Trends in Marine Mammal Bycatch in US Fisheries From 1990 to 2017

Bycatch is the greatest threat facing marine mammals globally. In the United States (US), there have been significant efforts to reduce marine mammal bycatch in commercial fisheries, particularly through the implementation of the 1994 amendments to the Marine Mammal Protection Act. However, trends in marine mammal bycatch in US commercial fisheries have not been evaluated in over a decade. We assessed trends in US marine mammal bycatch for 130 stocks of 51 species from 1990 to 2017 according to taxon, region and gear type based on National Marine Fisheries Service Stock Assessment Reports using the bycatch metric ‘mortalities and serious injuries’. We estimated that total annual marine mammal bycatch in US fisheries was 4296 ± 1789 individuals (mean ± SD), with the majority of bycatch occurring in gillnets (83%). The proportion of pinniped bycatch (52%) was slightly higher than that of cetaceans (48%). After decreasing from 1990 to 2002, there was no change in total marine mammal bycatch in all US fisheries from 2002 to 2017, although pinniped bycatch continued to decline throughout the series. Whereas reductions in marine mammal bycatch continued through 2017 in the Atlantic–Gulf of Mexico–Caribbean region, reductions in the Pacific and Alaskan regions had halted or reversed by 2006. Although a more fine-scale evaluation is needed to determine the exact drivers of changes in marine mammal bycatch, our results agree with previous studies in suggesting some success of US bycatch mitigation measures.