The Biscayne Bay Commercial Fisheries for Pink Shrimp (Farfantepenaeus duorarum). 1986-2005, and Relationship with Salinity and Freshwater Flow

Johnson, Darlene; Browder, Joan; Robblee, Michael; Hall, Jeremy; Hazra, Destiny; Brown-Eyo, Pamela

The Biscayne Bay bait (1986-2005) and food (1989-2005) shrimp fisheries were examined based on dealer-reported individual vessel-trip landings data. During the 1980's and early 1990's, landings were consistent, the bait shrimp fishery harvesting more than the food shrimp fishery. The number of trips and landings increased during the late 1990's through 2002 in both fisheries, after which levels of both landings decreased. During this time the food fishery harvests reached and exceeded harvest levels of those from the bait fishery. Statistical models were developed for each fishery to explore the relationship of bay salinity or freshwater flow to shrimp catch-per- unit-effort (cpue), assumed to reflect relative abundance over time of the shrimp available to each fishery. Catch/trip and catch/hour from individual trip landings records were used as the dependent variables (i.e., cpue), and month, salinity or freshwater flow, and moon illumination on the night of the trip were used as the independent variables in initial models. In addition, monthly or cumulative bait shrimp landings were used in the food shrimp models to examine the possible influence of the bait shrimp fishery on the food shrimp fishery. Catch/trip and catch/hour varied relatively little in the bait shrimp fishery compared to the food shrimp fishery. Within each fishery, catch/trip and catch/hour were highly correlated and were similar predictors. Month and salinity were the significant predictors of bait shrimp cpue, while moon illumination was not an important variable. Bait shrimp cpue had a curvilinear relationship with bay salinity. Moon illumination was the most important model variable in the food shrimp model, followed, in order of importance, by salinity, month, and bait shrimp landings. Food shrimp cpue and landings showed a bimodal relationship with salinity. Examination of sizes of shrimp caught confirmed that each fishery exploited a different size group of shrimp. The bait shrimp fishery typically caught shrimp less than 19 mm CL, while the food shrimp fishery harvested shrimp greater than 19 mm CL. An analysis of monthly bait shrimp cpue in relation to shrimp density, as determined from throw-trap sampling over a 3-yr period, indicated a strong statistical relationship with the throw-trap data lagged by 3 months, supporting the use of bait shrimp cpue as an index of abundance. Establishing the validity of bait shrimp cpue as an index of shrimp abundance would extend back in time (to 1986) the time series of relative shrimp abundance for analysis in relation to environmental variables such as salinity that might be affected by changes in water management.