513

Comparison of Vertebral Counts
of Atlantic Menhaden

by Fred C. June

Marine Biol:gi-a' Labcr2.c;y

LIBRARY

SEP 1 7 1965
WOODS HOLE, MASS.

SPECIAL SQENTIFIC REPORT-FISHERIES Na 513

UNITED STATES DEPARTMENT OF THE INTERIOR

Stewart L. Udall, Secretary

John A. Carver, Jr., Under Secretary

Stanley A. Cain, Assistant Secretary for Fish and Wildlife

FISH AND WILDLIFE SERVICE, Clarence F. Pautzke, Commissioner

Bureau of Commercial Fisheries, Donald L. MeKernan, Director

Comparison of Vertebral Counts
of Atlantic Menhaden

By
FRED C. JUNE

United States Fish and Wildlife Service
Special Scientific Report--Fisheries No. 513

Washington, D.C.
June 1965

CONTENTS

Page

Introduction 1

Materials and nnethods ^

Results 2

Variation of vertebral counts with fish length 2

Variation between sexes 2

Variation of a year class with age 2

Variation between year classes 6

Connparison of spawners and nonspawners within and between localities 7

Comparison of juveniles and spawners of the sanne year class 7

Discussion 8

Summary 11

Acknowledgments • 11

Literature cited 1 1

FIGURES

1. Map of the Atlantic coast of the United States showing locations mentioned in the text .

2. Monthly range and mean temperatures at three lightship stations along the Atlantic

coast of the United States, 1957-59

3

10

TABLES

1. Fork lengths, ages, and vertebral counts of Atlantic menhaden in samples from three

localities along the Atlantic coast of the United States, 1957-60 (number offish in
parentheses)

2. Comparison of mean vertebral counts of Atlantic menhaden of the 1955 and 1956 year

classes at successive ages

3. Comparison of mean vertebral counts of Atlantic menhaden of the 1955-57 year classes

4. Comparison of mean vertebral counts of juvenile Atlantic menhaden (age 0) from

estuarine nurseries north and south of Long Island, N.Y., with spring and autumn
spawners (older ages combined), respectively, of the same year class

5. Vertebral counts and fork lengths of juvenile Atlantic menhaden in samples from Long

Island, N.Y., estuarine nurseries, 1956-57

Comparison of Vertebral Counts of Atlantic Menhaden

By

FRED C. JUNE, Fishery Biologist (Research)

Bureau of Commercial Fisheries Biological Laboratory
Beaufort, N.C.

ABSTRACT

Comparison was made of vertebral counts of 6,048 adult Atlantic menhaden
(Brevoortia tyrannus) collected in three localities along the Atlantic coast of the
United States to determine if autumn and spring spawners were distinguishable by
this meristic character. Two discrete groups of spawners were identified: one
occurring in Cape Cod and Long Island waters in spring and the other in Long Island
and North Carolina waters in autumn. The mean vertebral count of spring spawners
was 46.914 and of autumn spawners, 47.031. Lower mean counts of spring spawners
in 3 successive year classes were shown to be similar to those of juveniles of the
same year classes occupying estuarine nurseries north of Long Island, while
higher mean counts of autumn spawners were similar to those of juveniles occurring
south of Long Island. The consistency of mean vertebral counts of spring and autumn
spawners for 3 consecutive years suggests that the observed differences are of
genotypic origin.

INTRODUCTION

Investigation of the population structure
forms one phase of a progran-i of research on
the Atlantic menhaden (Brevoortia tyrannus).
The purposes of this phase of research are to
identify any natural biological units within the
population and to determine the overlap in
time and space between them. Thus far,
morphological characters of juveniles have
been compared (June, 1958; Sutherland, 1963)
and the distribution of spawning (Higham and
Nicholson, 1964; Reintjes, 1961) has been
studied.

The above-mentioned studies have helped to
formulate hypotheses regarding population
structure, but there still are many unanswered
questions. Some of the nnore important ques-
tions concern the relationship of spring and
autumn spawners. When adults appear inshore
near Long Island, N.Y., and northward in
April or May, some are in spawning condition
(spawners), but the majority may be classified
as "spent", or otherwise sexually inactive
(nonspawners). The proportion of spawners
decreases through summer, but increases

■■- Now with the Bureau of Sport Fisheries and Wildlife,
Pierre, S. Dak.

again in September and reaches a maximum in
October (Higham and Nicholson, 1964). Prior
to their disappearance fronn northern waters
in October, spawners and nonspawners con-
gregate in large schools. These schools move
southward during November and congregate
off the North Carolina coast until mid- Decem-
ber (June and Nicholson, 1964). Before or
during the southward migration, however, the
composition of the schools changes, for among
the adult fish, only spawners appear off North
Carolina, and catches consisting of non-
spawners sometimes are taken in Long Island
waters immediately before and following the
southward nnigration of spawners. Except for
rare appearances, fish are absent from in-
shore waters through winter. In the following
April or May, they reappear inshore, and
mixed groups of spawners and nonspawners
again occur from Long Island northward
(Higham and Nicholson, 1964).

