Severity Scoring of Sperm Whale Behavioral Responses to an Operational Sonar Source Reveals Importance of Received Level and Source-Receiver Distance

Naval sonar’s impact on marine life is a long-standing concern. Our study conducted off Norway aimed to investigate the effect of both the received level (RL) and the source-receiver distance on whales’ responses to sonar. We cooperated with a naval frigate and used an experimental sonar (SOCRATES) to conduct controlled sonar exposure experiments at different source levels and at various distances from sperm whales (Physeter macrocephalus) tagged with acoustic-and-movement-recorder devices (Mixed-DTAGs). Behavioral responses were scored using a severity scale from 0 to 9. We showed that the probability and severity of scored responses increased significantly with higher RL (scored as SELmax, the maximum sound exposure level received during an exposure session) and lower distance (scored as DISTmin, the minimum source-receiver distance during an exposure session). Various behavioral responses occurred over a broad range of severities (1 to 7), including changes in vocal and dive behaviors, avoidance responses, cessation of feeding or resting activities, orientation responses, and changes in locomotion. Close proximity (< 1.5 km) of the source vessel without sonar transmission influenced whale behavior, although it was to a significantly lesser extent than with sonar. Dose-response relationships, including onset response RL and source-receiver distance, revealed that whale responsiveness declined more sharply with source-receiver distance than expected based solely on RL. The model predicted 14 km as a statistically significant distance threshold beyond which no responses would be expected independent of RL. At 225 dB re 1 µPa m source level, the model predicted that the estimated effective impacted area dropped from 1,770 km2 when only RL was considered to 27 km2 (factor 65) when both RL and source-receiver distance were considered. These findings underline the importance of taking into account both RL and source-receiver distance in assessing the impact of naval sonar on whale behavior.