Performance metrics for alternative management strategies for gray seal-commercial fishery interactions in the Northwest Atlantic

Abstract

The Potential Biological Removal (PBR) level serves as a threshold for management of marine mammal–fishery interactions under the US Marine Mammal Protection Act. The PBR protocol involves classifying fisheries based on the ratio of recent average bycatch mortality to PBR. A simulation-based framework is developed that quantifies the probability of incorrectly concluding that average bycatch mortality exceeds PBR (i.e., a false positive) or incorrectly concluding that average bycatch mortality is less than PBR (i.e., a false negative), with application to the US stock of gray seals in the northwest Atlantic, a stock for which human-caused mortality levels are approaching PBR. The application to this stock of northwest Atlantic gray seals is complicated by the transboundary nature of the population. Consequently, the analyses are based on a population model that includes the US stock and the component of the Canadian stock off southern Nova Scotia, fitted to available data using Bayesian methods. The total error (i.e., false positive or false negative) probability is found to be largely independent of the coefficient of variation (CV) for estimates of bycatch mortality, whereas this probability is an increasing function of the CV for estimates of abundance. For gray seals, a CV for the estimates of abundance of ∼0.2 appears to balance the reduction in the total error probability versus the cost of surveys with greater sampling effort and more precise estimates of abundance.