The purpose of this study was to see if a
comparison of mean vertebral counts would
show a relationship between (1) spawners and
nonspawners that appear together from Long
Island northward in April and May, (2) spring
and autumn spawners in that locality, (3)
autumn spawners in that locality and off
North Carolina, and (4) spring and/or autumn
spawners in those localities and the juvenile
stocks in adjacent estuarine nurseries.

MATERIALS AND METHODS

Fifty-two samples of adult Atlantic men-
haden, comprising 6,048 specimens, were
collected in spring (April- May) from pound
net catches in the vicinity of Cape Cod, Mass.,
and Long Island, N.Y., and in autumn (October-
December) from purse seine catches near
Long Island, N.Y., and Beaufort, N.C., from
1957 to 1960 (fig. 1). Collecting was limited to
periods when spring or autumn spawners
occurred in the catches in greatest nximbers.
Incidental catches of Atlantic nnenhaden by
pound nets provided the earliest and only
available samples in spring. Adverse weather,
which delayed the setting of pound nets in
waters north of Long Island during April and
May, thwarted attempts to secure a greater
number of samples in the Cape Cod area.
Although efforts were made to obtain at least
100 specimens in each sample, early season
pound net catches sometimes failed to provide
siifficient numbers. Also, sonne of the pre-
served material was lost during storage be-
cause of decomposition, separation of verte-
bral columns, and dissociation of identifying
labels. Individual samples available for study
contained from 41 to 313 specinnens.

At the time of collection, fish were examined
for sex and maturity. Most gonads were
weighed, and, following the general nnethod of
Higham and Nicholson (1964), males and fe-
males with a "gonad index" of 4.0 or more
were classified as spawners and those with an
index of 3.0 or less as nonspawners. Fish which
were in spawning condition in April and May
were classified as "spring spawners", those
in spavming condition from October to Decem-
ber as "autunrui spawners."

The fork length of each fish, in centimeters,
was recorded and a scale sannple taken,
following the methods described by June and
Reintjes (1959). The vertebral column, in-
cluding the skull and most of the caudal fin,
was removed, labeled, and preserved in 10
percent formalin.

Counts of vertebral numbers were nnade
from radiographs (Sutherland, 1958) and in-
cluded all vertebrae between, but not including
the occipital bone and the hypurals (June,
1958; Sutherland, 1963). Abnormal vertebrae
(found in 0.4 percent of the fish examined)
were counted if distinguished by a suture
between fused centra, or by the presence of a
neural or haemal spine. Columns in which
individual vertebrae could not be distinguished
were discarded (three specimens). Ages were
determined from scales, and the fish were
assigned to year classes as defined by Jtine
and Roithmayr (1960).

The correlation between vertebral counts
and fish length was tested for significance.
Vertebral counts were subjected to the vari-
ance-ratio and Bartlett's tests to test the
assumption of common variance, and an

analysis of variance was used to test the
significance among means of sexes, age groups,
year classes, spawning groups in the same
and different localities, and juveniles and
adults from adjacent localities (Snedecor,
1956).

Table 1 summarizes data on length, age,
and vertebral count of 6,048 spawners and
nonspawners in samples by locality and date.
Means and sums of squared deviations used in
the analyses also are given.

RESULTS

Variation of Vertebral Counts With Fish Length

To determine whether vertebral counts were
related to fish length, a correlation analysis
was nnade of the individual and combined
samples from each locality (spawners and
nonspawners were treated separately). Three
of the correlation coefficients were of border-
line significance (at the 5 percent level), and
the maximum variation attributable to length
in combined samples was suggested to be less
than 2 percent. Accordingly, length was not
considered in the analyses which follow.

