Humpback whale (Megaptera novaeangliae) entanglements in commercial fishing gear on the U.S. West Coast: Implications for management and stock structure under the Marine Mammal Protection Act

Abstract

In 2015, humpback whales (Megaptera novaeangliae) on the U.S. West Coast were separated into three Distinct Population Segments (DPSs) under the Endangered Species Act, but have remained one stock under the Marine Mammal Protection Act. The mismatch between stock structure and DPS structure under these two different laws has created several management challenges, particularly in terms of assessing allowable levels of human-caused mortalities. Entanglement in commercial fishing gear presents a significant source of mortality and injury for humpback whales, and entanglement reports on the West Coast have significantly increased since 2014. I address some of the implications of a future change in stock structure to match the DPS structure by assessing 1) which DPSs are most likely to be affected by entanglements, examining differential levels of mortality and serious injury, and 2) examining reductions in mortality needed to achieve management goals. I find that under a revised stock structure consistent with DPS designations, current entanglement rates exceed allowable mortality levels for the endangered Central America population.