The Effect of Cull Rings on Catch Rates of Blue Crabs Callinectes sapidus in the North Carolina Trap Fishery

Abstract

We sampled seven North Carolina estuaries (Currituck, Core, Albemarle, Bogue, and eastern Pamlico sounds and the Cape Fear and Pamlico rivers) to determine the separate and interacting effects of cull ring size and the number of cull rings per trap on the catch rate (catch per trap) of blue crabs Callinectes sapidus. Since a single minimum cull ring size is required in North Carolina and regional differences in blue crab shape may mediate its effectiveness, we also measured subsamples of male and mature female blue crabs to determine whether there were shape differences among estuaries. The cull ring sizes tested in Currituck and Core sounds were 58.7, 60.3, and 61.9 mm. The smallest two sizes were tested in the other five estuaries. We also varied the number of cull rings from two to four per trap. Thus, there were nine treatments in Currituck and Core sounds and six in the other estuaries. Three blue crab types dominated the catches: legal males (≥127 mm), sublegal males, and mature females. Increasing ring size and number did not significantly affect the catch per unit effort (CPUE) of legal males except in Currituck Sound, where they positively affected CPUE. Increasing ring size negatively affected the CPUE of sublegal males in all estuaries except eastern Pamlico Sound, where if had no effect. Increasing ring number negatively affected the CPUE of sublegal males in two estuaries: Currituck and Albemarle sounds. Increasing ring size negatively affected the CPUE of mature females in two pairwise cases in one estuary (Core Sound); ring size and number did not impact the CPUE of mature females in other estuaries. Despite the apparent differences in the shapes of blue crabs among estuaries, the influences of cull ring size and number were consistent. Our data indicate that a 1.6- or 3.2-mm increase in cull ring size would not negatively impact the catch rates of legal blue crabs but would increase the escapement of sublegal blue crabs.