Variation between Sexes

Because of unequal numbers of nnales or
females in some samples, differences in the
mean vertebral counts between samples fronn
the same locality might reflect differences
between the sexes. However, analysis of mean
vertebral counts of 12 samples, connprising
859 individuals of 3 separate year classes
(1955, 1956, and 1957), showed that none of
the samples had a statistically significant
difference between sexes. It was concluded
that variation associated with sex was random,
a conclusion which agrees with the findings of
June (1958) and Sutherland (1963) concerning
juveniles. Accordingly, males and females
were not separated in subsequent analyses.

Variation of a Year Class with Age

In all localities, the proportion of fish of
a given year class in samples changed as
the year class became older. Accordingly,
heterogeneity between mean vertebral counts
could arise from the varying numbers of fish
at different ages in the samples. Comparisons,
therefore, were made of the two most abundant
year classes (1955 and 1956) at successive
ages in each locality (table 2). Of the 12
possible comparisons (samples which con-
tained no fewer than 60 fish of a given age),
one showed a significant difference at the
5 percent level, and this was confined to
nonspawners of the 1955 year class in the
Long Island samples. In this case, there was
a small decrease in the mean vertebral count

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Table 2. — Comparison of mean vertebral counts of Atlantic menhaden of the 1955 and 1956 year

classes at successive ages

Locality-

Year
class

Mean vertebrae at age

Vertebrae difference
between ages

2

3

4

5

2-3

3-4

4-5

Cape God:
Spring spawners...

Nonspawners

Long Island :
Spring spawners...

Nonspawners. ...'...

North Carolina:
Autumn spawners. . .

1955
1956

1955
1956

1955
1956

1955
1956

1955
1956

Number

47.011
47.041

Number
46.843
47.050

46.800

47.028
47.092

47.000
47.062

Number

46.833
46.954

47.000
47.050

46.930
46.900

47.112
47.062

47.093

Number
46.851

46.933

46.921
46.930

Number

0.011
-.021

Number

-0.111

.000

-.100

-.034
.030

-.093

Number
-0.018

.067

.009
1 .182

"P is less than 0.05.

between ages 4 and 5, but in remaining com-
parisons the differences showed no consistent
trend.

The significant decrease with age in mean
vertebral count of nonspawners of the 1955
year class in the Long Island samples may
be due to (1) differential mortality as the
fish became older, (2) a shift in the spawning
cycle of individual fish with advancing age,
or (3) chance. Because the difference in the
mean vertebral count between ages was sig-
nificant only in the 1955 year class in one
locality, it is unlikely that this source of
variation altered the results of subsequent
tests.

Variation between Year Classes

Samples from the different localities in-
cluded fish of more than one year class.
Therefore, if mean vertebral counts of the
various year classes represented in the
samples differed significantly, the nnean
counts of sanriples simply might reflect differ-
ences in age composition.

In every instance except one (nonspawners.
Long Island) mean counts of both spawners
and nonspawners of the 1956 year class were
slightly higher than those of the 1955 and
1957 year classes (table 3). Differences as
great as 0.145 vertebra are shown for mean
counts between year classes (1956 and 1957
year classes in Cape Cod samples); however,
no difference was statistically significant.

Table 3. — Camparison of mean vertebral counts of Atlantic menhaden
of the 1955-57 year classes

Locality

Mean vertebrae for ye

ar class

Maximum

difference

in

1955

1956

1957

vertebrae

Number

Number

Number

Number

Gape Cod:
Spring spawners. . .
Nonspawners

46.841
46.973

46.905
47.050

46.760
47.016

0.145
.077

Long Island:
bpring spawners...
Nonspawners

46.923
47.033

46.906
47.080

47.000

.017
.083

North Carolina:
Autijmn spawners. . .

47.022

47.052

47.037

.030

Sutherland (1963) found the mean vertebral
counts of juveniles of the 1956 year class
significantly higher than those of the 1957,
1958, and 1959 year classes. Examination of
daily and nnean monthly surface water tem-
peratures and salinities (Bunnpus, 1957; Day,
1959a, 1959b, 1960) at lightship and shore
stations along the Atlantic coast showed no
striking anomalies during the period of early
development of eggs and larvae in 1956.
There was no pattern of correlation between
the vertebral counts and the hydrographic
data. An explanation of this phenomenon naay
involve one or nnore of the following points:
(1) selection by spawning fish of temperature
and/or salinity strata other than the surface
waters considered, (2) variations in the tinne
and place of spawning, (3) differential sur-
vival in the early life history stages, and

(4) differential genotypic variation between
year classes.

