How effectively do horizontal and vertical response strategies of long-finned pilot whales reduce sound exposure from naval sonar?

- Behavioural strategies of cetaceans in response to noise are complex and variable.
- We observed pilot whales surfacing in synchrony with intense sonar pulse arrivals.
- Received sound levels during this and other strategies were measured and modelled.
- The simulated responses had different effectiveness in reducing received levels.

The behaviour of a marine mammal near a noise source can modulate the sound exposure it receives. We demonstrate that two long-finned pilot whales both surfaced in synchrony with consecutive arrivals of multiple sonar pulses. We then assess the effect of surfacing and other behavioural response strategies on the received cumulative sound exposure levels and maximum sound pressure levels (SPLs) by modelling realistic spatiotemporal interactions of a pilot whale with an approaching source. Under the propagation conditions of our model, some response strategies observed in the wild were effective in reducing received levels (e.g. movement perpendicular to the source's line of approach), but others were not (e.g. switching from deep to shallow diving; synchronous surfacing after maximum SPLs). Our study exemplifies how simulations of source-whale interactions guided by detailed observational data can improve our understanding about motivations behind behaviour responses observed in the wild (e.g., reducing sound exposure, prey movement).