Sperm whale depredation on longline surveys and implications for the assessment of Alaska sablefish

Abstract

Sperm whales (Physeter macrocephalus) in the Gulf of Alaska depredate (remove or damage fish caught on fishing gear) on the annual National Marine Fisheries Service’s longline survey. Depredation can reduce sablefish (Anoplopoma fimbria) catch rates and increase uncertainty in survey-derived estimates of sablefish biomass. Using 27 years of longline survey data, this study: 1) evaluates fixed- and mixed-effects generalized linear models to estimate the effects of sperm whale depredation on the sablefish survey abundance; and 2) evaluates the impact of accounting for sperm whale depredation in the sablefish stock assessment. Model evaluation and simulations showed that mixed-effect models were far superior to fixed-effect models in terms of precision and confidence interval coverage. The estimated reduction in sablefish catch rate due to depredation was approximately 15%, which was considerably higher than previous estimates. Correcting for sperm whale depredation in the assessment resulted in a 2% increase in estimated female spawning biomass in the terminal year and a 3% higher quota recommendation, valued at approximately US $3 million. Accounting for sperm whale depredation in the sablefish assessment should be done in concert with estimating the increase in fishing mortality caused by depredation in the commercial fishery.