South Florida's seagrass and mangrove dominated estuaries serve prominently as nursery habitats for the pink shrimp, Penaeus duorarum, and support two offshore fisheries. The Tortugas fishery is the larger of the two and is the largest commercial fishery in Florida, with landings of approximately 9 million pounds annually. The Sanibel fishery is smaller, with a harvest averaging about 4 percent of the Tortugas Grounds. During the middle to late 1980's, a sharp decline in Tortugas landings occurred roughly coincident with seagrass die-off and the subsequent onset of extensive and persistent algal blooms in Florida Bay. These events focused increased attention on the interdependence of south Florida's estuaries and the productivity of Florida's offshore shrimping grounds and the role that the pink shrimp might play in restoration related research activities for Florida Bay.

Linkages between inshore estuaries and offshore fishing grounds are complex. Initially, following seagrass die-off, it was convenient to hypothesize that loss of habitat and declining environmental conditions in Florida Bay explained fishery declines. This approach was supported by tagging studies in the 1960's, which indicated that western and southwestern Florida Bay, the middle Florida Keys, Whitewater Bay, Coot Bay, and the Ten Thousand Islands served as nursery grounds, supporting the Tortugas fishery. Florida Bay was considered the principal inshore nursery ground. However, over the last several years, harvest has improved to near historic levels on the Tortugas Grounds, while conditions in Florida Bay have not changed significantly. This suggests that assuming a principal role for the Bay may be inappropriate. Other evidence supports this as well. Recruitment of pink shrimp into the offshore fishery occurs throughout the year with two peaks of recruitment, a Fall recruitment period measured from July through December peaking between August and October, and a Spring recruitment period measured from January through June, peaking between March and May.

Analysis of harvest data indicate that fishery decline in the 1980's has been associated with a loss of the Fall recruitment peak. Available data on inshore abundance of juvenile pink shrimp in Florida Bay and Whitewater Bay suggests the prominence of single, late Summer to early Winter (August-December) peaks of recruitment. These shrimp would presumably contribute to the Spring recruitment peak in the offshore grounds.

Solving these questions regarding the relative importance of inshore nursery areas in south Florida and the timing of movements between inshore nurseries and offshore spawning grounds is considered critical to determining the cause of recent declines in Florida's pink shrimp fishery as well as to setting and evaluating restoration objectives for the Bay. In this project, we propose to use natural stable C, N, and S isotope ratios to link inshore pink shrimp stocks with the offshore Tortugas and Sanibel fisheries. Our objectives are to: 1) determine if inshore pink shrimp populations differ isotopically among south Florida nursery areas and if shrimp entering the Tortugas (fall and spring peaks) and Sanibel fisheries differ isotopically; 2) determine the relative importance of various inshore nursery areas to the Tortugas and Sanibel grounds using isotope tags to establish linkage; and 3) determine the relative importance of pink shrimp source nursery areas for the Fall and Spring peak recruitment periods in the Tortugas fishery, using isotope analysis of young recruits collected in different seasons.

Shrimp acquire carbon isotopes from their diets, which differ according to habitat in south Florida, with ¹³C-enriched values found in shrimp from seagrass meadows, intermediate values found in shrimp from offshore, and ¹³C-depleted values in shrimp from mangrove areas. We have thus far analyzed 60 small offshore shrimp in the 3 to 11 g wet weight range (70-120 mm TL), and find that most shrimp (75 percent) caught in both the Tortugas and Sanibel fisheries exhibit ¹³C characteristic of shrimp that have spent extended periods feeding and growing offshore. This may mean that on average, most shrimp recruit offshore at sizes considerably smaller than 5 g wet weight (80 mm TL), or that shrimp are growing offshore without an inshore life stage. The remaining 25 percent of the shrimp clearly do not have offshore, intermediate ¹³C values, and most (20 of the 25 percent) show an inshore signal consistent with migration from seagrass meadows such as those sampled in Johnson Key Basin. The remaining 5 percent of the offshore-collected shrimp have ¹³C-depleted values similar to those observed in shrimp collected in open bays that are lined with mangroves, such as Coot and Whitewater Bays. Based on these limited initial analyses, we come to the preliminary conclusion that seagrass areas may supply 80 percent of recruiting pink shrimp, and open bays and mangroves may supply the remaining 20 percent. We will be analyzing some 120 samples this summer, beyond the 60 already processed, to see if this preliminary conclusion holds for shrimp caught offshore in the main Spring harvest.

¹Southeast Environmental Research Program, Florida International University, OE Building, Room 148, University Park Campus, Miami FL 33199.

²U.S. Geological Survey, BRD, Florida International University, OE Building, Room 148, University Park Campus, Miami FL 33199.

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