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NOAA Technical Report NMFS SSRF- 701

ae at
Rey é 30% :

Seasonal Abundance and Distribution
of Zooplankton, Fish Eggs, and

Fish Larvae in the Eastern

Gulf of Mexico, 1972-74

EDWARD D. HOUDE and NICHOLAS CHITTY

SEATTLE, WA
AUGUST 1976

nNoa@a NATIONAL OCEANIC AND ; 4 National Marine
ATMOSPHERIC ADMINISTRATION Fisheries Service

NOAA TECHNICAL REPORTS
National Marine Fisheries Service, Special Scientific Report —Fisheries Series

The major responsibilities of the National Marine Fisheries Service (NMFS) are to monitor and assess the abundance and geographic distribution of fishery
resources, to understand and predict fluctuations in the quantity and distribution of these resources, and to establish levels for optimum use of the resources.
NMFS is also charged with the development and implementation of policies for managing national fishing grounds, development and enforcement of domestic
fisheries regulations, surveillance of foreign fishing off United States coastal waters, and the development and enforcement of international fishery agreements
and policies. NMFS also assists the fishing industry through marketing service and economic analysis programs, and mortgage insurance and vessel construction
subsidies. It collects, analyzes, and publishes statistics on various phases of the industry. { Miki

The Special Scientific Report—Fisheries series was established in 1949. The series carries reports on scientific investigations that document long-term
continuing programs of NMFS, or intensive scientific reports on studies of restricted scope. The reports may deal with applied fishery problems. The series is
also used as a medium for the publication of bibliographies of.a specialized scientific nature. F

NOAA Technical Reports NMFS SSRF are available free in limited numbers to governmental agencies, both Federal and State. They are also available in
exchange for other scientific and technical publications in the marine sciences. Individual copies may be obtained (unless otherwise noted) from D83, Technical
Information Division, Environmental Science Information Center, NOAA, Washington, D.C. 20235. Recent SSRF's are:

619 Macrozooplankton and smal! nekton in the coastal waters off Vancouver
Island (Canada) and Washington, spring and fall of 1963. By Donald S. Day,
January 1971, iii + 94 p., 19 figs., 13 tables.

620. The Trade Wind Zone Oceanography Pilot Study. Part IX: The sea-
level wind field and wind stress values, July 1963 to June 1965. By Gunter R.
Seckel. June 1970, iii + 66 p., 5 figs.

621. Predation by sculpins on fall chinook salmon, Oncorhynchus tshawyt-
scha, fry of hatchery origin. By Benjamin G. Patten. February 1971, iii + 14
p.. 6 figs., 9 tables.

622. Number and lengths, by season, of fishes caught with an otter trawl
near Woods Hole, Massachusetts, September 1961 to December 1962. By F. E.
Lux and F. E. Nichy. February 1971, iii + 15 p., 3 figs., 19 tables.

623. Apparent abundance, distribution, and migrations of albacore, Thunnus
alalunga, on the North Pacific longline grounds. By Brian J. Rothschild and
Marian Y. Y. Yong. September 1970, v + 37 p., 19 figs., 5 tables.

624. Influence of mechanical processing on the quality and yield of bay
scallop meats. By N. B. Webb and F. B. Thomas, April 1971, iii + 11 p., 9
figs., 3 tables.

625. Distributionof salmon and related oceanograpic features in the North
Pacific Ocean, spring 1968. By Robert R. French, Richard G. Bakkala, Masanao
Osako, and Jun Ito. March 1971, iii + 22 p., 19 figs., 3 tables.

626. Commercial fishery and biology of the freshwater shrimp, Macrobra-

chtum, in the Lower St. Paul River, Liberia, 1952-53. By George C. Miller.

February 1971, iii + 13 p., 8 figs., 7 tables.

627. Calico scallops of the Southeastern United States, 1959-69. By Robert
Cummins, Jr. June 1971, iii + 22 p., 23 figs., 3 tables.

628. Fur Seal Investigations, 1969. By NMFS, Marine Mammal Biological
Laboratory. August 1971, 82 p., 20 figs., 44 tables, 23 appendix A tables, 10
appendix B tables.

629. Analysis of the operations of seven Hawaiian skipjack tuna fishing
vessels, June-August 1967. By Richard N. Uchida and Ray F. Sumida. March
1971, v + 25 p., 14 figs., 21 tables. For sale by the Superintendent of
Documents, U.S. Government Printing Office, Washington, D.C. 20402.

630. Blue crab meat. I. Preservation by freezing. July 1971, iii + 13 p., 5
figs., 2 tables. Il. Effect of chemical treatments on acceptability. By Jurgen H.
Strasser, Jean S. Lennon, and Frederick J. King. July 1971, iii + 12 p., 1 fig.,
9 tables.

631. Occurrence of thiaminase in some common aquatic animals of the United
States = Canada. By R. A. Greig and R. H. Gnaedinger. July 1971, iii + 7
p.. 2 tables.

632. An annotated bibliography of attempts to rear the larvae of marine fishes
in the laboratory. By Robert C. May. August 1971, iii + 24 p., 1 appendix I
table, 1 appendix II table. For sale by the Superintendent of Documents, U.S.
Goverment Printing Office, Washington, D.C. 20402.

633. Blueing of processed crab meat. II Identification of some factors involved
in the blue discoloration of canned crab meat Callinectes sapidus. By Melvin E.
Waters. May 1971, iii + 7 p., 1 fig., 3 tables.

634. Age composition, weight, length, and sex of herring, Clupea pallasii,
used for reduction in Alaska, 1929-66. By Gerald M. Reid. July 1971, iii + 25
p., 4 figs., 18 tables.

Continued on inside back cover.

635. A bibliography of the blackfin tuna, Thunnus atlanticus (Lesson).
Grant L. Beardsley and David C. Simmons. August 1971, 10 p. For sale by
Superintendent of Documents, U.S. Government Printing Office, Washin
D.C. 20402.

636. Oil pollution on Wake Island from the tanker R. C. Stoner. By Regi
M. Gooding. May 1971, iii + 12 p. 8 figs., 2 tables. For sale by
Superintendent of Documents, U.S. Government Printing Office, Washin
D.C. 20402.

637. Occurrence of larval, juvenile, and mature crabs in the vicinit;
Beaufort Inlet, North Carolina. By Donnie L. Dudley and Mayo H.
August 1971, iii + 10 p., 1 fig., 5 tables. For sale by the Superintenden
Documents, U.S. Government Printing Office, Washington, D.C. 20402.

638. Length-weight relations of haddock from commercial landings in
England, 1931-55. By Bradford E. Brown and Richard C. Hennemuth. Aug
1971, v + 13 p., 16 figs., 6 tables, 10 appendix A tables. For sale by
a ae tags of Documents, U.S. Government Printing Office, Washing
D.C. 20402.

639. A hydrographic survey of the Galveston Bay system, Texas 1963-66.
E. J. Pullen, W. t. Trent, and G. B. Adams. October 1971, v + 13 p.,
figs., 12 tables. For sale by the Superintendent of Documents, U.
Government Printing Office, Washington, D.C. 20402.

640. Annotated bibliography on the fishing industry and biology of the bl
crab, Callinectes sapidus. By Marlin E. Tagatz and Ann Bowman Hall. Aug
1971, 94 p. For sale by the Superintendent of Documents, U.S. Governme
Printing Office, Washington, D.C. 20402.

