Estimation of Trap Selectivity for Male Snow Crab (Chionoecetes opilio) Using the SELECT Modeling Approach with Unequal Sampling Effort

An experiment was conducted to study trap selectivity for male snow crab (Chionoecetes opilio) in St. Mary's Bay, Newfoundland. The finishing effort of large-mesh commercial traps and small-mesh control traps was not equal because two commercial traps were fished for every control trap. We show how the SELECT (Share Each Length's Catch Total) method extends to this situation and, more generally, to selectivity trials in which catches are sampled in unequal proportion. Application to the crab data showed that a good fit was provided by the logistic selection curve under the assumption of unequal fishing efficiencies of commercial and control traps. The carapace widths at the retention probabilities of 0.25, 0.50, and 0.75 were estimated to be 86.4, 94.6, and 102.9 mm, respectively, and the retention probability at the minimum legal size of 95 mm was estimated to be 0.51. The commercial traps had almost twice the fishing efficiency of the control traps and consequently caught more legal-sized crabs per trap.