survey.  Proc  Am.  Phil.  Soc,  Vol.  79,  pp.  9-25,  11 
textfigs. 

Savage,  R.  E. 

1931.  The  Relation  between  the  Feeding  of  the  Herring  off  the  East  Coast  of  Eng- 
land and  the  Plankton  of  the  Surrounding  Waters.  Fishery  Invest.,  Min. 
Agri.  Fish.  (Great  Brit.),  Ser.  II,  Vol.  XII,  No.  3,  88  pp.,  32  tables,  10  pis.,  23 
textfigs. 

1937.  The  Food  of  North  Sea  Herring  1930-1934.  Fishery  Invest.  Min.  Agr.  Fish. 
(Great  Brit.),  Ser.  II,  Vol.  XV,  No.  5,  57  pp.,  16  textfigs.,  29  tables. 


bigelow  and  seahs:  north  atlantic  zooplankton  studies        377 

Schmidt,  Johannes 

1912.  Introduction.  Rep.  Dan.  Ocean.  Exped.  1908-1910  to  the  Mediterranean  and 
Adjacent  Seas,  Vol.  1,  Pt.  1,  pp.  1-75,  12  textfig.s. 

Sharpe,  Richard  W. 

1911.  Notes  on  the  marine  Copepoda  and  Cladocera  of  Woods  Hole  and  adjacent 
regions,  including  a  synop.sis  of  the  genera  Harpacticoida.  Proc.  U.  S.  Nat. 
Mus.,  Vol.  38,  1911,  pp.  405^36,  20  textfigs. 

Stephensen,  K. 

1924.  Hyperiidea-Amphipoda  (Part  2,  Paraphronimidae,  Hyperiidae,  Dairellidae, 
Phronimidae,  Anchylomeridae).  Rep.  Dan.  Ocean.  Exped.  1908-1910  to  the 
Mediterranean  and  Adjacent  Seas,  Vol.  II,  Biol.,  D.  4,  pp.  71-149,  20  textfigs., 
15  charts. 

Steuer,  Adolf 

1910.  Planktonkunde.  723  pp.,  365  textfigs.,  1  pi.  Leipzig  and  Berlin. 

St0rmer,  Leif 

1929.  Copepods  from  the  "Michael  Sars"  Expedition,  1924.  Rapp.  et  Proc.-Verb., 
Cons.  Perm.  Int.  Expl.  de  la  Mer,  Vol.  LVI,  57  pp.,  13  textfigs. 

SUDLER,  MeRVIN  T. 

1899.  The  Development  of  Penilia  schmackeri  Richard.  Proc.  Boston  Soc.  Nat. 
Hist.,  Vol.  XXIX,  pp.  109-131,  3  pis. 

Sumner,  Francis  B.,  Osburn,  Raymond  C,  and  Cole,  Leon  J. 

1913.  A  biological  survey  of  the  waters  of  Woods  Hole  and  vicinity.  Pt.  II,  Sect.  III. 
A  catalogue  of  the  marine  fauna  of  Woods  Hole  and  vicinity.  Bull.  U.  S. 
Bureau  of  Fisheries,  Vol.  XXXI,  Pt.  II,  pp.  547-794. 

Thiel,  Max  Egon 

1938.  Die  Chaetognathen-Bevolkerung  des  Slidatlantischen  Ozeans.   Wiss.  Ergebn. 

Deutsch.  Atlantisch.  Exped "Meteor"  1925-1927,  Vol.  XIII,  110  pp., 

62  textfigs. 

Walford,  Lionel  A. 

1938.  Effect  of  Currents  on  Distribution  and  Survival  of  the  Eggs  and  Larvae  of  the 
Haddock  (Melangranius  aeglefinis)  on  Georges  Bank.  Bull.  U.  S.  Bureau  of 
Fisheries,  Vol.  XLIX,  No.  29,  pp.  1-75,  textfigs.  1-50. 

Wheeler,  William  Morton 

1901.  The  free-swimming  copepods  of  the  Woods  Hole  region.  Bull.  U.  S.  Fish 
Comm.,  Vol.  XIX,  1889  (1901),  pp.  157-192,  30  textfigs. 

Wilson,  Charles  B. 

1932.  The  copepods  of  the  Woods  Hole  region.   Bull.  Smithsonian  Inst.,  U.  S.  Nat. 

Mus.,  158,  635  pp.,  316  textfigs.,  41  pis. 
1932a.  The  copepod  crustaceans  of  Chesapeake  Bay.   Proc.  U.  S.  Nat.  Mus.,  Vol. 

80,  Art.  15,  54  pp.,  5  pis. 

WiMPENNY,  R.  S. 

1929.  Preliminary  Observations  on  the  Fat-Content  of  the  Plankton  on  the  English 
Herring  Grounds  in  the  North  Sea.  Fishery  Invest.  Min.  Agri.  Fish.  (Great 
Brit.),  Ser.  II,  Vol.  XI,  No.  5,  23  pp.,  9  tables. 


378  memoir:  museum  of  comparative  zoology 

1937.  The  Distribution,  Breeding  and  Feeding  of  .some  important  Plankton  Organ- 
isms of  the  South-Wcst  North  Sea  in  1934.  Pt.  I.  Calanus  finmarchicus 
(Gunn.),  Sagitla  setosa  (J.  Mtiller),  and  Sagitta  elegans  (Verrill).  Fishery- 
Invest.  Min.  Agri.  Fish.  (Great  Brit.),  Ser.  II,  Vol.  XV,  No.  3,  pp.  3-53,  24 
te.\tfigs.,  14  tables. 

WiNsoR,  Charles  P.,  and  Walford,  Lionel  A. 

1936.  Sampling  Variations  in  the  Use  of  Plankton  Nets.  Jour.  Cons.,  Cons.  Perm. 
Int.  Expl.  de  la  Mer,  Vol.  XI,  No.  2,  pp.  190-204,  1  te.xtfig. 

ZiMMER,  Carl 

1909.  VI.  Die  nordischen  Schizopoden.  Nordisches  Plankton,  Zwolfte  Lief.,  No. 
VI,  1909,  pp.  1-178,  383  textfigs. 


PUBLICATIONS 


OF  THE 


MUSEUM  OF  COMPARATIVE    ZOOLOGY 


AT  HARVARD  COLLEGE. 


There  have  been  published  of  the  Bulletin  Vols.  I  to  LXXXV; 
of  the  Memoirs,  Vols.  I  to  LV. 

A  price  list  of  the  publications  of  the  Museum  will  be  sent  on  appli- 
cation to  the  Director  of  the  Museum  of  Comparative  Zoology,  Cambridge, 
Mass. 


3   2044   066 


Date  Due