641. Use of threadfin shad, Dorosoma petenense, as live bait during expe
mental pole-and-line fishing for skipjack tuna, Katsuwonus pelamis, in Ha
By Robert T. B. Iversen. August 1971, iii + 10 p., 3 figs., 7 tables. For sale
the Superintendent of Documents, U.S. Government Printing Office, Washi
ton, D.C. 20402.

642. Atlantic menhaden Brevoortia tyrannus resource and fishery—analysis
decline. By Kenneth A. Henry. August 1971, v + 32 p., 40 figs., 5 append
figs., 3 tables, 2 appendix tables. For sale by the Superintendent of Document
U.S. Government Printing Office, Washington, D.C. 20402.

643. Surface winds of the southeastern propical Atlantic Ocean. By John
Steigner and Merton C. Ingham. October 1971, iii + 20 p., 17 figs. For sale
the Superintendent of Documents, U.S. Government Printing Office, Washi
ton, D.C. 20402.

644. Inhibition of flesh browning and skin color fading in frozen fillets
yelloweye snapper (Lutzanus vivanus). By Harold C. Thompson, Jr., and Mai
H. Thompson. February 1972, iii + 6 p., 3 tables. For sale by the Superinte
dent of Documents, U.S. Government Printing Office, Washington, D.C. 2

645. Traveling screen for removal of debris from rivers. By Daniel W. Bat
Ernest W. Murphey, and Martin G. Beam. October 1971, iii + 6 p., 6 figs.,
table. For sale by the Superintendent of Documents, U.S. Government Printi
Office, Washington D.C. 20402.

646. Dissolved nitrogen concentrations in the Columbia and Snake Rivers
1970 and their effect on chinook salmon and steelhead trout. By Wesley J. Ebe
August 1971, iii + 7 p., 2 figs., 6 tables. For sale by the Superintendent-
Documents, U.S. Government Printing Office, Washington D.C. 20402.

647. Revised annotated list of parasites from sea mammals caught off t
west coast of North America. By L. Margolis and M. D. Dailey. March 1972,
+ 23 p. For sale by the Superintendent of Documents, U.S._Governme:
Printing Office, Washington, D.C. 20402.

NOAA Technical Report NMFS SSRF-/01

Seasonal Abundance and Distribution
of Zooplankton, Fish Eggs, and

Fish Larvae in the Eastern

Gulf of Mexico, 1972-74

EDWARD D. HOUDE and NICHOLAS CHITTY

SEATTLE, WA
AUGUST 1976

UNITED STATES NATIONAL OCEANIC AND National Marine
DEPARTMENT OF COMMERCE we ATMOSPHERIC ADMINISTRATION Fisheries Service 6
Elliot L. Richardson, Secretary Robert M. White, Administrator Robert W. Schoning, Director %

The National Marine Fisheries Service (NMFS) does not approve, rec-
ommend or endorse any proprietary product or proprietary material
mentioned in this publication. No reference shall be made to NMFS, or
to this publication furnished by NMFS, in any advertising or sales pro-

motion which would indicate or imply that NMFS approves, recommends
or endorses any proprietary product or proprietary material mentioned
herein, or which has as its purpose an intent to cause directly or indirectly
the advertised product to be used or purchased because of this NMFS
publication.

CONTENTS

Introduction
Methods
INEST ogo cec 00 goo COOH map ORO eh Clk aaates SiR eI cir a e ke a oe ary ie a ego A
_ USER 7 LISS SUISSE et ee
Seasonality of abundance
SOP EDSETIS DLE: SL DCCs cs aR BS ee cg ee
Distribution relative to the 50-m depth contour
ea DAIA DEO eA MERON RENE Joc n= (0) 20) 5 ore I ew aca 8) 27e Ae Tales aivshacpnje'uls aie odds s¥lemaces
Pee GUTS AONE MIOOPICAl VATIADICS . 5 o.-. 5 5. cec< 5» opspa/niecssaiviais na ayocn os jee tyes vendnedienceccecsseuees
Relationships of biological variables to temperature and salinity
Acknowledgments
Literature cited

Figures

Chart of eastern Gulf of Mexico ichthyoplankton survey area
Contour charts of zooplankton volumes
Cantonimchavesoensne mia BUNGANCe mete riisei ties eine siels © cies misiave roe eels evs S's Bee ve ue adie ve ayediane
CaontounchariesomisimarvacaDUNGAaNCe sce + <4. 2c a6 ci se ls 6 Ohne Be dirs sowie edeat dase oeaceameaens
Frequency distribution of zooplankton volumes
PrequencyMiscrinnilonior msnierp ADUNGANCE <... 6... 2 oes ee cs cles eo Sake She sicle Do obese ee nccdee ds
Frequency distribution of fish larvae abundance
Mean zooplankton volumes
NU GET TEI GEE ASG CTCD 0-0-5 6 hoe Onn COO OT TOTES OGIO Ge DSTORE CG ce eINC ences eine ay Senne ee
hme Vieanbashtlar aera pln Gane wey were yey eof che daie = ayaie vas «ci cvayshaiebsyevsusveleiegs ieiaseleretaresdiduoyseecereiie%e wife goyee © a
11. Comparison of mean zooplankton volumes for stations north and south of lat. 27°15'N ................
12. Comparisons of mean fish egg and fish larvae abundances north and south of lat. 27°15'N
13. Comparison of mean zooplankton volumes between stations less than and more than50m...:.........
14. Comparisons of mean fish egg and fish larvae abundances between stations less than and more
than 50m

5S OO Se ST eNOS

Tables
1. Cruises and number of stations analyzed for zooplankton volumes and ichthyoplankton, 1972-74 .......
2. Mean zooplankton volumes and numbers of fish eggs and larvae for each cruise ......................
3. Summary of plankton, fish eggs, and fish larvae data for each cruise in northern and southern areas ...
4. Summary of plankton, fish eggs, and fish larvae mean catches for each cruise in depths less than and
REALE Tat ANU IT Weeray eee see a ep raae, eeratse U e scsi weiss avaiable av suclolofelens sicueterern oWdleaniay a Pace eugie aes. ous
5. Summary of simple correlations among biological variables ................. 0.0 cee eee eee eee eee

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Seasonal Abundance and Distribution of

Zooplankton, Fish Eggs, and Fish Larvae in the
Eastern Gulf of Mexico, 1972-74’

EDWARD D. HOUDE and NICHOLAS CHITTY?

ABSTRACT

Zooplankton volumes and abundance of fish eggs and fish larvae were determined for stations on 12
cruises to the western Florida continental shelf. Contour charts of zooplankton volumes and of
ichthyoplankton abundance are presented. A marked seasonality was observed for zooplankton and
ichthyoplankton, highest zooplankton volumes and ichthyoplankton abundance occurring during May
through September. Zooplankton volumes were highest and spawning by fishes most intense in the northern
half of the study area (north of lat. 27°15'N). Fish larvae abundance (number under 10 m? of sea surface)
was highest at stations deeper than 50 m. Simple correlations among biological variables showed fish egg
abundance-zooplankton volumes and fish egg abundance-fish larvae abundance to be positively correlated on
most cruises. No clear relationships were observed between abundance or concentration of biological

variables and temperature or salinity.

