18 
BULLETIN OF THE BUREAU OF FISHERIES 
GROWTH 
Because of the fact that young are constantly being added to the population 
sampled by the fishing, it is extremely difficult to approximate the growth. As the 
earliest spawned shrimp continually increase in length, and small of a relatively uniform 
size are constantly appearing, the range of sizes, of course, increases ; consequently the 
growth is not represented by the mean or modal lengths, which each month are based 
on a new assortment of ages. It is clear that the increase of the means is much slower 
than the growth of shrimp of any particular age. 
The advancing upper size limit of the entire group should measure the growth of 
the older shrimp but is difficult in practice to determine. No statistical measure 
exists which will be comparable for all these types of compound frequency curves. 
In discussing the series of length frequencies, we have used the mode, or most abundant 
size as determined by inspection, for the average, and for the upper and lower limits 
a point where the ordinate has fallen to 1 percent of the total number of measure- 
ments. Admittedly inaccurate, this may serve, however, as a first approximation 
for the purposes of the present discussion. 
The length-frequency data obtained with a standard trawl in the ordinary fishing 
grounds is ill adapted to show growth. For the present, therefore, only the most 
general features will be sketched and a more detailed analysis postponed to a future 
time when another season’s records are available. 
The average size of the young at the time of appearance in July 1931 was about 
90 mm, with a range from 70 to 105. (See fig. 10.) At this time there is no significant 
difference in size between the sexes; in fact a clear differentiation does not occur until 
the following April. Through July and August growth is rapid; and by the 1st of 
September the most common size is about 130 mm, while the largest exceed 160 mm. 
The lower limit rises slightly and remains at approximately 80 mm until the first of 
the following year. We may consider that with the present gear about 70 mm is the 
minimum size entering the commercial catch to any extent. 
During September, October, and November the average length fluctuates between 
120 and 130 mm. During this period the females seem slightly the larger. This is 
the time of the fall maximum of abundance and the peak of the average commercial 
catch. From a value of 130 mm early in November the average size falls slightly, 
reaching a low point of about 120 mm near the first of the year. The difference in 
size between the sexes also disappears. This corresponds in a general way with the 
low period of the commercial catch. 
Obviously these decreases of average size must result from behavior, which is 
differential with regard to size — the large and the small shrimp must frequent dif- 
ferent localities or react differently in other ways. It is suggestive that the winter 
decrease of size is accompanied by a very marked decrease of abundance. During 
the low temperatures of the winter, movements or habits of the shrimp are changed 
in such a way as to remove the majority of them from the field of the commercial 
fishery on the Georgia coast. 
Following the first of the year (1932) the lower limit begins slowly to rise and by 
the 1st of April has increased from 80 to 100 mm. It is apparent that by January the 
last shrimp spawned in the preceding summer have entered the commercial catch and 
that the increase of the lower limit now reflects the growth. This seems the more 
probable, since the average advances at the same rate from 120 to 140 mm during 
this time. 
