LIFE HISTORY OF THE COMMON SHRIMP 
19 
During the following months of April, May, June, and July 1932, accompanying a 
steady increase of water temperature from 56° F. in March to 86 in July there is a 
remarkable exhibition of growth. This is manifested in three ways. The steady 
ensuing growth is differential, being more strongly expressed in the females than the 
males; there is a surprising reduction of the size range; and the sex products mature 
rapidly. 
As will be recalled, this period coincides with the time during which mature 
shrimp are taken. These first appear among the largest; but soon the smallest have 
exceeded the minimum size for sexual maturity, maturing becomes general, and by the 
1st of July no immature of any size can be found in this group of shrimp. 
The progress of growth is very interesting. During the spring, in each sex a dis- 
tribution of wide range without a sharp mode is converted into a compact group with 
little variation of size and a well-marked mode. This may be caused by either or 
both of two processes — first, the more rapid growth of the smaller and younger 
members; second, the disappearance of the larger individuals that have spawned. 
Whatever the process, its orderly progress is striking. In fact, a frequency distribu- 
tion from May or June may readily be identified by its characteristic shape. 
Although up to April there has been no significant difference in size between the 
sexes as evidenced from the frequency curves (except for the short period in the fall 
of 1931 already noted), the growth of the females, beside producing a characteristi- 
cally compact and homogenous group of this sex, rapidly outstrips that of the males 
until in June, as previously stated, the sexes differ in length by 8 percent, the males 
averaging 144 and the females 156 mm. By this time the range has been so reduced 
that the ninth decil of the males (the largest males remaining after the upper 10 
percent have been cut off) is below the average of the females and the first decil of 
the females is above the average length of the males. 
FATE OF ADULTS 
A question of great interest and importance both from the theoretical and the 
practical standpoint is the fate of the larger group present in July. The smaller 
group of shrimp, traced into earlier months, was found to have been spawned by the 
larger, and the larger to have overwintered from young of the previous spring or 
summer. Traced onward from July the smaller group persists through the winter 
and spawns the following spring and summer. Does the group of larger shrimp also 
survive the winter and take part in a second spawning? 
Again our evidence is indirect but impressive. We shall consider the length fre- 
quency and sexual maturity data in the light of the abundance of the shrimp. No 
accurate measure of the abundance of the shrimp in the waters of Georgia at different 
seasons is available. An approximation, however, is furnished by the number of 
shrimp taken in experimental trawling. By these useful, although imperfect, data 
let us follow the abundance of the group of small shrimp from the time of their entrance 
into the commercial catch until their disappearance. The numbers increase rapidly, 
being constantly augmented by new young of later and later hatchings. The maxi- 
mum of abundance is reached in the fall, usually September or October, agreeing in 
time with the fall peak of the commercial catch. (See fig. 11.) Although the fall 
peak of the total catch tends to be overemphasized by the intensity of the fishing at 
this season and in general economic factors prevent a complete correspondence, 
nevertheless the fall catch rests on a period of marked abundance of shrimp. From 
