| Summary: | Anthropogenic noise sources range from intermittent to continuous, with seismic and navy sonar technology developing applications with more continuous transmission schemes. Continuous active sonar (CAS) may be used at a lower source level than traditional pulsed active sonar (PAS), but with potential for greater cumulative exposure over time and increased auditory masking concern. We conducted at-sea experiments to contrast the relative effects of navy PAS vs. CAS on sperm whale behaviour using animal-attached sound and movement-recording tags (n=16 individuals) in Norway. Changes in foraging effort and proxies for foraging success and cost during sonar exposures were assessed while accounting for baseline variation (individual effects, time of day, bathymetry, blackfish [killer whale or pilot whale] presence) in generalised additive mixed models. We found no reduction in time spent foraging during exposures to medium-level PAS (MPAS) transmitted at the same source sound pressure level (SPL) as CAS. In contrast, time spent in a non-foraging active behaviour state increased by a factor of 2.4 and 3.3 during CAS and higher source level PAS (HPAS) (Wald test, F=9.9-19.8, p<0.003) at similar received sound exposure levels (SEL; squared pressure integrated over signal duration), providing clear support for received SEL over SPL as the response predictor. The responses were relatively short in duration and less than 8% of the individual-average time budget was affected. On-going analysis of acoustic indicators of prey search (clicking depth, inter-click intervals) and prey capture attempts (buzz click rates, duration) will test for changes in echolocation behaviour that may indicate responses to auditory masking. The importance of exposure context including the potential impact of presence of blackfish is discussed. Our results highlight the benefit of using SEL to account for noise sources with different signal duration and/or duty cycle, and the need to consider masking effects for more continuous sources.
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