INTRODUCTION

A fisheries assessment survey of sardinelike fishes in
the eastern Gulf of Mexico was carried out from 1971 to 1974
to estimate spawning biomass from the seasonal abundance
and distribution of planktonic eggs and larvae (Houde 1973,
1974, in press). A total of 12 cruises was completed during
1972-74 from which both plankton samples and hydrographic
data were collected. Zooplankton abundance and the abun-
dance of both fish eggs and fish larvae were examined to de-
termine which areas of the western Florida continental shelf
(Fig. 1) were highly productive with respect to plankton and
which areas might be important spawning areas for fishes.
Abundance of these organisms was examined seasonally and
in relation to temperature and salinity in the eastern Gulf.

Most previous reports on plankton abundance from the
western Florida shelf area included few sampling stations or
were not carried out long enough to determine seasonality of
production. Hopkins (1973) recently has reviewed plankton
investigations in the eastern Gulf. Plankton abundance from
shelf waters on a transect west of Charlotte Harbor (lat.
26°30'N) in 1946-47 was reported by King (1949). Arnold
(1958) summarized results of plankton-ichthyoplankton col-
lections made throughout the Gulf of Mexico in 1951-53. His
collections included 63 stations from the western Florida
shelf that were sampled quantitatively. Soviet-Cuban
fishery investigations in 1963-64 included some determina-
tions of total plankton biomass from most parts of the Gulf of
Mexico. These data were summarized by Bogdanov et al.
(1968) and Khromov (1969). Recently, Austin and Jones
(1974) reported on the physical oceanography and plankton
biomass relationship on the Florida Middle Ground, a
‘productive shelf area located northwest of Tampa Bay.
Their data represented monthly observations from June
1969 through August 1970 and included a single fixed station
at lat. 28°30'N, long. 84°14’W, as well as three to five

1 Contribution from the Rosenstiel School of Marine and Atmospheric
Science, University of Miami, Miami, FL 33149.

? Division of Biology and Living Resources, Rosenstiel School of
Marine and Atmospheric Science, University of Miami, 4600 Ricken-
backer Causeway, Miami, FL 33149.

stations on a transect along lat. 28°30'N. Although earlier
reports are incomplete in many respects, and sometimes
incomparable due to collecting gear differences, they do
offer a basis for comparison with our plankton distribution
data which give more complete seasonal and areal coverage
of the eastern Gulf.

Identification of organisms was limited to the broad
categories of zooplankton, fish eggs, and fish larvae. Thus,
our results were used only in a relative sense, to define
areas of high or low productivity on the western Florida
shelf. The eggs and larvae of clupeid fishes were identified
and future papers will describe their distribution and
abundance, as well as estimates of stock abundance based on
them (Houde in press). Results published here will be useful
when considering possible effects of man’s activities in the

FLORIDA

28°

1] northern sector
27°15 southern sector

a, 86° 84° 82° go°

Figure 1.—Chart of eastern Gulf of Mexico ichthyoplankton survey area.
Dots indicate stations.

eastern Gulf of Mexico, because the area has important
commercial fisheries and is presently a focus of oil explora-
tion.

METHODS

Data based on 12 cruises during 1972-74 are included in
this report (Table 1). Plankton volumes were determined
from a total of 482 stations and fish eggs and larvae were
collected at 483 stations. Stations were located on a grid
extending from lat. 24°30'N to lat. 29°30'N and from the
10-m contour near the west Florida coast to approximately
the 200-m contour at the edge of the continental shelf (Fig.
1). The two cruises in 1974 included some stations in water
as shallow as 4 m to compare plankton biomass and fish
spawning there with offshore areas. The number of stations
occupied on cruises ranged from 13 to 51 (Table 1).

Plankton collections were made with the paired 61 cm
Bongo net plankton sampler (Posgay et al. 1968). One net
was fitted with 505-um Nitex mesh from which fish eggs and
larvae were sorted. The second net was 333-um Nitex mesh
and it was used to provide zooplankton for volume determin-
ations; this net meets the specifications of a reference net
prescribed by the National Academy of Sciences (1969) for
determining zooplankton biomass. A TSK (Tsurumi-Seiki
Kosakusho) flowmeter was secured in the mouth of the
505-u~m mesh net to determine volume filtered during each
tow. Tows were double oblique from within 5 m of bottom to
surface, or from 200-m depth to surface at deep stations.
Towing speed was 2 to 3 knots and winch speeds were
controlled to ensure that all depths were sampled propor-
tionately. A Benthos time-depth recorder was attached to
the plankton sampler to record tow characteristics. Tow
times varied from 1 min at stations shallower than 50 m to
20 min at the deepest stations. Volumes filtered during a
tow varied from approximately 100 m? at shallow stations
to 400 m? at the deepest stations.

All plankton collected by the nets was retained and
preserved in 10% buffered seawater Formalin. In the
laboratory all fish eggs and larvae were sorted from the
505-um mesh net samples. After samples had been stored for
at least 2mo to allow for changes in plankton volume due to
preservation, volumes were determined using methods
similar to those described by Thrailkill (1969) and Kramer et
al. (1972). Zooplankton volumes are displacement volumes,

Table 1.—Cruises and number of stations analyzed for zooplankton
volumes and ichthyoplankton, 1972-74. 8B = RV Bellows, GE = RV
Gerda, IS = RV Columbus Iselin, and CL = RV Calanus.

Plankton Fish egg
volume and larvae

Cruise Dates stations stations
8B 7201 & GE 7202 1-11 Feb. 1972 26 30
GE 7208 1-10 May 1972 28 30
GE 7210 12-18 June 1972 13 13
IS 7205 9-17 Sept. 1972 34 34
IS 7209 8-16 Nov. 1972 49 50
IS 7303 19-27 Jan. 1973 50 51
IS 7308 9-17 May 1973 49 49
IS 7311 27 June-6 July 1973 51 51
IS 7313 3-13 Aug. 1973 50 50
IS 7320 6-14 Nov. 1973 51 51
CL7405 28 Feb.-9 Mar. 1974 36 32
CL7412 1-9 May 1974 45 42
Totals 482 483

reported as cubic centimeters per 1,000 m* of seawater
filtered by the collecting net. Contour charts (Fig. 2)
illustrate zooplankton concentrations for each cruise. These
are mean values for the whole water column at a location
since discrete depth strata were not sampled.

Zooplankton displacement volumes were determined
after removing nonplanktonic organisms (e.g., seaweeds,
large fish) and large planktonic organisms (>5 cc; mostly
coelenterates). The remaining zooplankton organisms were
of 5 ce or less in volume. This fraction of the plankton
sampled was aspirated onto a 35-um plankton mesh filter in
a Buchner funnel under a 22 psi vacuum until less than one
drop of water per 15 s was drawn from the sample. Samples
with large numbers of jellylike organisms were aspirated for
shorter times to avoid reduction in plankton volume due to
body fluid loss. Volume was then determined by water
displacement in a graduated cylinder. The zooplankton
volumes given in this report, therefore, are for organisms
less than 5 ec. The most numerous organisms were crusta-
ceans, molluscs, and chaetognaths, all of which are impor-
tant in food webs that sustain pelagic fish resources.

Numbers of fish eggs and larvae were reported as
numbers under 10 m? of sea surface, giving an estimate of
abundance for each station. Contour charts of the abundance
of fish eggs and larvae (Figs. 3, 4) show the distribution
based on this method. Mean concentrations of fish eggs and
larvae at a station also can provide useful information. These
values are not illustrated or reported here but can be
obtained from our abundance charts and a bathymetric chart
of the eastern Gulf by multiplying our contoured abundance ©
values by 10/d (where d equals depth of tow in meters), to
give numbers per 100 m?. Tow depths and volumes filtered
for stations on each cruise are available in a comprehensive
data report (Houde et al. 1976). If calculated, concentrations
of fish eggs or larvae at a given station represent the mean
concentration over the tow depth of that station because
discrete depths were not sampled.