Since differences between the nnean verte-
bral counts of three year classes were not
significant, variation between year classes
was not considered in the following analyses.

Comparison of Spawners and Nonspawners
within and between Localities.

The first major hypothesis tested in this
study was that the spawners and nonspawners,
which occurred together in Cape Cod and Long
Island waters in spring, constituted two dis-
crete groups of fish. Vertebral counts of
spawners from each of the two localities
first were tested for evidence of significance
between sample means. Results of these
analyses gave no indication of heterogeneity
within localities. Further analysis indicated
that spawners from the two localities probably
were drawn from a homogeneous population.
Similar comparisons of nonspawners, both
within and between localities, led to the same
conclusions. Accordingly, the vertebral counts
of spawners from the two localities were
pooled and compared with the pooled counts
of nonspawners from both localities. A highly
significant F value of 47.74 (Pq q, = 6.64)

indicates that it is unlikely that the samples
of spawners and nonspawners collected in
Cape Cod and Long Island waters in spring
were nnennbers of a homogeneous population.

Vertebral counts of 1,989 spawners ranged
from 45 (10 fish) to 49 (1 fish), with the
majority having 47 vertebrae (1,441 fish);
mean counts of samples varied from 46.830
to 47.000. Vertebral counts of 2,014 non-
spawners also ranged from 45 (1 fish) to 49
(1 fish), with most having 47 vertebrae (1,509
fish); mean counts of samples ranged from
46.882 to 47.150. The difference between the
grand means of spawners (46.914) and non-
spawners (47.027) was 0.113 vertebra.

The foregoing findings led to the second
major hypothesis, nannely, that the non-
spawners which occurred in Cape Cod and
Long Island waters in spring and the spawners
which occurred in Long Island and North
Carolina waters in autunnn constituted a
homogeneous population.

Mean counts of samples within each locality
were tested for homogeneity and showed no
significant differences in either locality. The
autumn spawners from Long Island and North
Carolina were then compared and gave no
indication of heterogeneity. Finally, both
groups of autumn spawners were connpared
with the nonspawners taken in Cape Cod and
Long Island waters in spring, and differences
were not significant.

Vertebral counts of 407 autumn spawners
from Long Island ranged from 46 (36 fish)
to 49 (1 fish), with most having a count of 47

(315 fish); mean counts of samples ranged
between 46.949 and 47.107. A total of 1,638
autumn spawners from North Carolina had
counts ranging from 45 (2 fish) to 49 (1 fish),
with the majority having 47 (1,226 fish); mean
counts of samples varied from 46.972 to
47.101. The grand means for Long Island and
North Carolina autumn spawners were 47.052
and 47.030, respectively, as compared with
d count of 47.027 for nonspawners taken in
Cape Cod and Long Island waters in spring.

Finally, comparison was made of the mean
vertebral counts of spring spawners and
autumn spawners (nonspawners taken in Cape
Cod and Long Island waters in spring were
included with autumn spawners from Long
Island and North Carolina). As might be ex-
pected, results of the analysis showed highly
significant heterogeneity between the nneans
(F = 69.22; Pq q, = 6.64). The grand mean

vertebral count for 1,989 spring spawners
was 46.914 and for 4,059 autumn spawners,
47.031, a difference of 0.117 vertebra.

The conclusions drawn from the foregoing
analyses of vertebral counts were that

(1) spring spawners in Long Island and Cape
Cod waters constituted a homogeneous group
which was distinct from nonspawners,

(2) autumn spawners in Long Island and North
Carolina waters and nonspawners in Long
Island waters in spring (autumn spawners)
constituted a honaogeneous group, and (3)
spring and autumn spawners constituted two
discrete groups of fish. The term group is
used here, as defined by Marr (1957), to de-
scribe "a fraction of a population with dis-
tinctive characteristics, the nature of which
(phenotypic or genotypic) has not yet been
determined."

Comparison of Juveniles and Spawners of the
Same Year Class

From a study of mean vertebral counts,
June (1958) and Sutherland (1963) found that
juvenile Atlantic menhaden inhabiting estuarine
nurseries along the Atlantic coast were sep-
arable into two groups--one occurring north
of Long Island, N.Y., and the other south of
there. Of further interest in the present study,
therefore, is a comparison of the mean verte-
bral counts of spring and autumn spawning
adults of a given year class with juveniles of
the same year class (age 0) from these two
general nursery areas to determine if some
relationship could be established.

