Mortality and Exploitation Rate Estimates for the Recreational Bay Scallop Fishery off the Gulf Coast of Florida, USA

Abstract

Historically, the bay scallop Argopecten irradians supported recreational and commercial fisheries along Florida's Gulf Coast. Declines in density led to a ban on commercial fishing and tighter regulation of recreational fishing. In support of sustainable management, we derived estimates of mortality and exploitation from a multiyear series of preharvest and postharvest surveys of bay scallop density at five locations; catch curves from two locations during 3 years; and an aerial survey of recreational effort. Surveys at locations not subject to harvest yielded natural mortality estimates from which we derived estimates of fishing mortality and exploitation. Based solely on preharvest and postharvest surveys, we estimated the seasonal exploitation rate (u) as 0.0684. Based on all relevant surveys of bay scallop density, we calculated means and 95% confidence intervals for actual seasonal mortality (A = 0.6365 ± 0.06833), daily instantaneous total mortality (Z = 0.0153 ± 0.00293), daily instantaneous natural mortality (M = 0.0145 ± 0.00314), and daily instantaneous fishing mortality (F = 0.0013 ± 0.0055; 0.0000 replaced the negative lower confidence limit in further calculations). We used the means and confidence limits for M and F to calculate a mean and range for u (mean = 0.0575, range = 0.0000–0.2335). Effort on weekends and holidays was significantly greater than effort on weekdays. Data on effort and harvest generated u-values of 0.1244 at Homosassa and 0.1667 at Steinhatchee. Overall, our results confirm that natural mortality is substantially higher than fishing mortality for bay scallops. Simulations indicated that current levels of harvest, current bag limits, and potential variations in catches are unlikely to extirpate bay scallops if initial densities are at least 25 scallops/600 m² and if u stays within the calculated range. Our data did not indicate significant compensatory or depensatory mortality.