Contour charts for zooplankton, fish eggs, and fish
larvae were produced using the method proposed by
Simpson (1959). Isopleths of zooplankton concentrations
were drawn for the 25, 50, 100, 400, and 800 cc/1,000 m? con-
tours (Fig. 2). Those for fish eggs and larvae were drawn at
50, 100, 400, 800, and 1,600 under 10 m2 of sea surface (Figs.
3, 4).

Temperatures and salinities were obtained at each sta-
tion by casts of 1.7 liter Niskin bottles equipped with
reversing thermometers. A mechanical bathythermograph
also was dropped at each station. An STD (salinity-temper-
ature-depth) recorder was used on some cruises. Salinities
from Niskin bottle samples were determined on a salinity
bridge.

Analyses of hydrographic data and contour plots of
these data were provided us by physical oceanographers at
the State University System of Florida Institute of Ocean-
ography and the Rosenstiel School of Marine and Atmo-
spheric Science.” 4

3State University System of Florida, Institute of Oceanography, St.
Petersburg, Fla. 1975. Final Report to Bureau of Land Management.
Contract No. 08550-CT4-16. Compilation and summation of historical and
existing physical oceanographic data from the eastern Gulf of Mexico, 97
p. plus 7 appendices.

‘Hydrographic data from cruises discussed in our report are
available through the National Oceanographic Data Center, Washington,
D. C. Data can be retrieved from the MAFLA (Mississippi-Alabama-
Florida) file.

88-7201

GE -7202 GE-7208
FEBRUARY 1972 \ ; MAY 1972

ZOOPLANKTON Ice per 1000m3) ZOOPLANKTON (cc per 1000m3)

86°W. B4ASWw 82°W.

GE-7210 IS-7205

JUNE 1972 oN SEPTEMBER 1972 30°N
ZOOPLANKTON (cc per 1000m3)

ZOOPLANKTON (cc per 1000 m3)

B°N

6°N 26°N

aN 24°N

Figure 2.— Contour charts of zooplankton volumes (cubic centimeters per 1,000 m°) for 12 cruises in the eastern Gulf of Mexico, 1972-74.

Sow i oY

>.

18-7209 1S-7303
NOVEMBER 1972 \ Son JANUARY 1973
ZOOPLANKTON icc per 1000m3) ZOOPLANKTON cc per 1000m3)

50m.

ox

24°N

IS-7308 :
MAY 1973 N JUNE-JULY 1973
ZOOPLANKTON (cc per 1000m3)

Is-7311

ZOOPLANKTON (cc per 1000m3)

26°N

4°N

Figure 2.— Continued.

Is 7313 IS-7320
AUGUST 1973 NOVEMBER 1973
ZOOPLANKTON icc per 1000m3) ZOOPLANKTON (cc per 1000m3)

86°W. B4°W 82°W.
KS
SS. BK cL-7405 Is 7412
MARCH 1974 30°N MAY 1974
oS e e ZOOPLANKTON (cc per 1000m3) ZOOPLANKTON (cc per 1000m3)
7 g
»

28°N

+ * - 126°N
\
1 >100
\
|
2 200,
i)
i
z
: 200’

24°N

Figure 2.— Continued.

88-7201 GE-7208
GE -7202 MAY 1972
FEBRUARY 1972 EGGS (number per 10m?)
EGGS (number per 10m?)

GE-7210 IS-7205
JUNE 1972 SEPTEMBER 1972
EGGS (number per 10m2) EGGS (number per 10m?)

Figure 3.—Contour charts of fish egg abundance (numbers under 10 m? of sea surface) for 12 eastern Gulf of Mexico cruises, 1972-74.

86°W

Is-7209
NOVEMBER 1972
EGGS (number per 10m?)

IS-7308
MAY 1973 30°N
EGGS (number per 10m?)

28°N

26°N

22°N

Figure 3.—Continued.

g4°w srw
i)
A
EF ARE IS-7303
JANUARY 1973 we ee
EGGS (number per 10m2) \\
10
> .
26°N
{|
26°N

24°N

IS-7311
JUNE-JULY 1973
EGGS (number per 10m2)

Saw Bow

= a 3
= ihe. « IS 7313
a 1, AUGUST 1973

tGGS per Wins

numer

ITS 7320
NOVEMBER 1973
EGGS (number per 10m?)

CL 7405
MARCH 1974 30°N
EGGS (number per 10m2)

28°N

126°N

24°N

Figure 3.— Continued.

IS-7412
MAY 1974
EGGS (number per 10m2)

88-7201

GE-7202 GE -7208
FEBRUARY 1972 MAY 1972

LARVAE (number per 10m2) LARVAE (number per 10m?)

GE-7210 IS-7205

JUNE 1972 3O°N SEPTEMBER 1972 30°N

LARVAE (number per 10m2) LARVAE (number per m2)

28°N

26°N

24°N 24°N

Figure 4.—Contour charts of fish larvae abundance (number under 10 m? of sea surface) for 12 eastern Gulf of Mexico cruises, 1972-74.

IS-7209 ‘ J IS 7303
NOVEMBER 1972 r JANUARY 1973
LARVAE (number per 10m?) - LARVAE (number per 10m?)

B6°~W Baw
A
SS
IS-7308 FAAS IS-7311
MAY 1973 30°N JUNE-JULY 1973 \ 30°N
LARVAE (number per 10m2! i BD 50 LARVAE (number per 10m2) _—\
Son
e ° e e
ben

|26°N

24°N

Figure 4.—Continued.

10

86"W. 84°W 82°W

SS
Las pa, 1S.7313 18-7320
AUGUST 1973 \ SO°N NOVEMBER 1973
BD LARVAE (number per 10m2) LARVAE (number per 10m2)

28°N

26°

24°N

CL-7405 CL-7412
MARCH 1974 30°N MAY 1974 30°N
LARVAE (number per 10m2) LARVAE (number per 10m2)

28°N

26°N

24°N

Figure 4.— Continued.

11

RESULTS

Nature of Distributions

Distributions of zooplankton, fish eggs, and fish larvae
are patchy in the eastern Gulf (Figs. 2-4). Means, standard
errors of the means, and coefficients of variation were cal-
culated for each kind of observation for each cruise (Table 2).
For the 12 cruises, mean zooplankton volumes ranged from
69.1 to 287.8 cc/1,000 m* in the study area. The mean value
for all cruises combined was 153.4 cc/1,000 m*. Mean
numbers of fish eggs under 10 m? of sea surface ranged from

163.1 to 927.1 for the 12 cruises, and the pooled mean for ~
combined cruises was 510.1. Fish larvae mean abundance |
ranged from 55.3 to 825.4 under 10 m? of sea surface and the ||
pooled mean was 435.5. |

The relatively high standard errors and high coeffi-
cients of variation (Table 2) reflect patchiness in distribution })
of these organisms in the eastern Gulf. The coefficient of |
variation is a relative measure of the variability in abun-
dance or concentration. For plankton volumes, the coeffi-
cient ranged from 56.3 to 187.2% on the 12 cruises, |
averaging 91.8%. The range for fish egg abundance was 58.4 |
to 174.1%, averaging 99.4%. For fish larvae abundance, the |

Table 2.— Mean zooplankton volumes (cubic centimeters per 1,000 m‘), and numbers of fish eggs and larvae
(numbers under 10 m’ sea surface) for eastern Gulf of Mexico cruises, 1972-74.