In table 4, the mean vertebral counts of
spring spawners in three year classes
(1955-57) are compared with the mean counts
of juveniles of the same year class at age 0
from estuarine nurseries north of Long Island,
and autumn spawners with juveniles from
estuarine nurseries south of Long Island. Of
six possible connparisons, all but two (both

Table K. — Comparison of mean vertebral counts of juvenile Atlantic menhaden (age 0) from estu-
arine nurseries north and south of Long Island, N.Y., with spring and autumn spawners (older
ages combined), respectively, of the same year class

Mean vertebrae

Year class

Juveniles

north of

Long Island

(age 0)

Spring
spawners

(older
ages com-
bined)

Difference

Juveniles

south of

Long Island

(age 0)

Autumn
spawners

(older
ages com-
bined)

Difference

1955

Number

46.666
46.658
46.645

Number

46.889
46.959
46.760

Number

^ 0.223

^ .301

.115

Number

47.083
47.090
47.014

Number

47,022
47,052
47.037

Number
^ 0.061

1956

^ .038

1957

.023

P is less than 0.01.
P is less than 0.05.

involving the 1957 year class) were found to
differ significantly (two were significant at the
5 percent level and two at the 1 percent level).
Differences between means ranged fronn 0.023
(1957 year class, autumn spawners) to 0.301
(1956 year class, spring spawners). In four of
the comparisons, there were increases in the
nnean vertebral counts with age; in two, de-
creases.

In most cases, there were statistically sig-
nificant differences between the mean vertebral
counts of juveniles of a given year class
(age 0) inhabiting estuarine nurseries north
and south of Long Island, and spring and
autunnn spawners of the same year class,
respectively, occurring in an adjacent ocean
locality. It is, however, evident that the n-iean
counts of spawners and of juveniles in the
respective localities followed a similar pat-
tern; that is, lower mean counts were asso-
ciated with spring spawners and juveniles
occurring north of Long Island, and, con-
versely, higher counts with autumn spawners
and juveniles occurring south of Long Island.
The observed differences between juveniles
and adults could have resulted from (1) changes
in mean vertebral counts with age as men-
tioned in an earlier section, (Z) recruitnnent
of larvae into estuarine nurseries from ocean
spawning grounds other than where spawners
were collected, and (3) inadequate sampling
of the spawning and/or juvenile populations
within the respective localities.

DISCUSSION

In our studies of the Atlantic menhaden
population structure, we are concerned with
whether observed differences between groups
of fish (for exannple, spring and autunnn
spawners) reflect discrete natural biological

units within the population. Probably the nnost
that we can hope to do through morphological
studies is to gain sonne insight into the con-
sistency of quantitative differences between
groups of fish. Such information would lead
to a better understanding of population struc-
ture.

Results of studies by June ( 1958) and Suther-
land (1963) revealed consistent differences in
nnean vertebral counts (as well as other
nneristic characters) of juveniles in five suc-
cessive year classes (1955-59) inhabiting
estuaries north and south of Long Island.
Juveniles identifiable with both groups were
shown to be living together in Long Island
estuaries. June (1958) found differences in
nneristic counts of juveniles in Long Island
estuaries associated with fish length and
hypothesized that the differences reflected the
intermingling of progeny from autumn and
spring spawnings. Sutherland (1963) noted that
fish length was a possible source of variation
within samples, but did not consider it in the
analysis of his data. Examination of his raw
data (table 5) reveals, however, that classifi-
cation of individual sannples from Long Island
waters, as to northern or southern groups,
also can be made on the basis of length. Those
samples identified with the southern group
comprised fish that were consistently larger
than those identified with the northern group.
In every instance where both groups were
represented in the same estuary, the smaller
fish occurred in samples collected late in the
season, and, in most cases, there was little
or no overlap in the length distributions.
There is little evidence, however, that progeny
from the autunnn spawning contribute appre-
ciably to the juvenile stocks in estuarine
nurseries north of Long Island. Numerous
attempts to collect larvae and juveniles in

Table 5. --Vertebral counts and fork lengths of juvenile Atlantic menhaden in samples from
Long Island, N.Y. , estuarine nurseries, 1956-57''-

Locality and date

Fish

in

sample

Vertebrae

-45

-ie

47

48

49

Average

Length

Range

Mean

1956:
Reeves Bay

Aug. 2

Oct. 10

Quantuck Creek

Aiig. 3

Oct. 9

Carmans River

Aug. 3

Oct. 8

1957:

