From Presence to Behaviour: Dolphin Acoustic Responses to Noise and Environment in the North Channel

This study investigated how environmental conditions and anthropogenic underwater noise influence acoustically derived indicators of dolphin occurrence, activity, and behaviour in the North Channel. Using passive acoustics monitoring (PAM), acoustic data was collected using a stationary hydrophone deployed between May and October 2024. Dolphin vocalisations were processed using automated detection in PAMGuard and a confidence-based validation framework, with multiple acoustic metrics modelled against sound pressure level (SPL) frequency bands and environmental predictors.

Shipping-attributed noise dominated the soundscape, but low- and especially mid-frequency anthropogenic SPL were more consistently and strongly associated with reduced dolphin acoustic indicators. Environmental predictors, particularly diel phase and seasonal drivers including temperature and productivity, contributed more to overall model fit than noise levels, and acoustic activity exhibited clear temporal patterns across both diel and seasonal scales. Importantly, the ecological interpretation varied across metrics: occurrence-based indicators reflected broad-scale habitat use, whereas behavioural proxies revealed stronger responses to anthropogenic noise.

These findings demonstrate that reliance on occurrence data alone may underestimate the behavioural impacts of noise on dolphins. Integrating multiple acoustic indicators provides a more complete understanding of how dolphins respond to anthropogenic disturbance and can support informed management of noise impacts in coastal environments.