Number of Standard Coefficient of
Cruise Cruise date observations Mean error variation (%)
Zooplankton volumes
(ce per 1,000 m‘)

8B 7201 & GE 7202 Feb. 1972 26 69.1 9.7 (ilar
GE 7208 May 1972 28 107.1 16.7 82.4
GE7210 June 1972 13 287.8 149.4 187.2
IS 7205 Sept. 1972 34 181.0 21.0 67.8
IS 7209 Nov. 1972 49 119.3 9.9 58.3
IS 7303 Jan. 1973 50 152.9 13.3 61.4
IS 7308 May 1973 49 204.3 25.0 85.8
IS 7311 June-July 1973 51 120.2 11.9 70.9
IS 7313 Aug. 1973 50 194.9 31.2 113.1
IS 7320 Nov. 1973 51 131.3 10.4 56.3
CL7405 Mar. 1974 36 92.2 122 79.3
CL7412 May 1974 45 219.1 54.6 167.1

Total 482 153.4 8.5 91.8

Fish eggs
(number under 10 m? sea surface)

8B 7201 & GE 7202 Feb. 1972 30 517.6 164.5 174.1
GE 7208 May 1972 30 577.7 119.9 113.7
GE 7210 June 1972 13 527.2 85.4 58.4
IS 7205 Sept. 1972 34 578.8 72.5 73.1
IS 7209 Nov. 1972 50 186.5 21.9 83.0
IS 7303 Jan. 1973 51 874.9 58.8 112.1
IS 7308 May 1973 49 872.2 129.7 104.1
IS 7311 June-July 1973 51 450.2 50.2 79.6
IS 7313 Aug. 1973 50 725.1 158.5 154.6
IS 7320 Nov. 1973 51 163.1 18.4 80.7
CL 7405 Mar. 1974 32 291.1 42.6 82.7
CL7412 May 1974 42 927.1 109.0 76.2

Total 483 510.1 30.3 99.4

Fish larvae
(number under 10 m? sea surface) i

8B 7201 & GE 7202 Feb. 1972 30 226.6 46.2 111.7
GE 7208 May 1972 30 414.3 47.5 62.8
GE 7210 June 1972 13 341.5 130.9 138.1
IS 7205 Sept. 1972 34 825.4 112.1 79.2
IS 7209 Nov. 1972 50 295.5 45.6 109.0
IS 7303 Jan. 1973 51 301.2 45.1 106.9
IS 7308 May 1973 49 613.5 62.5 71.3
IS 7311 June-July 1973 51 551.5 77.8 100.8
IS 7313 Aug. 1973 50 581.6 94.8 115.2
IS 7320 Nov. 1973 51 346.0 55.5 114.5
CL7405 Mar. 1974 32 55.3 12.6 129.3
CL7412 May 1974 42 519.2 89.7 112.0

Total 483 435.5 19.5 104.2

12

coefficient ranged from 62.8 to 138.1%, averaging 104.2%.
Frequency distributions of abundance from pooled data of all
cruises also demonstrate the patchy nature of the distribu-
' tions. Small or modest catches were observed most fre-
quently (Figs. 5-7), but very high abundances or concen-
_ trations of organisms occurred at a few stations during each
cruise. Frequency distributions for individual cruises,
though not illustrated here, closely resembled those for the
pooled data, demonstrating that the patchy nature of the
distributions is not a seasonal phenomenon but occurs
throughout the year.

Seasonality of Abundance

Zooplankton volumes and spawning by fishes show a
distinct seasonality in the eastern Gulf (Table 2; Figs. 8-10).
Plankton volumes are largest and fish eggs and fish larvae
are most abundant in the spring and summer months (May
through September cruises). Fall and winter (November
through March cruises) abundance is less, but the decrease
is more apparent for eggs and larvae than for zooplankton
volumes. Mean fall-winter zooplankton volumes were less
than 125 ec/1,000 m; while spring-summer means increased
to 200 cc/1,000 m? or more. A sharp drop in mean zoo-

NUMBER OF STATIONS

x
PLANKTON (cc per I000M®>)

Figure 5.—Frequency distribution of zooplankton volumes, based on
samples from all stations made during 12 eastern Gulf of Mexico cruises,
1972-74.

i"

Y
g
Z
Y

60
Gg

Z
g
g
Z

NUMBER OF STATIONS

Z
y
Z

Z

Z D
Z Z
j
j
500

Z
ay
YY
AY
AY
100-

200

y
Y
Z
a

n
; AUBAUY Y AA
100 38° 398 $88" 288° SOS E88" $63 BBS 8S" woe BES 1S isos" os" IFS too 9
EGGS (No per 10 M7)

Figure 6.—Frequency distribution of fish egg abundance based on
samples from all stations made during 12 eastern Gulf of Mexico cruises,
1972-74.

NUMBER OF STATIONS

Figure 7.—Frequency distribution of fish larvae abundance based on
samples from all stations made during 12 eastern Gulf of Mexico cruises,
1972-74.

bey
a
es

MEAN (+1 Standard Error) PLANKTON VOLUME (ce per 1900m3)

on
[e}

PRMPARMG Jans Alcs, -OUNIND ISIE IMMATIM: al 1° TAS Sh OT INSDieU im Foi) Anni
1972 1973 1974
DATES OF CRUISES

Figure 8.—Mean zooplankton volumes for 12 eastern Gulf of Mexico
cruises, 1972-74.

SH EGGS (under 10m2 sea surface)

se 8 8 8 8 38 8 3

MEAN NUMBER (+1 Standard Error)

FEFMAMJIJIASONODOSF MAMJJSASONODS FMAM
1972 1973 1M
DATES OF CRUISES

Figure 9.—Mean fish egg abundance for 12 eastern Gulf of Mexico
cruises, 1972-74.

—oe = ere en ee Ee eee

ON J FMAM J JAS OND JSF MAM

1973 1974
DATES OF CRUISES

MEAN NUMBER

Figure 10.—Mean fish larvae abundance for 12 eastern Gulf of Mexico
cruises, 1972-74.

plankton volume was observed in June-July 1973, along with
similar decreases in ichthyoplankton abundance. There was
a recovery in abundance by August 1973. Mean abundance
of fish eggs was less than 300 under 10 m? of sea surface

during fall-winter but exceeded 550 during the spring-sum-
mer months. Fish larvae mean abundance was less than 350
under 10 m? in fall-winter, increasing to more than 500 in
spring-summer.

North-South Distribution

Zooplankton and ichthyoplankton were examined for
each cruise in relation to latitude. The study area was arbi-
trarily divided into halves at lat. 27°15'N (Fig. 1). Catches
north and south of this line were compared to determine if
abundance differed greatly between the two areas.

Mean abundance of eggs and larvae and zooplankton
volumes usually were greatest north of lat. 27°15'N (Table
3; Figs. 11, 12). Only fish larvae had a higher pooled mean
abundance (number under 10 m?) in the southern half of the
area for all cruises combined. The contour charts of
organism distribution (Figs. 2-4) do not readily show that
mean abundance tended to be higher north of lat. 27°15'N
because of the patchy nature of the distributions.

