Noise levels received by endangered killer whales Orcinus orca before and after implementation of vessel regulations

Abstract

Whale watching is often conducted from motorized vessels, which contribute to underwater noise pollution and can disturb marine mammals. Protective measures can ameliorate some effects of disturbance, but it is crucial to empirically assess the effectiveness of such measures, particularly for endangered species. We quantitatively compared noise exposure to endangered southern resident killer whales before and after US federal vessel regulations were established to protect this population from disturbance by vessels and sound. We expected to see a reduction in noise exposure to this population from vessel sound propagation loss due to a doubling of the minimum viewing distance relative to a prior state law. Noise levels were empirically measured from digital acoustic recording tags (DTAGs) suction-cup attached to killer whales in transboundary critical habitat. We collected concurrent vessel data during DTAG deployments to relate to received noise levels at the animal. Results of a linear mixed model analysis that included 10 explanatory variables in candidate models revealed that noise was best predicted by animal ID, vessel count, vessel speed category, and year. Vessel count and speed category were positive predictors of noise levels. Vessel regulations (before vs. after implementation), country, and average vessel distance were not significant predictors of noise levels, although only 1 yr of baseline data limited assessment. These findings inform managers about the effectiveness of current regulations for viewing killer whales and are applicable to other cetacean species that are exposed to vessel noise from whale-watching activities.