Research Document 2022/058
Areas of elevated risk for vessel-related physical and acoustic impacts in Southern Resident Killer Whale (Orcinus orca) critical habitat
By Thornton, S.J., Toews, S., Burnham, R., Konrad, C.M., Stredulinsky, E., Gavrilchuk, K., Thupaki, P., and Vagle, S.
Abstract
Primary threats to the endangered Southern Resident Killer Whale (SRKW) population are reduced prey availability, acoustic and physical disturbance, contaminants, and vessel strike. Successful threat mitigation is contingent on knowledge of spatiotemporal overlap of whale presence with areas of elevated risk.
A co-occurrence framework was used to illustrate areas within the critical habitat where SRKW are at higher risk for physical and acoustic disturbance and strike from vessels from May to October. The SRKW sightings data analyses indicate that the locations with the highest SRKW frequency of occurrence are Swiftsure Bank, Haro Strait, the Fraser River estuary. Individual and group follow data were used to add behavioural context to sightings data and identify Swiftsure Bank and Haro Strait as key foraging locations, while Juan de Fuca Strait is mainly used for travel.
Collision risk was determined using a combination of Automatic Information System (AIS) vessel data to assess presence of large commercial ships, and aerial surveys adding to records of the presence of smaller or recreational vessels.
The acoustic implications of vessel presence were explored by examining the noise additions in frequency ranges used for SRKW communication (500 Hz to 15 kHz) and echolocation (15-100 kHz). A reference level of ‘minimum ambient’ noise was derived from the 1% quietest conditions obtained from acoustic mooring recordings in the study area. The ranges at which calls and clicks could travel in minimum ambient conditions were then calculated. Loss of communication and echolocation ranges were expressed as percent reductions from minimum ambient. Noise from AIS Class A vessels resulted in significant loss of both echolocation and communication range in SRKW critical habitat. Echolocation range loss of greater than 50% was identified in key foraging locations, and range loss increased with foraging depth. Echolocation range loss from small vessels was also observed, with increased impacts occurring on weekends.
These analyses will inform recovery measures to reduce acoustic impacts on SRKW in critical habitat, and provide a framework for future investigations on threats to recovery. Moving forward, inclusion of prey and contaminants data into the co-occurrence analysis will inform our understanding of cumulative effects, and will support management actions for the survival and recovery of the population.
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