MORTALITY RATES IN POPULATIONS OF PINK SHRIMP, PENAEUS DUO- 

 RARUM, ON THE SANIBEL AND TORTUGAS GROUNDS, FLORIDA' 



By T. J. COSTELLO AND DONALD M. ALLEN, Fishery Biologists 

 Bureau of Commercial Fisheries Biological Station, Miami, Fla. 33149 



ABSTRACT 



Mark-recovery experiments were made to obtain esti- 

 mates of fishing and natural mortalities as a portion of 

 studies related to the life history of commercial shrimps 

 in the Gulf of Mexico. In two experiments, groups of 

 pink shrimp were injected with biological stains and 

 released into the Sanibel and Tortugas fisheries off the 

 southwest coast of Florida. Marked shrimp were re- 

 captured by commercial shrimp fishermen. 



Mortality estimates were derived from analysis of 



marked shrimp recoveries during the first 10 and 8 

 weeks of the Sanibel and Tortugas experiments, re- 

 spectively. In the Sanibel population, fishing mortality 

 was estimated to have been 6.8 percent for each 2-week 

 period, and all other losses in the population were 14.8 

 percent; for the Tortugas population, fishing mortality 

 was estimated to have been 13.1 percent for each 2- week 

 period, and all other losses 19.7 percent. 



In the past 3 decades, biologists have probed 

 at various aspects of the hfe histories of commercial 

 shrimps in the Gidf of Mexico. Recently, large- 

 scale, mark-recovery experiments were made to 

 estimate rates of fishing and natm-al mortaUties 

 in shrimp popidations. A mark-recovery experi- 

 ment, in which biological stains were the marking 

 agent, was undertaken on the Tortugas pink 

 shrimp {Penaeus duorarum) trawUng grounds west 

 of Key West, Fla., in September 1961. Develop- 

 ment of an appropriate recovery system brought 

 return of 21.1 percent of the marked shrimp. These 

 shrimp were in commercial catches and returned 

 by fishermen at shrimp landing ports. Analysis of 

 data produced estimates of the rate of fishing and 

 natural mortaUty in the Tortugas pink shrimp 

 population (Kutkuhn, 1966). Two similar experi- 

 ments were carried out in south Florida waters in 

 1962 and 1963. Emphasis was placed on obtaining 

 a complete tabulation of fishing effort and recover- 

 ing a high percentage of the marked shrimp that 



' Contribution No. 229, Bureau of Commercial Fisheries Biological Labora- 

 tory, Galveston, Texas 77552. 

 Published March 1968. 



FISHERY bulletin: VOLUME 66, NO. 3 



were recaptured by commercial gear. One experi- 

 ment was on the Sanibel grounds south of Sanibel 

 Island, and the other on the Tortugas grounds 

 (fig. 1). These two experiments form the basis of 

 this report. 



THE SANIBEL AND TORTUGAS FISHERIES 



In both fisheries, trawUng gear is similar to 

 that used elsewhere in the Gulf of Mexico (BuUis, 

 1951). Trawhng is at night because pink shrimp 

 usually remain buried during the day. Other 

 species of penaeid shrimp in the catches are of 

 minor commercial importance. 



The area known as the Sanibel grounds com- 

 prises about 2,000 km.^ (600 square nautical miles) 

 of trawlable bottom in two sections south and 

 northwest of Sanibel Island, Fla. Most fishing is 

 on the southern portion of the grounds between 

 the 11- and 18-m. depths. The fishery began in 

 1954 and has produced about 272,000 kg. (600,000 

 pounds) (tails) of pink shrimp annually. Peak 

 catches are made from March through May each 

 year when 35 to 90 vessels participate in the 



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