shrimp should be increased, that the harvesting of 

 any age group (or "brood") should be delayed 

 until, by virtue of a high growth potential, maxi- 

 mum (or at least a greater) weight or biomass is 

 reached. Phrased more simply, the question 

 posed and heretofore not answered satisfactorily 

 is: At what size per individual should an age 

 group of shrimp begin to be harvested so as to 

 realize the maximum yield from that group during 

 its life span? 



This question has important economic implica- 

 tions and requires that we understand the relation- 

 ships between, and can measure, the rates of two 

 concurrent biological processes: (1) Growth, the 

 increase in shrimp size per individual per unit time, 

 and (2) mortality, the numbers of shrimp dying 

 per unit time (particularly due to natural causes). 



Over the years, fishery biologists have found 

 population growth and mortality parameters 

 highly elusive. Recently, however, as part of its 

 long-range shrimp research program, the Bureau of 

 Commercial Fisheries perfected a very satisfactory 

 process of marking shrimp that now permits 

 such parameters to be approximated by the useful 

 mark-recapture technique. The marking system 

 proving so successful is the stain-injection method. 

 Menzel (1955) is generally credited as having con- 

 ceived its use on shrimp; Dawson (1957) as having 

 substantiated its potential by means of laboratory 

 experimentation; and Costello (1959, 1964) and 

 Costello and Allen (1960) as having developed its 

 large-scale application afield. 



This paper first presents the results of a mark- 

 recapture experiment with a rather well-defined 

 stock of pink shrimp, Penaeus duorarum Burken- 

 road, which supports an important commercial 

 fishery in southwest Florida, the widely known 

 Tortugas fishery. After documenting the re- 

 quired estimates of growth and mortality, the 

 paper then describes their synthesis through a 

 theoretical yield equation to attempt an answer 

 to the question stated earlier. In so doing, it 

 draws in small part on the results of yet another 

 mark-recapture experiment conducted with the same 

 species, but for a different purpose, just prior to the 

 one of primary interest. 



Though it was classified as merely preliminary 

 and expanded efforts were to be scheduled only if 

 it achieved any reasonable degree of success, the 

 Tortugas experiment proved successful well be- 

 yond our somewhat modest expectations. The 



quality of the resulting data is reflected in the kind 

 of treatment they are given here. Such extensive 

 treatment, regardless of the experiment's outcome, 

 had not been anticipated in advance. 



TORTUGAS MARK-RECAPTURE 

 EXPERIMENT 



DESCRIPTION AND REGULATION OF FISHERY 



Lying west of Key West, Fla., reaching little 

 beyond the 30-fathom contour, and extending not 

 much farther northward than the 25th parallel, 

 the Tortugas trawling grounds seasonally ex- 

 perience some of the most intense shrimp fishing 

 seen in the Gulf of Mexico (fig. 1). Stimulating 

 this activity is the appearance of large numbers of 

 subadult pink shrimp which emerge in closely 

 spaced surges from the shallow "nursery" grounds 

 in Florida Bay to the east. 1 Although matur- 

 ing shrimp may enter the fishing grounds at all 

 seasons, it is in September-March that migrations 

 are greatest and fishing is heaviest. Practically 

 all fishing occurs at night when the pink shrimp 

 is most active and hence most vulnerable. Like 

 its close relative the brown shrimp, P. aztecus 

 Ives, this species usually burrows and is therefore 

 comparatively inaccessible during the hours of 

 daylight. 



Most trawlers operating on the Tortugas and 

 adjacent grounds are of the characteristic Florida 

 design (fig. 2). For all practical purposes they 

 are nearly uniform in size and power, and employ 

 fishing gear of roughly the same specifications 

 (Iversen and Idyll, 1959; Kutkuhn, 1962). 



The Tortugas fishery together with a lesser one, 

 the Sanibel fishery, which is centered roughly 100 

 nautical miles to the north, did not become well 

 established until the early 1950's (Idyll, 1957). A 

 question often raised and now being examined is 

 whether populations supporting both fisheries are 

 genetically discrete, or whether they are con- 

 tinuous — even though continuity may prevail 

 only at widely spaced life history stages. Evi- 

 dence acquired to date tends to support the as- 

 sumption made in this study that the Tortugas 

 fishery largely depends on a stock of pink shrimp 

 which is essentially separate from that sustaining 

 the Sanibel fishery (Costello and Allen, 1960; 

 1961). 



I Idyll (1957), Dohkin (1961), and Cummings (1961) should he consulted for 

 a detailed account of pink shrimp life history. 



314 



U.S. FISH AND WILDLIFE SERVICE 