Skiljnans Creek
Aug. 10

Reeves Bay

July 13

Sept. 4

Sept. 4

Quantuck Creek

July 14

Aug. 10

Sept. 4

Carmans River

Aug. 9

Sept. 4

Number

100
97

98
95

100
98

100

100

100

59

100

94

100

94
96

Number

Niimber

2

30

7
19

2

39

10

20
25

20

15
42
53

39
41

Number

80
63

63
73

76
57

74

77
68
34

69
48
46

52
52

Niimber

18
3

28
3

22

1

16

16
3

1

Number

Number

2 47.16
^ 46.70

2 47.21
2 46.83

2 47.20
^ 46.56

47.06

46.83
46.70
46.71

47.01
46.56
46.48

46.59
46.57

Mm.

70- 97
45- 67

61- 85
26- 61

65-104
45- 56

111-127

38- 63
37- 77

86-105
25- 50
74- 93

34- 68
48- 77

Mm.

78.0
53.9

72.4
41.0

84.0
50.0

118.6

48.3
51.7

94.1
33.2
82.3

50.4
59.5

"'■ Vertebral counts from Sutherland (1963) appendix tables 1 and 2.
^ Southern group.
^ Northern group.

northern estuaries from spring to early sunn-
mer 1955-59 were unsuccessful, although
larvae and recently metamorphosed juveniles
were found in abundance beginning in midsum-
mer. The foregoing findings suggest that the
larger juveniles in Long Island estuaries were
the progeny of an autumn or winter spawning,
while the smaller juveniles resulted from a
spring or early summer spawning.

While information on the spatial and tem-
poral distribution of spawning is inconnplete,
there is sufficient evidence to demonstrate
that, south of Long Island, spawning is con-
fined to autumn and winter. Massmann,
Norcross, and Joseph (1962) reported larvae
in plankton tows in ocean waters off Virginia
from Novennber to May. McHugh, Oglesby,
and Pacheco (1959) found larvae abundant in

Chesapeake Bay in late April and early May.
Reintjes (1961) reported the occurrence of eggs
and larvae along the North Carolina coast
from Novennber to March, but none during
other months of the year . Higham and Nicholson
(1964) reported fish in spawning condition in
catches off the North Carolina coast during
autumn and winter only.

Although spawning apparently takes place
throughout the period that the fish occur in
inshore waters north of Long Island, there
are two peaks--one in spring and the other in
autumn. Little spawning is indicated between
June and August. Perlmutter (1939) found
maximum numbers of eggs and larvae in Long
Island waters in May and only scattered
numbers during summer. Wheatland (1956)
and Richards (1959) collected eggs and larvae

in Long Island Sound fronn May to October,
and foimd greater numbers in spring and
autumn. Herman (1963) obtained eggs and
larvae in Narragansett Bay, R.I., from May
through August and in late October, and he
postulated a split spawning season in that
locality, with autumn spawning being more
productive. Scattergood, Trefethen, and Coffin
(1951) reported that spawning fish occurred
in Maine and Massachusetts waters in August
and September. Higham and Nicholson (1964)
found fish in spawning condition in purse
seine catches north of Long Island from June
to October and concluded that spawning in
those waters reached a peak in October, with
the suggestion of an earlier peak in May,

It seems clear, then, that two distinct
groups of adults occur together from Long
Island northward--one which spawns in spring,
and the other in autumn. In this study, samples
of spawners and nonspawners were collected
simultaneously from pound net catches in Long
Island and Cape Cod waters in spring, audit is
known that mixed schools of spring and autumn
spawners live together in these waters through
summer (Higham and Nicholson, 1964). They
are subject, therefore, to the same environ-
ment. If the reproductive cycle were governed
primarily by environment, then it would be
expected that the fish would mature and spawn
at the same time, and variations in these
responses would be random. Morphological
differences also would be obscured. Results
of the present study show, however, that ir-
respective of the locality in which autumn
spawners were taken, their mean vertebral
counts were similar. This was also true for
spring spawners. But comparisons of the mean
counts of autumn spawners with those of
spring spawners showed significant hetero-
geneity.

