Implications of wind and vessel noise on the sound fields experienced by southern resident killer whales Orcinus orca in the Salish Sea

Abstract

The soundscape of critical habitat for southern resident killer whale (SRKW) Orcinus orca in the Salish Sea, the waters around southern British Columbia, Canada, and northern Washington State, USA, is shaped by wind and wave noise as well as heavy commercial and recreational vessel traffic loads. First, we used recordings from 6 passive acoustic moorings to characterize the acoustic landscape experienced by SRKW in this region, focusing on the frequencies used for communication and echolocation. Mid-frequency wind noise was prevalent in winter sound fields, whereas higher-frequency noise levels associated with increased numbers of recreational vessels increased during summer. Commercial vessel presence was consistent, with acoustic inputs prevalent in the western part of the study area. The potential implications of these additions on SRKW acoustics use were then explored for the frequency band 1-40 kHz to represent communication calls and at 50 kHz to consider echolocation. The inputs of wind were extrapolated from modelled hourly wind speed measures and commercial shipping noise. The noise impact was expressed as a proportional reduction of communication and echolocation extent compared to maximum acoustic ranges at ‘minimum ambient’ levels, void of vessel and abiotic noise. The reductions calculated were substantial, with the presence and impact of vessel noise greater than wind-derived additions and the greatest impacts around shipping lanes. Impacts were found for SRKW foraging areas, with implications for group cohesion and feeding success. This interpretation of the influence of natural and vessel noise more clearly demonstrates the potential implications of altered soundscapes for SRKW.