operation usually undertaken aboard the fishing 

 vessel when most of the catch consists of medium- 

 size or larger shrimp. Regardless of where be- 

 heading takes place, however, the chances of 

 detecting marked specimens in the process are 

 always very good to excellent. Under the con- 

 ditions prevailing in most processing plants they 

 are highest. All recovered shrimp not found at 

 sea by vessel crews were later retrieved by plant 

 personnel. 



So far as is known, only a small fraction of all 

 landings made within the study period escaped 

 such close scrutiny, this occurring when landings 

 infrequently had to be shipped directly to canneries 

 located on the northern Gulf coast. 



SANIBEL MARK-RECAPTURE 

 EXPERIMENT 



Though specifically designed for another pur- 

 pose, the Sanibel experiment yielded growth data 

 that proved useful in corroborating comparable 

 data derived from the Tortugas experiment. By 

 way of providing a brief description, the former 

 experiment began about 9 months before the latter 

 and extended over the period December 1960 to 

 June 1961. In an attempt to determine the mi- 

 gration patterns of juvenile pink shrimp as they 

 leave estuaries in the vicinity of Sanibel Island, 

 Fla., 32,900 shrimp were marked with Trypan blue 

 stain and released in Pine Island Sound. Com- 

 mercial fishermen subsequently recaptured 155 

 (or 0.5 percent) at the locations indicated in 

 figure 1. 



Individuals making up the experimental popula- 

 tion were first graded to uniform size though not 

 nearly so carefully as those selected for the Tortu- 

 gas experiment. A sample of 463 specimens taken 

 at the experiment's start revealed a mean total 

 length and corresponding weight of 75.3 mm. and 

 3.5 g., respectively. Sexes occurred in the esti- 

 mated ratio of 46 males to 54 females. 



ESTIMATION OF GROWTH 

 METRIC CONVERSIONS 



Solution of the yield equation employed in a 

 later section postulates that the average growth of 

 each individual making up the population of 

 interest be expressible in terms of weight, and that 



the average relationship between an individual's 

 weight and length be firmly established. 



Since it is ordinarily impractical to measure pre- 

 cisely in the field the weight of animals as small as 

 shrimp, sampled individuals for the most part must 

 be measured as to length at the sampling site, or 

 preserved (rarely frozen) and later measured, again 

 according to some length criterion, at a more con- 

 venient time and place. Subsequently required in 

 either case are appropriate means for converting 

 length to weight units. 



Three situations that are met in the course of 

 studies like the present one, and in shrimp research 

 generally, may thus be defined. The first entails 

 reciprocal conversion of the linear size indices 

 commonly used in shrimp work, viz, total and 

 carapace length. In this study, for example, 

 total length was the criterion used to grade 

 shrimp selected for the Tortugas experiment. 

 But the sample from which the actual size-range 

 limits were determined had to be preserved 

 (formalin) for later measurement under more 

 favorable conditions. Since the total length of 

 shrimp so treated cannot be accurately measured 

 because of the distorting and hardening effects of 

 the preservative, carapace length, which remains 

 unimpaired, provides the most practicable index 

 of shrimp size. The statistical relationship 

 between total length and carapace length serves 

 to transform either linear dimension to the other 

 and proves particularly useful when comparing 

 the results of research in which the methods of 

 measurement differ from one study to the next. 



The two remaining situations prevail where 

 interest centers on the transformation of linear 

 to weight units, i.e., (1) from total length 

 to total weight when length measurements 

 can be made at the time of sampling and (2) from 

 carapace length to total weight when the sample 

 material must first be preserved. 



Over the period August 1960 to January 1961, 

 and independent of the experiments reported 

 herein, weight and corresponding length measure- 

 ments were obtained from slightly more than 1,600 

 pink shrimp sampled from commercial catches 

 taken along the east and southwest Florida coast 

 (Biscayne Bay and the Tortugas-Marquesas area, 

 respectively). An indication of their statistical 

 strength in terms of sample distribution by sex 

 and relative size is given in table 3. 



DYNAMICS OF A PENAEID SHRIMP POPULATION 



321 



