Figure 15. — Release sites for marked pink shrimp 

 released on the Tortugas grounds. 



To obtain estimates of mortality, we re- 

 leased three separate groups of distinctively 

 marked pink shrimp in an intensively fished 

 part of the grounds (fig. 15). To obtain growth 

 information, we stained and released near 

 the eastern edge of the fishing grounds a 

 fourth group, consisting of small shrimp of 

 a restricted size range (3.8 to 4.1 in.). De- 

 tails concerning each group appear in table 6. 



Calculations of rates of fishing and natural 

 mortality are based on the number of marked 

 shrinnp recaptured by the fishery. The two 

 causes of death can be separated when an 

 accurate record of fishing effort is available. 

 Our interviews with the vessel crew members 

 provide information on the amount of fishing 

 effort and where marked shrimp are caught. 



Figure 16 illustrates fluctuations in fishing 

 effort and in the number of marked shrimp 

 recovered during the first 80 days of the 

 Tortugas experiment. The nunnber of re- 

 coveries of marked shrimp is related to 



Figure 16. — Comparison of the number of marked shrimp 

 caught and fishing effort, January 25 to April 14, 1965. 



fishing effort. Estimates of fishing and natural 

 mortality can be calculated for each of the 

 three groups when we identify the fishing 

 intensity applicable to each. 



The movement of marked shrimp appeared 

 to be slow and northwesterly from the release 

 sites. During the second nnonth of the experi- 

 ment (30 to 59 days at large), 93 percent of 

 marked shrimp recovered had moved less than 

 20 miles; and during the third nnonth, 88 per- 

 cent were taken within 20 miles of the release 

 areas. We can more accurately assess the 

 rates of movement after we analyze the ex- 

 perimental data. 



Growth data obtained from the group of snnall 

 shrimp released on the eastern edge of the 

 fishing grounds showed a difference in the 

 growth of male and female shrimp. During 



Table 6. — Descriptive data pertaining to groups of marked shrimp released and recaptured on the 

 Tortugas fishing grounds, February to April 1965 



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