Abstract
ABSTRACTAnthropogenic noise sources range from intermittent to continuous, with seismic and navy sonar technology moving towards near-continuous transmissions. Continuous active sonar (CAS) may be used at a lower amplitude than traditional pulsed active sonar (PAS), but potentially with greater cumulative sound energy. We conducted at-sea experiments to contrast the effects of navy PAS versus 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 and control exposures were assessed while accounting for baseline variation [individual effects, time of day, bathymetry and blackfish (pilot/killer whale) presence] in generalized additive mixed models (GAMMs). We found no reduction in time spent foraging during exposures to medium-level PAS (MPAS) transmitted at the same peak amplitude as CAS. In contrast, we found similar reductions in foraging during CAS (d.f.=1, F=8.0, P=0.005) and higher amplitude PAS (d.f.=1, F=20.8, P<0.001) when received at similar energy levels integrated over signal duration. These results provide clear support for sound energy over amplitude as the response driver. We discuss the importance of exposure context and the need to measure cumulative sound energy to account for intermittent versus more continuous sources in noise impact assessments.
Highlights
Our experiments were designed to contrast the effect of pulsed versus continuous active sonar (PAS versus Continuous active sonar (CAS)) on sperm whale behaviour, and compare peak signal amplitude versus energy (SPL versus SEL) as well as duty cycle as response predictors
No effect of medium-level PAS (MPAS) was found, even though it was transmitted at the same source level amplitude as CAS
The higher SEL led to stronger responses for sonar received at the same sound pressure level (SPL), while we found no evidence for different responses to CAS versus pulsed active sonar (PAS) when received at similar SELs
Summary
Anthropogenic sources vary in amplitude, spectral and temporal patterns, ranging from intermittent ( pulsed) to more continuous sounds (Hildebrand, 2009; Shannon et al, 2016) Understanding how these different types of sound exposures influence physiology and behaviour is crucial for assessing and mitigating noise pollution impacts. More continuous introduction of energy into the marine environment may allow the use of lower source levels (in terms of peak amplitude), with similar or even greater cumulated exposure levels This could lead to more severe environmental impact, especially by increasing the risk of auditory masking because more continuous sound transmissions provide fewer silent periods and opportunities for auditory recovery
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