8 
BULLETIN OF THE BUREAU OF FISHERIES 
sexes, so that the proportion of sexes, uniform during the winter, shows wide fluctu- 
ation. Far more mature shrimp of both sexes are found outside than inside. 
The shrimp is most abundant in shallow coastal waters near river mouths or 
deltas. It is omnivorous, feeding on a variety of animals and plants. 
LIFE HISTORY 
NATURE OF DATA 
First in importance are the length frequencies obtained by measuring shrimp 
taken by the standard commercial gear, the otter trawl. All of the shrimp measured 
in Texas were samples from the catch of fishermen; the same is true of part of the 
data from Louisiana. The remainder of the shrimp were obtained with standard 
gear from boats operated by members of the staff in Louisiana, Georgia, and North 
Carolina. This experimental fishing differs from the commercial fishing solely in the 
localities fished. The fishermen obviously cannot afford to fish in localities yielding 
small catches, while in the experimental fishing these localities give information fully 
as valuable as those where many shrimp are taken. Of these length-frequency series 
the most important are those for Penaeus setiferus, although similar but far less 
complete data are available for Penaeus brasiliensis, Xiphopenaeus kroyeri, and 
Trachypenaeus constrictus. The series from Georgia is, because started earlier and 
unmarred by breaks, the most valuable. 
The total length measured from the tip of the rostrum to the tip of the telson 
has been used as the standard measurement. This measurement was chosen over 
other linear measurements such as carapace length and over weight and volume after 
a study of the variability of measurements obtained in various ways. Weight and 
volume are subject to great variation because of the adhering water on a body as 
irregular in shape as the shrimp. These measurements consume much time and 
require apparatus not easily used aboard small boats. In obtaining the total length 
the shrimp is placed, ventral surface down, on the scale of a special measuring board 
with the rostrum in contact with a fixed block. The measurement is read and 
recorded to the nearest millimeter. For plotting and analysis the measurements 
are first tabulated, then grouped into 5-mm classes in such a way that the mid-point 
always falls on a 3 or an 8. For example, all shrimp measuring from 131 to 135 mm 
(both figures inclusive) are grouped together in a class interval with a mid-point of 
133 mm and so plotted in a frequency polygon. Similarly, all from 136 to 140 mm 
are grouped with a mid-value of 138. 
Measurements other than total length have been made for special purposes and 
in smaller numbers; most notable are those for racial studies now in progress. 
The measurements from the four localities, covering, in the case of Georgia, over 
2 years, now number more than 150,000 and therefore represent an adequate basis 
for statistical treatment. (See table 3.) 
