Exposure of whales to entanglement risk in Dungeness crab fishing gear in Oregon, USA, reveals distinctive spatio-temporal and climatic patterns

Abstract

Entanglement in fishing gear presents a major threat to marine mammals worldwide and a pressing concern for distinct populations of whales off the US West Coast. The lack of understanding of their fine-scale distribution in relation to fishing activity limits management efforts, specifically in Oregon. Based on year-round predictions of rorqual whale densities and fishing effort compiled from logbooks, we assess co-occurrence between commercial Dungeness crab fishing gear and whales over a decade (2011–2020) as an indicator of exposure to entanglement risk. Generalized Additive Models including temporal, climatic, and ocean upwelling predictors were used to investigate variations in exposure. Exposure peaked in April, at the onset of the upwelling season when whales were predicted to occur in greater numbers and closer to shore. Exposure remained constant until the end of the crab season in nearshore waters <40 fathoms (73 m) and decreased past these depths. Across years, exposure was lower during the marine heatwave (2014–2016) when fishing was more active nearshore and whales were predicted to be less abundant. Exposure was higher before (2011–2013) and after (2017–2020) the heatwave, which correspond to negative phases of the Pacific Decadal Oscillation associated with stronger upwelling, indicating more productive conditions favorable to whales. A recent increase in exposure was also due to a slight shift in fishing effort towards deeper waters. These findings illustrate the use of fine-scale species distribution models to assess space-use conflicts in dynamic marine ecosystems and can be used to guide fisheries management to reduce entanglement risk in Oregon.