The seasonal difference in spawning times
of individuals in northern waters indicates a
fundamental difference in response to the
environment. Spring and autumn spawnings
(and early development of young) apparently
take place under entirely different temperature
conditions. In figure Z are plotted the m.onthly
nneans and the daily ranges of surface water
temperature for 1957-59 in localities where
samples of fish used in the present study were
collected. Temperatures near Cape Cod and
Long Island not only were similar, but the
means in April and May were from 4° to 13°C.
lower than those in September and October,
with overlap in ranges occurring only in May
(Ambrose Lightship was off station in May each
year.). Temperatures near Long Island in
September and October were similar to those
off North Carolina inNovember and December.
This suggests that the southward migration of
autumn spawners from Long Island to the
Carolinas may be governed by their physiologi-
cal response to tenmperature. This argument
seems to be supported by the lack of significant

^ 25

g 20
q:
^ 15

^ 10
(t 5

UJ

BUZZARDS LIGHTSHIP
Lot. 4I''2I'N. Long. 71" 03' W.

AMBROSE LIGHTSHIP -•
Lot. 40° 27'N. Long. 73° 49' W.

J I I I I L

J I

15 -

10 -■
5

FRYING PAN SHOALS LIGHTSHIP
Lot. 33° 28' N. Long. 77° 33' W.

■ ' ■ I I 1 1 —

_i-

-L.

F Nfl A M J J A
MONTH

N

Figure 2. — Monthly range and mean temperatures at
three lightship stations along the Atlantic coast of the
United States, 1957-59.

variation in the meristic counts of juveniles
that were presumed to have originated from
the autumn spawning from Long Island south-
ward (Sutherland, 1963),

There is evidence of other biological dif-
ferences between autumn and spring spawners.
For example, the mean ages and lengths of
spring spawners differed from those of autumn
spawners (table 1). Allowing for the advance in
age that takes place betweenautumn and spring
each year (arbitrarily on January 1), I found
that both the mean age and mean length of
autumn spawners off North Carolina were
significantly lower than those of the spring
spawners taken in Cape Cod and Long Island
waters in spring. There was, however, no
significant difference between autumn spawners
and nonspawners taken in northern waters in
spring. This suggests that autumn spawners
either suffered a greater fishing mortality or
were recruited at a younger age and at a
shorter length than spring spawners, or both.

10

There also appears to be a consistent difference
in the relative abundance of the two spawning
groups. Higham and Nicholson (1964), for
example, showed that from Long Island north-
ward, somewhere between 12 and ISpercentof
the fish in purse seine catches in October 1958,
1959, and I960 were sexually inactive. In the
present study, the proportion of spring
spawners in individual samples collected in
these samie waters in May was between 13 and
ZO percent of about the same total numbers of
fish caught in each corresponding October. The
relatively small numbers of nonspawners in the
autumn catches and spring spawners in spring
catches north of Long Island indicate that
spring spawners probably constituted a much
snnaller group than autumn spawners.

The similarity in the vertebral counts of
juveniles in northern estuaries and of spring
spawners from adjacent ocean waters further
emphasizes the distinctness of a northern group
of fish which appears to be physiologically
and genetically adapted to life under conditions
of lower temperature. Thus, the inverse re-
lation between vertebral number and tennper-
ature noted for many clupeoid fishes (Barlow,
1961) does not seenn to hold for Atlantic men-
haden.

In the present study, the hypothesis was that
no more than two discrete groups of spa%vners
were present simultaneously in the different
localities, and the data confirm that this con-
clusion, in general, agrees with the known
facts concerning the occurrence of spring- and
autumn- spawned juveniles in estuarine
nurseries and the seasonal occurrence of
sexually active fish in the commercial catches.
Sonne spawning, however, occurs in waters
north of Long Island through sunnmer (Herman,
1963). Furthermore, spawning by younger,
smaller fishoccurs from North Carolina south-
ward beginning in September or October (in
advance of the arrival of the larger, older
spawning fish from farther northward). The
relationship of these additional spawning groups
should be investigated as soon as practicable
since they must be considered in answering
the question of population structure.

In view of the results of this study, I
recomnnend that the meristic studies be ex-
panded to include all known spawning groups.
Other meristic and/or morphometric
characters also should be included to
provide estimates of the relative proportions
of discrete subpopulation components in mixed
samples of fish. Growth, serological, and
fecundity studies also should be undertaken
since these will yieldr ancillary information
that, when combined with results of meristic
studies, might lead to an understanding of the
structure of the Atlantic menhaden population.

SUMMARY

1. Vertebral counts of 6,048 adult Atlantic
menhaden (Brevoortia tyr annus) collected off
Cape Cod, Mass., Long Island, N.Y., and
Beaufort, N.C., from 1957 to 1960 were analyzed
to determine if autumn (October-December)
and spring (April-May) spawners were dis-
tinguishable by this meristic character.

