Figure 14. — Staining shrimp for mark-recapture studies. 



Major research of this program seeks to 

 determine the optimum size at which shrimp 

 should be harvested. An answer to this ques- 

 tion can be obtained rather easily when we 

 have good estimates of the rates of growth 

 and mortality of shrimp. Good estimates, 

 however, are difficult to obtain because shrimp 

 live in a dynamic environment where growth 

 and mortality change continually in response 

 to seasonal differences in temperature, food 

 supply, predation, fishing pressure, etc. Each 

 of the projects within the program is obtain- 

 ing information needed to describe some 

 aspects of shrimp growth and mortality. 



A further means to ensure that fishermen 

 are able to realize maximum returns from 

 shrimp available for harvest is to provide 

 advance indications of where and when good 

 catches can be made. We have made sub- 

 stantial progress in predicting the abundance 

 of brown shrimp from the numbers of post- 

 larvae caught 2 or 3 mo. earlier. Forecasting 

 the abundance of pink and white shrimp has 

 not been perfected, but we have evidence to 

 suggest that this too can be accomplished. 



Richard J, Berry, Program Leader 



MARK-RECAPTURE EXPERIMENTS 



Modern theories of fishery management 

 consider the simultaneous losses and addi- 

 tions in weight that occur in a population and 

 attempt to find the point in time at which 

 these two opposing factors are of equal mag- 

 nitude. This point represents the average age 

 at which individuals in a population should 

 be harvested to obtain the greatest yield in 

 weight. A practical nnethod for estimating 

 rates of losses and additions to shrimp popu- 



lations is by mark-recapture experiments 

 in which biological stains are the marking 

 agent. Records of the total fishing effort 

 expended by the fishery in catching marked 

 shrimp are used to estimate population losses 

 resulting from fishing and natural mortalities. 

 Gains in population weight are calculated 

 from the growth of the marked shrimp caught 

 in the fishery. The following paragraphs de- 

 scribe such a mark-recapture experiment 

 on the Tortugas grounds in early February 

 1965. 



The Tortugas shrimping grounds include 

 an area about 50 by 80 miles off the south- 

 western coast of Florida. Water depths on 

 the grounds range from about 9 fath. on the 

 east to 30 fath. on the west. The fishing fleet, 

 which includes 200 to 400 vessels between 

 October and May, concentrates in depths of 

 10 to 20 fath. in an area that covers about a 

 quarter of the total fishing grounds. 



Previous mark-recapture experiments on 

 the Tortugas grounds formed a basis for 

 the present study. We knew that shrimpmoved 

 slowly in a northwesterly direction over the 

 grounds and that marked shrimp are available 

 for capture during a 2- to 4-mo. period after 

 release. Therefore, stained shrimp released 

 in the eastern portion of the grounds in early 

 February should be recaptured from that time 

 until the fleet moves elsewhere in late May. 

 Because measures of fishing and natural 

 mortality obtained from prior experiments 

 have included a range of values, we wished 

 to obtain several independent estimates from 

 this study in order to determine average 

 values under existing conditions. Further ob- 

 jectives were to obtain information on how 

 much shrimp grew during early spring and 

 how they dispersed over the fishing grounds. 



21 