Mean zooplankton volumes were highest in the north-
ern sector for 7 of the 12 cruises (Table 3; Fig. 11). Four of
the five exceptions occurred during spring and summer
cruises. Mean fish egg abundance was highest in the north
during 6 of the 12 cruises (Table 3; Fig. 12). The pooled
mean abundance of eggs for all cruises was highest for the

Table 3.—Summary of plankton, fish eggs, and fish larvae data for each cruise, after dividing the cruise area
into northern (above lat. 27°15'N) and southern (below lat. 27°15 N) portions. Tabulated values are means.
Values in parentheses are the numbers of stations that were sampled.

Plankton volume Fish eggs Fish larvae
(ec/1,000 m*) (No./10 m’) (No./10 m?)
Cruise Month North South North South North South
1972
GE 7202 & 76.9 65.9 722.3 221.8 162.9 297.1
8B 7201 Feb. (10) (15) (13) (15) (13) (15)
GE 7208 May 103.6 111.0 497.2 617.8 406.7 418.1
(10) (18) (10) (20) (10) (20)
GE 7210 June 142.2 352.5 656.4 469.8 340.3 342.1
(4) (9) (4) (9) (4) (9)
IS 7205 Sept. 218.1 158.1 764.2 464.0 7164.4 863.2
(13) (21) (13) (21) (13) (21)
IS 7209 Nov. 132.1 108.0 166.2 205.2 206.6 377.9
(23) (26) (24) (26) (24) (26)
1973
IS 7303 Jan. 164.4 143.1 451.3 307.0 219.3 374.3
(23) (27) (24) (27) (24) (27)
IS 7308 May 220.9 188.4 845.2 898.1 477.2 744.4
(24) (25) (24) (25) (24) (25)
IS 7311 June-July 108.2 131.9 415.5 481.4 466.6 633.0
(25) (26) (25) (25) (25) (26)
IS 7313 Aug. 255.9 141.5 909.2 492.4 621.9 541.2
(25) (25) (25) (25) (25) (25)
IS 7320 Nov. 145.1 118.1 150.7 175.1 288.9 401.0
(25) (26) (25) (26) (25) (26)
1974
IS 7405 Mar. 80.0 101.5 191.1 391.1 60.4 50.2
(19) (17) (16) (16) (16) (16)
IS 7412 May 157.3 278.8 926.1 884.5 497.2 545.8
(22) (21) (23) (19) (23) (19)
Pooled means 157.1 150.3 545.4 463.4 380.5 486.9
(223) (256) (226) (225) (226) (255)

14

(n,s5)

Z
Z

(23,22 8)

132)

MEAN PLANKTON VOLUME (cc71000m*)

*2 DAS SAASSY("7, 20.6)

| Sa SI

CRUISE DATES

Figure 11.—Comparison of mean zooplankton volumes for stations
sampled north of lat. 27°15'N and stations south of that latitude for 12
eastern Gulf of Mexico cruises, 1972-74. Numbers of stations and
standard errors of the mean are given in parentheses above bar graphs.

northern sector (Table 3). There was no apparent seasonal
pattern that explained the occurrence of the highest mean
egg abundance in the southern sector for six cruises (Fig.
12). The mean abundance of fish larvae was highest in the
northern sector during only 2 of the 12 cruises (Table 3;
Fig. 12).

Spawning by fishes seems to be more intensive in the
northern sector, but larval abundance usually is not highest
there. Higher mortality rates in the north, transport out of
that area toward the south, or transport of larvae into the
southern sector from outside the sampling area could
account for the observed distributions. Mean zooplankton
volumes usually were highest in the northern sector but
differences between sectors were not often great, pooled
means differing by only 7 cc/1,000 m* between the two
areas (Table 3).

The seasonality of abundance that was observed pre-
_ viously, when the whole shelf area was considered, also was
| apparent in each of the sectors (Figs. 11, 12). Zooplankton
_ volumes were lowest in winter and highest in summer for
both the northern and southern sectors, although the degree
_ of increase during summer was greatest in the southern
_ sector. Abundance of ichthyoplankton also reflected the
| seasonal pattern of relative scarcity in winter and increased
abundance in summer in each of the sectors.

Distribution Relative to the 50-m Depth Contour

The 50-m isobath divides the shelf study area into
approximate halves (Fig. 1). Zooplankton volumes and
ichthyoplankton abundance and concentrations were ex-
amined in relation to this depth contour to determine if they
differed between nearshore and offshore areas (Table 4;
Figs. 13, 14). The nature of water masses, rather than depth
is a factor determining zooplankton and ichthyoplankton
abundance, but the 50-m depth is a convenient, common
contour for comparing cruises. Gulf Loop Current water
may occasionally intrude onto the west Florida continental

15

(25,252.68)
(23,1606)
(n,s5)

(24, 232 4)
(25, (27.2)
(19,146 0)

~
€
° =~

= s
2 =
3 9009 $ Rate a
eS ae
o = a . i =
a S = 2 = 3 North
aero A lis | Ie 23 r
Fe = (Ast it =e 37 é
pele [BA

. eee

Zwortis |Y A it Z
> a |G A Z
z Z 4 Z Zi
ry Z Z 4 South
2 A | 4

FEB. may JUN, SEPT, NOV, MAY JUN> JUL

72 72 72 72 73 73

(27, 63.4)

4
g
Y
Z

Py:
2 =
“9

a
may
4

REEMA SASS (25, 100.0)
RSS (26 94.2)

KSSH (2! 6 45)

MEAN NUMBER OF FISH LARVAE UNDER 10m?

SSS (26, 69.8)

Qe Uh

KISS

RQny (15, 801)

=
<
a
©
N
UM,

. Ne May JUN
i“) 73 7

Figure 12.— Mean fish egg and fish larvae abundance. Comparisons are
made for stations sampled north of lat. 27°15 N and stations south of that
latitude for 12 eastern Gulf of Mexico cruises, 1972-74. Numbers of
stations and standard errors of the mean are given in parentheses above
bar graphs.

2 RSS (9. 1831)

‘o

rv

=
z
‘|

aE

SEPT.
72

Ne

UL, | AUG.
73

i

F
7

~

2

)

shelf where depths range from 50 to 200 m (Maul 1974) and
the character of zooplankton and ichthyoplankton could be
influenced by such intrusions (Austin 1974). We did not
determine if differences in plankton species composition
occurred during this study.

Mean volumes of zooplankton were greatest at stations
less than 50 m deep on 8 of the 10 cruises where stations
in both depth zones were sampled. The two exceptions
occurred in February 1972 and June-July 1973 when the
mean volumes of zooplankton were slightly higher over the
outer shelf. Pooled mean zooplankton volumes for the 10
cruises were calculated for each of the two depth zones
(Table 4). Mean volume at depths shallower than 50 m
exceeded that at deeper stations by a factor of 1.78. For the
10 cruises in which both depth zones were sampled, mean
zooplankton volumes at stations shallower than 50 m
exceeded 150 cc/1,000 m? seven times, but mean plankton
volume exceeded that amount on only one cruise at stations
deeper than 50 m.

