Noise impacts on Southern Resident killer whales: a comparison of noise levels before and after U.S. vessel regulations

Disturbance from vessels and noise is one of several threats to the recovery of the Southern Resident killer whale population. In the U.S., vessel regulations were developed to protect these endangered killer whales from vessel and associated noise disturbance, particularly given the extent of whale-watching activities in the Salish Sea. Under the current regulations, most vessels are prohibited from approaching within 200 yd and intercepting the path within 400 yd of any killer whale in the inland waters of Washington State. In this study, we measured noise levels from suction cup-attached acoustic tags (DTAGs) attached to Southern Resident killer whales and compared noise levels before and after vessel regulations went into effect to determine if there was a reduction in noise exposure to this population. During tag deployments, we also collected detailed geo-referenced vessel data that were likely related to expected vessel noise exposure relative to the focal (tagged) whale. Received noise levels (dBrms re 1microPa, 1-40 kHz band) were significantly different across years but, unexpectedly, the highest noise levels occurred in a year after vessel regulations went into effect. Of the vessel factors considered in a linear mixed-model analysis, both vessel count and speed, but not distance, explained differences in noise levels with the highest noise level year having higher average vessel speeds and counts. Changes in whale-watching vessel practices after regulations went into effect may explain these findings. The results, along with those of other related studies, inform the evaluation of the effectiveness of U.S. vessel regulations for viewing killer whales, although significant management challenges remain