Estimates and potential drivers of protected species bycatch in the California set gillnet fishery

Abstract

The identification of efficient management strategies that reduce protected species bycatch while also minimizing impacts on fishing livelihoods is a global conservation challenge. Identifying such strategies requires understanding levels of bycatch relative to management targets as well as the relationship between bycatch risk and potential management actions. In this study, we use ratio estimation to reconstruct bycatch of select marine mammal and seabird species in the California ≥ 3.5” set gillnet fishery from 1981 to 2022 and random forest models to identify potential drivers and hotspots of bycatch risk. We find that bycatch has dropped precipitously since the 1980s as a result of management-induced reductions in fishing effort, but at significant costs to fisheries participation and revenues. Recent marine mammal bycatch ranges from 0.1 % to 4.0 % of the potential biological removal and marine mammal populations are recovering. Spatial-temporal correlates of bycatch risk were more important than fishing-related correlates of risk, suggesting that spatial-temporal closures would reduce bycatch more reliably than mesh size or soak time restrictions. For each species, we identified 1–3 hotspots of bycatch risk as candidates for temporary seasonal closures. Bycatch risk for harbor seal (Phoca vitulina) and California sea lion (Zalophus californianus), the species with the greatest bycatch risk, is especially high from April 1st to June 15th, suggesting that hotspot closures during this 2.5-month time period could be particularly efficient. Our study also highlights the value of competing multiple sample balancing approaches to identify methods that best predict rare bycatch events.