Mean fish egg abundance usually was higher at stations
less than 50 m deep but mean larval abundance was always
higher at the deeper stations for the 10 cruises. One reason
for this observation is that fish larvae range in age from 1
day to more than 3 wk. Fish eggs, by contrast, usually hatch
within 2 days of spawning in the eastern Gulf, precluding the
possibility of numbers accumulating over a period in any
area. Therefore, abundance of eggs would tend to be highest
where the eggs were spawned in greatest numbers. Other
reasons that larval abundance may be relatively low at
shallow stations are that a higher proportion of shallow-
water species are demersal and furthermore their larvae
become demersal at a younger age than larvae of demersal,
offshore species.

Seasonal fluctuations of zooplankton and ichthyoplank-
ton abundance were most apparent at stations less than 50 m
deep. Plankton volumes usually increased in summer at
stations located over deeper water (Fig. 13) but the relative
increase in zooplankton was less than at shallower stations.

Table 4.—Summary of plankton, fish eggs, and fish larvae mean catches for each cruise, dividing the cruise
into portions greater than 50 m depth and less than 50 m depth.

Plankton volume Fish Eggs Fish Larvae
(ce/1,000 m° ) (No./10 m*) (No./10 m*)
Cruise Month <50m >50 m <50m >50 m <50 m >50m
1972
8B 7201 & Feb. 68.8 69.6 420.1 880.3 153.2 336.7
GE 7202 (15) (12) (18) (12) (18) (12)
GE 7208 May 158.2 48.2 820.0 300.6 348.8 489.1
(15) (13) (16) (14) (16) (14)
GE 7210 JUNC wenn nnn nnn nnnnnnnnee No stations were greater than 50 m deep ---------------------
IS 7205 Sept. 195.7 120.7 559.9 640.1 742.9 1,093.7
(26) (8) (26) (8) (26) (8)
IS 7209 Nov. 144.6 67.1 198.4 161.0 134.8 638.0
(33) (16) (34) (16) (34) (16)
1973
IS 7303 Jan. 181.1 107.1 419.0 362.2 146.1 645.3
(33) (17) (34) (17) (34) (17)
IS 7308 May 237.6 128.9 955.2 684.0 545.8 767.0
(34) (15) (34) (15) (34) (15)
IS 7311 June-July 118.2 124.2 483.8 379.7 328.7 997.0
(34) (17) (34) (17) (34) (17)
IS 7313 Aug. 232.6 114.9 857.2 368.4 363.1 1,045.8
(34) (16) (34) (16) (34) (16)
IS 7320 Nov. 158.3 100.6 364.3 138.1 121.5 698.2
(32) (18) (32) (18) (32) (18)
1974
CL 7405 1) C\ No stations were greater than 50 m deep----------------------
CL 7412 May 215.1 191.7 937.4 354.9 508.7 655.5
(41) (3) (39) (3) (39) (3)
Pooled means 178.9 100.7 606.4 402.8 342.2 739.2
(297) (135) (301) (136) (301) (136)

_ ue
= c =
2 iS =
: Z
300 2 Y
4
>50m
a: 3
—E a
° o
= a
Q =
~
8 200 = J
2 b
2 =
=
°
>
z
oO 3 =
Ee & =
E
= 100 § 3
< = ©
= ’ ~
a : :
= E e
2 °
< o 8
w A A
2 .
= 5 §
3 2
= *
°
A = z
FEB. MAY JUN. SEPT, NOV. JUN= JUL AUG, NOV MAR, MAY
72 72 72 72 72 73 7 3 73 73 74 74

CRUISE DATES

Figure 13.—Comparison of mean zooplankton volumes between stations
less than 50 m deep and those more than 50 m deep for 12 eastern Gulf of
Mexico cruises, 1972-74. Numbers of stations and standard errors of the
means are given in parentheses above bar graphs.

16

(39,15 3)

8

8

3 RSSSSin. 5) | on

o
°
3

MEAN NUMBER OF FISH EGGS UNDER 10m?
io stotlons > SOm were sampled
jo stolons >» SOm were sompled

ae 32, 42.8)

BES SNNANSSS RSS

E
z
Eas
3
x
x

AR.
4

— : othe
5 =
2 Z a
Z = Z Z
g Ca A Y =
800 Be * A Y 2 of
3 Ss Jeon. ve 4A ce
5 oes Z y AZ ag = 1%
x a = 8, 4 Z sy a : Z
Fd a S Se Z Y Ag =Y 5 4
a bea oY we Y Y eg a E g
8) 4007 SS YA is Z =f "yy YA a Z
5 manifest YW IY st |
Fy Ali“g ii |A@ | Bee
> 1H 1A it |A Y A \Y x: 1
: IZ IA IE 1A [A IY AY n: 1G
= Fea, ‘v JUN, SEPT, NOV. Ne JA. AUG. NOV. MAR - rts
72 72 72 72 72 73 73 3 73 73 +

CRUISE DATES

Figure 14.—Mean fish egg and fish larvae abundance. Comparisons are
made between stations less than 50 m deep and those more than 50 m
deep for 12 eastern Gulf of Mexico cruises, 1972-74. Numbers of stations
and standard errors of the means are given in parentheses above bar
graphs.

|

Most of the observed seasonality in larval fish abundance
was attributable to changes at stations shallower than 50 m
(Fig. 14).

Specific Areas of Abundance

The patchy nature of zooplankton distribution in the
eastern Gulf made it difficult to determine if any areas
consistently had high biomass. The contour charts (Fig. 2)
do suggest that an area on the southwest Florida shelf,
encompassed by lat. 25°30'N to 27°30'N and long. 82°30’W
to 84°00’W, usually had high zooplankton volumes. This
observation is similar to that of Bogdanov et al. (1968) and
Khromov (1969) who observed year-round high plankton
standing stocks in this area during 1963 and 1964.

Another area of relatively high plankton volumes was
located on the shelf in the northeast Gulf (lat. 28°N to 29°N
and long. 84°W to 85°W). Our data show this area to be
rich in zooplankton at all seasons but richest in summer,
thus differing from data of Bogdanov et al. (1968) and
Khromov (1969), who reported high plankton biomass
there only in winter. Because their observations apparently
included phytoplankton biomass, as well as zooplankton, our
data may not be directly comparable. Austin and Jones
(1974) also reported a high zooplankton biomass in this
region and cited possible influences of the Loop Current and
shelf circulation on production. Their data, based on
monthly collections from June 1969 through August 1970,
showed plankton biomass for this area of the Gulf to be
lowest in February and March, then increasing rapidly
through May and June, and reaching a peak in fall.

Zooplankton volumes frequently were high in small
areas near the coast, but none of these areas had consis-
tently high volumes. Such areas included coastal stations
west of Charlotte Harbor (lat. 26°30 N), Tampa Bay (lat.
27°30'N) and Cedar Key (lat. 29°00'N). Nutrients from local
terrestrial runoff may have a major influence on plankton
production in these areas.

Contour charts of fish egg abundance (Fig. 3) show less
uniformity than those for zooplankton volumes. Egg abun-
dance was low during fall and winter; the only area that had
relatively high numbers of eggs at those seasons was located
between Charlotte Harbor and Tampa Bay at depths of 20 to
100 m. During spring and summer, spawning is intense but
no areas of unusually great spawning activity were located.
Patches of high abundance (>1,600 eggs under 10 m’) often
occurred and these usually were found where depths ranged
from 20 to 50 m.

Abundance of fish larvae (number under 10 m’) was
greatest at depths from 50 to 100 m (Fig. 4). Contour lines of
larval fish abundance tended to lie parallel to isobaths.
Greatest abundance of larvae occurred between lat. 26°30'N
and 28°30'N. Localized patches of great abundance some-
times were observed for larvae, but these were less common
than for fish eggs.