2. Variation between sexes and within and
between year classes was found to be negligible.

3. Two discrete groups of spawners--one
occurring in Cape Cod and Long Island waters
in spring and the other occurring in Long Is-
land and North Carolina waters in autumn- -
were identified on the basis of mean vertebral
count. The mean count of spring spawners
was 46.914, and of autum spawners, 47.031,

4. Comparison of mean counts of spawners
in three successive year classes (1955-57)
with those of juveniles in the same year
classes from estuarine nurseries showed that
lower mean counts were associated with spring
spawning north of Long Island.

5. The tendency for mean vertebral counts
of reproductively isolated groups to follow a
distinct pattern for several years suggests that
the observed differences are genotypic in
origin.

ACKNOWLEDGMENTS

Doyle F. Sutherland, with James F. Guthrie,
collected most of the specimens and made the
vertebral counts and measurements upon which
this study was based. Joseph R. Highann, Jr.
aged the fish.

LITERATURE CITED

BARLOW, GEORGE W.

1961. Causes and significance of morpho-
logical variation in fishes. Syst. Zool.

10 (3): 105-117.
BUMPUS, DEAN F.

1957, Oceanographic observations, 1956,

east coast of the United States. U.S.

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233, 132 p.
DAY, C. GODFREY.

1959a, Oceanographic observations, 1957,

east coast of the United States. U.S.

Fish Wildl. Serv., Spec. Sci. Rep. Fish.

282, 123 p.
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318, 119 p.

11

eggs and larvae of
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and WILLIAM R.

DAY, C. GODFREY--Continued

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east coast of the United States, U.S.

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359, 114 p.
HERMAN, SIDNEY S.

1963. Planktonic fish
Narragansett Bay.
8 (1): 103-109.

HIGHAM, JOSEPH R.,
NICHOLSON.

1964. Sexual maturation and spawning of
Atlantic menhaden. U.S. Fish Wildl.
Serv., Fish. Bull. 63: 255-Z71.

JUNE, FRED C.

1958. Variation in meristic characters of
young Atlantic nnenhaden, Brevoortia
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Mer, Rapp. et Proc. Verb. Reun. 143
(2): Z6-35.

JUNE, FRED C, and WILLIAM R.

NICHOLSON.

1964. Age and size composition of the nnen-
haden catch along the Atlantic coast of
the United States, 1958; with a brief
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Fish Wildl. Serv., Spec. Sci. Rep. Fish.
446, 34 p.
JUNE, FRED C, and JOHN W. REINTJES.

1959. Age and size composition of the men-
haden catch along the Atlantic coast of
the United States, 1952-55; with a brief
review of the commercial fishery. U.S.
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317, 65 p.

JUNE, FRED C, and CHARLES M. ROITH-
MAYR.

1960. Determining age of Atlantic men-
haden from their scales. U.S. Fish
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342.

MARR, JOHN C.

1957. The problem of defining and recogniz-
ing subpopulations of fishes. In JohnC.
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NORCROSS, and EDWIN B. JOSEPH.
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coastal waters. Chesapeake Sci. 3 (1):
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McHUGH, J. L., RAY T. OGLESBY, and
ANTHONY L. PACHECO.

1959. Length, weight, and age composition
of the menhaden catch in Virginia waters.
Limnol. Oceanogr. 4 (2): 145-162.
PERLMUTTER, ALFRED.

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of Long Island, 1938. Section I, An
ecological survey of young fishandeggs
identified from tow-net collections. N.Y.
Conserv. Dep. 2: 11-71.
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1961. Menhaden eggs and larvae from M/V
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RICHARDS, SARAH W.

1959. Oceanography of Long Island Sound,

VI. Pelagic fish eggs and larvae of

Long Island Sound. Bull. Binghann

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SCATTERGOOD, LESLIE W., PARKER S.

TREFETHEN, and GARETH W. COFFIN.

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Iowa State College Press, Annes, 534 p.
SUTHERLAND, DOYLE F.

1958. Use of diagnostic X-ray for determin-
ing vertebral numbers of fish. U.S.
Fish Wildl. Serv., Spec. Sci. Rep. Fish.
244, 9 p.

1963. Variation in vertebral numbers of
juvenile Atlantic menhaden. U.S. Fish
Wildl. Serv., Spec. Sci. Rep. Fish.
435, 21 p.
WHEATLAND, SARAH B.

1956. Oceanography of Long Island Sound,
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MS. #1449

12

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