M (coefficient of true natural mortality). By 

 definition, X includes M, together with losses 

 from all other causes except fishing. Losses due 

 to migration from the area of fishing, and mor- 

 tality attributable to marking, handling, or the re- 

 lease procediu-e can contribute a considerable loss 

 to the experimental group of animals. This fact 

 must be given consideration in the use of X as 

 an estimate of M. 



Consideration of possible management implica- 

 tions calls for recognition that operation of trawls 

 may have complex effects upon a resident shrimp 

 population (Lindner, 1936). Also, cessation of 

 trawling affects the population. The mortahty 

 estimates we have given were calculated from 

 data assembled while a sizable fishery was in 

 progress. The coefficient of true natural mortality 

 may shift considerably when a regulatory measure, 

 e.g., closure of the fishery, is applied. 



F, as a function of fishing effort, fluctuates 

 over short periods of time. The value of M also 

 changes Ln response to such factors as varying 

 predation by migratory schools of fish. The values 

 derived here, however, for shrimp of the sizes in 

 the experiment, may establish the approximate 

 levels for offshore pink shrimp fisheries. These 

 parameters, together with supplementary infor- 

 mation and recommendations by Lindner (1966), 

 may be used as a basis for management of the 

 valuable Sanibel and Tortugas resources. 



ACKNOWLEDGMENTS 



The crew of the research vessel Silver Bay and 

 management personnel of the south Florida shrimp 

 industry helped with the field experiments. 



Ralph P. Silliman, Vaughn C. Anthony, R. A. 

 Fredin, and Albert C. Jones, all of the Bureau of 

 Commercial Fisheries, helped with various analyt- 

 ical procedures and edited the manuscript. 



LITERATURE CITED 



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1962. Survival of stained, tagged, and unmarked 

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1964. A record of the microsporidian, Thelohania 

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Joyce, Edwin A., Jr. 



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