Relationships Among Biological Variables

Complex statistical analyses were not undertaken to
examine relationships among the plankton and ichthyo-
plankton categories. Simple correlation coefficients among
pairs of variables for each cruise were calculated, however,
to determine if any of them had a significant linear associa-
tion (Table 5). All of the correlations were calculated using
log 1. -transformed data because most of the cruise frequency
distributions for abundance or concentrations were nearly
normally distributed after transformation.

atf/

Table 5.—S of simple correlations among biological variables.
ae correlation coefficients were calculated using log ,. -transformed
ta.

Plankton Plankton Fish eggs
(ec/1,000 m3) (ce/1,000 m°) (No./10 m2)
by by by

fish eggs fish larvae fish larvae
Cruise (No./10 m*) (No./10 m?) (No./10 m?)
8B 7201 &
GE 7202 +0.281 +0.218 +0.389*
GE 7208 +0.631** +0.097 +0.087
GE 7210 -0.172 +0.727** -0.329
IS 7205 +0.283 +0.109 +0.610**
IS 7209 +0.038 -0.179 +0.021
IS 7303 +0.022 -0.118 +0.172
IS 7308 +0.205 -0.056 +0.070
IS 7311 -0.074 +0.347* +0.252
IS 7313 +0.171 -0.241 -0.012
IS 7320 +0.211 -0.000 +0.376**
CL 7405 -0.044 +0.450** +0.346
CL 7412 +0.309* +0.359* +0.258

* Significant; P<0.05
** Highly significant; P<0.01

The correlations between zooplankton volumes and fish
egg abundance were positive for nine cruises,. but signifi-
cantly so (P< 0.05) for only two of these. This suggests that
there is a relationship between areas of intense spawning
and high zooplankton abundance at some times. The
correlations between fish egg abundance and fish larvae
abundance, though positive on 10 of the 12 cruises, were
significant in only three cases. The fact that only positive
correlation coefficients were significant among those that
were calculated (Table 5) supports the belief that there is at
times a real association among these variables in the eastern
Gulf.

Relationships of Biological Variables to
Temperature and Salinity

There were no obvious associations among temperature
and salinity conditions and the distribution of organisms.
Such associations almost certainly would be species depen-
dent; because we did not identify organisms to the species
level or sample discrete depth strata, these associations
were not observed. Even in summer 1973, when a lens of low
salinity water (<30%..) was located over part of the shelf
study area, predominantly from Mississippi flood runoff, the
effects on the plankton were not obvious. Zooplankton
volumes, fish egg abundance, and larval fish abundance
were all reduced during the June-July 1973 cruise (Figs.
8-10), but had recovered by August 1973, although surface
salinities remained low in the area. Because this low salinity
water was present only from the surface to 15-m depth, it is

possible that many planktonic organisms that remained
below that depth were not seriously affected. It is not known
what effects this low salinity lens had on spawning, egg
development, or larval survival for those species that
normally are found near the sea surface.

ACKNOWLEDGMENTS

Particular thanks go to Steven Berkeley, Lise Dowd,
John Klinovsky, Betsy Sullivan, and Gregg Waugh, all of
whom helped to prepare and reduce data for this report. We
thank Murice Rinkel of the State University System of
Florida Institute of Oceanography, and James Price and C.
N. K. Mooers of the Rosenstiel School of Marine and
Atmospheric Science for providing assistance in digitizing
and plotting hydrographic data. Financial support for this
project was provided by NOAA Sea Grant 04-3-158-27 to the
University of Miami and by Bureau of Land Management
Contract No. 08550-CT4-16.

LITERATURE CITED

ARNOLD, E. L., JR.
1958. Gulf of Mexico plankton investigations: 1951-53. U.S. Fish
Wildl. Serv., Spee. Sci. Rep. Fish. 269, 53 p.
AUSTIN, H. M.
1974. Biological indicators of oceanographic phenomena. JnR. E.
Smith (editor), Proceedings of marine environmental implications
of offshore drilling in the Eastern Gulf of Mexico, p. 175-186.
State Univ. Syst. Fla., Inst. Oceanogr., St. Petersb.
AUSTIN, H.M., and J. I. JONES.
1974. Seasonal variation of physical oceanographic parameters on
the Florida Middle Ground and their relation to zooplankton bio-
mass on the West Florida shelf. Fla. Sci. 37:16-32.
BOGDANOV, D. V., V. A. SOKOLOV, and N. S. KHROMOV.
1968. Regions of high biological and commercial productivity in
the Gulf of Mexico and Caribbean Sea. Oceanology 8:371-381.
HOPKINS, T. L.
1973. Zooplankton. Jn J. I. Jones, R. E. Ring, M. O. Rinkel, and
R. E. Smith (editors), A summary of knowledge of the Eastern
Gulf of Mexico 1973, p. II F-1 to II F-6. State Univ. Syst. Fla.,
Inst. Oceanogr., St. Petersb.
HOUDE, E. D.
1973. Estimating abundance of sardine-like fishes from egg and
larval surveys, Eastern Gulf of Mexico: preliminary report.
Proc. Gulf Caribb. Fish. Inst., 25th Annu. Sess., p. 68-78.

18

1974. Research on eggs and larvae of fishes in the Eastern Gulf of
Mexico. JnR. E. Smith (editor), Proceedings of marine environ-
mental implications of offshore drilling in the Eastern Gulf of
Mexico, p. 187-204. State Univ. Syst. Fla., Inst. Oceanogr.,
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In press. Abundance and potential for fisheries development of
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HOUDE, E. D., S. A. BERKELEY, J. J. KLINOVSKY, and C. E.
DOWD.

1976. Ichthyoplankton survey data report. Summary of egg and
and larvae data used to determine abundance of clupeid fishes in
the Eastern Gulf of Mexico. Univ. Miami, Sea Grant Tech.
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KHROMOV, N. S.

1969. Distribution of plankton in the Gulf of Mexico and some
aspects of its seasonal dynamics. Jn A. S. Bogdanov (editor),
Soviet-Cuban fisheries research, p. 36-56. (Transl. from Russian
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KING, J. E.

1949 (1950). A preliminary report on the plankton of the west
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KRAMER, D., M. J. KALIN, E. G. STEVENS, J. R. THRAILKILL,
and J. R. ZWEIFEL.

1972. Collecting and processing data on fish eggs and larvae in the
California Current region. U.S. Dep. Commer., NOAA Tech.
Rep. NMFS CIRC-370, 38 p.

MAUL, G. A.

1974. The Gulf Loop Current. Jn R. E. Smith (editor), Proceed-
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NATIONAL ACADEMY OF SCIENCES.

1969. Recommended procedures for measuring the productivity of
plankton standing stock and related oceanic properties. Biol.
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Counc., Natl. Acad. Sci., Wash., D. C., 59 p.

POSGAY, J. A., R. R. MARAK, and R. C. HENNEMUTH.

1968. Development and test of new zooplankton samplers.
Comm. Northwest Atl. Fish., Res. Doc. 68/34, 7 p.

SIMPSON, A. C.

1959. The spawning of the plaice (Pleuronectes platessa) in the
North Sea. Fish. Invest. Minist. Agric. Fish Food (G.B.), Ser.
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THRAILKILL, J. R.

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