LIFE HISTORY OF THE COMMON SHRIMP 
9 
Table 3. — Shrimp ( Penaeus setiferus) measured during semimonthly periods in the three major regions 
of investigation 
Period 
Feb. 1. 
Feb. 2_. 
Mar. 1. 
Mar. 2.. 
Apr. 1.. 
Apr. 2.. 
May 1_. 
May 2.. 
June 1_. 
June 2 _. 
July 1.. 
July 2. _ 
Aug. 1.. 
Aue. 2.. 
Sept. 1. 
Sept. 2. 
Oct. 1.. 
Oct. 2__ 
Nov. 1_. 
Nov. 2.. 
Dec. 1.. 
Dec. 2.. 
1931 
Subtotal. 
Georgia 
Males 
547 
1,021 
476 
559 
819 
732 
1,222 
1,012 
604 
81 
453 
427 
918 
1,730 
1.551 
1.552 
664 
1,329 
632 
1, 158 
1,737 
19, 227 
Fe- 
males 
638 
1 , 111 
563 
627 
830 
821 
1,314 
1,287 
1,010 
411 
411 
373 
521 
1,398 
1,467 
1,720 
752 
1,389 
679 
1,323 
1, 862 
20,507 11,775 
Louisiana 
Males 
155 
37 
530 
969 
964 
1,996 
1, 972 
1, 151 
1,339 
1,994 
319 
349 
Fe- 
males 
139 
50 
476 
788 
889 
2,004 
2, 028 
1,308 
1, 261 
2,206 
362 
451 
11,962 
Texas 
Males 
495 
575 
311 
494 
69 
222 
433 
315 
488 
598 
588 
394 
463 
439 
348 
Fe- 
males 
536 
525 
289 
506 
131 
178 
367 
310 
512 
602 
612 
406 
537 
561 
452 
6, 232 6, 524 
Period 
Georgia 
Louisiana 
Texas 
Males 
Fe- 
males 
Males 
Fe- 
males 
Males 
Fe- 
males 
1932 
Jan. 1 
456 
528 
328 
317 
178 
222 
Jan. 2 
476 
561 
172 
228 
Feb. 1 
301 
343 
777 
823 
Feb. 2 
275 
338 
193 
207 
Mar. 1 
507 
514 
485 
607 
291 
309 
Mar. 2 
457 
526 
361 
378 
186 
214 
Apr. 1 
1,260 
1,322 
272 
265 
420 
480 
Apr. 2_ _ . _ 
1,348 
1,592 
126 
82 
101 
99 
May 1 _ 
125 
212 
326 
285 
118 
282 
May 2_ 
662 
931 
233 
248 
393 
407 
June l 
349 
596 
73 
49 
177 
223 
June 2 
314 
688 
248 
290 
101 
99 
July 1 
373 
356 
237 
270 
314 
286 
July 2 
963 
683 
255 
287 
306 
294 
Aug. l._. - _ 
1,017 
704 
217 
204 
792 
608 
Total 
28, 110 
30, 401 
14, 936 
15, 244 
10, 751 
11,305 
Beside length measurements, all observers have recorded the conditions of the 
gonads, permitting these specimens to be classified not only by length but also by 
stage of sexual maturity. 
The field examination has been by naked eye, but in addition all the observers 
have made microscopic examination of part of the specimens and preserved material 
for future study as a check on the field classification. 
Smaller shrimp than those taken by the commercial gear have been seined or 
taken by small beam trawls or "try nets” with bobbinet casings. These shrimp, 
ranging from 7 to 20 mm, have been measured under the low power of the microscope 
with an eyepiece micrometer. 
Of a different nature is the material obtained by plankton nets. The study of 
the larval stages thus obtained is not yet complete. 
INTERPRETATION OF DATA 
RECOGNITION OF AGE GROUPS 
The interpretation of the length-frequency data from Georgia upon which, as 
our most complete series, most of our knowledge of the life history is based, may con- 
veniently begin with those for July 1931. Here there is a well-marked bimodality of 
the curve due to the presence of two completely separated groups differing markedly 
in size. This is plainly shown in figure 6, in which the smaller group with a mode 
at about 95 mm extends from 65 to 125 for both males and females, while the larger 
group, centered about 153 mm for the males and 175 for the females, extends for the 
males from 140 to 170 mm and for the females from 150 to 190 mm. There is thus a 
gap of 20 mm between the two groups. At no other time of the year can two com- 
pletely separated groups be found. Examination of the other records show an 
essentially similar condition during July in Louisiana and during August in Texas. 
