Fishery bulletin of the Fish and Wildlife Service - NOAA

STUDIES ON THE STRIPED BASS OF THE ATLANTIC COAST 21
Such a disproportionate number of females to males is of course most unusual,
and it seems unlikely that this condition prevails among the total population of the
Atlantic coast. The examination of 29 small bass from Delaware Bay in November
1937 showed approximately 46 percent were males. A sample of 126 bass ranging
in size from 21 to 42% cm., from Albemarle Sound, N. C., in March and April 1938
was composed of 31.7 percent male fish. There is also evidence that the composition
of the spawning populations of striped bass is predominantly male (p. 19). A
theoretical explanation of the strikingly low percentage of males in northern waters
is included in the section under migrations (p. 44).
In studies of the age at maturity, miscroscopic examination of the gonads presented
the most plausible method of procedure in northern waters. The fact that
ripe 8 individuals were not available in Connecticut precluded the possibility of
studying the age groups making up a spawning population. Gonads from 109 female
striped bass ranging in size from 3'2 to 110 cm. were collected at various intervals
from April through November 1936 and 1937. Of these, 46 were fixed in Bourn's
fluid and slices from the anterior, middle, and posterior region of each one were cleared
in toluene. 9 These were sectioned, stained with Delafield's hematoxylin and eosin,
and mounted. Samples of up to 50 ova from each of the three regions of the gonads
from which slices were taken were then measured by means of an ocular micrometer.
It was soon found that samples from the anterior, middle, and posterior parts of each
ovary contained eggs of the same general sizes, and that there was no significant
difference between the ova of these regions, no matter at what stage of development the
gonads were. Thereafter only sections from the middle of each ovary were studied.
The remaining 63 ovaries from striped bass collected from April through November
1936 and 1937 were preserved in a solution of 10 percent commercial formalin and
water. Slices from the middle of each one of these gonads were then macerated
mechanically, until the eggs either floated free or could be easily teased from the
surrounding epithelium. Samples of up to 50 ova from each ovary were then measured
under a dissecting microscope by means of an ocular micrometer. The measurements
on the eggs from 109 ovaries by these 2 methods gave comparable results
throughout.
A study of the measurements of the eggs from striped bass of different sizes almost
immediately revealed that there were two easily distinguishable types of ovaries.
(See fig. 13.) The first type had eggs whose diameters consistently averaged 0.07
mm. There were occasionally eggs as large as 0.18 mm. in diameter, but more commonly
the largest eggs measured 0.11 mm. The second type contained eggs of two
definite size categories; there were small eggs of the same size as all those that were
seen in the first type of ovary, averaging 0.07 mm. in diameter, and there were large
eggs averaging 0.216 mm. in diameter or greater, the extreme size that has been
encountered being 0.576 mm. It is a reasonable assumption, especially in view of
Scofield's (1931) work, that those ovaries containing only small eggs represent immature
fish, and that those ovaries having eggs of both small and large size come
from fish that are mature, in the sense that the large eggs are those that will be produced
the following spawning season. A possible criticism of this assumption is that
part of the material examined might have been composed of ovaries from fish that
had just completed spawning, and that such ovaries might, therefore, contain only
eggs of the small size. On the basis of the distinction between mature and immature
individuals proposed above, these fish would then be considered miniature, a conclusion
that would be entirely erroneous. There is no evidence, however, that ovaries
from fish that had completed spawning immediately before were included in the
material. It has already been pointed out that spawning individuals were not found
in the waters from which this material was collected, and it is most unlikely that
any freshly spawned bass were studied for the purpose of determining the age of maturity.
Moreover, by far the greater part of the collection of gonads of striped bass
of different sizes took place in the summer and fall, by which time spawning is known
to be long since past. Another possible criticism of this method of determining the
age at maturity of striped bass is that some of the material may have come from fish
that were not spawning the following year, for this species is not necessarily an annual
1 The word "ripe" Is used throughout to connote flowing milt or eggs.
• Oil of wlntergreen and other clearing agents were also used at first, but In general toluene gave the most satisfactory results.