Commercial ship versus whale watch boat noise: relative effects on Southern Resident killer whales

Underwater noise may be impacting the population recovery of critically endangered Southern Resident Killer Whales (SRKW). This study used an SRKW-Noise exposure simulation model to compare noise effects from large (AIS-enabled) commercial vessels with whale watch boats during summer (May-September) within their principal Salish Sea habitat range use. It predicted moderate or low behavioural responses (BRs) using SRKW-specific dose-response relationships and, if no BRs were triggered, the extent of residual high frequency echolocation click masking. BRs were considered to result in lost foraging due to switches in behaviour or via strong masking effects. The Monte-Carlo simulation used a fine-scale acoustic model to predict broadband sound pressure levels (SPL, BR analysis) and power spectral density (PSD) at 50 kHz (click masking analysis) for AIS-enabled commercial vessels. To derive equivalent data from whale watch boats, we combined data from Holt et al. (2009), SoundWatch and Beam Reach Sustainable School. SRKW habitat used a 10-year synthesis of effort-correct observer sightings. Overall, noise from AIS-enabled vessels was estimated to contribute 93% of overall BR-related potential lost foraging time, with whale watch boats contributing the remaining 7%, despite mean estimates of 6.1hr of boats with each whale per day. Lower SPLs of slow moving boats had low probabilities of exceeding BR thresholds, while large commercial vessel often exceed these thresholds. Lost foraging time per whale was estimated as a median 3.2hr per day when whales were present (13.4% of day). Echolocation click masking effects accumulated an additional 1.7hr of lost foraging time and was strongly dominated by noise predicted from slow (2.5-8 knot) whale watch boats, noting high model uncertainty due to PSD, speed and proximity assumptions. Overall, lost foraging time totaled 20.3% of each whale day (4.9hr), with ~2/3 due to AIS-enabled commercial vessels, highlighting mitigation measures for both vessel types should be considered