Multi-Scale Temporal Patterns of Small Cetacean Behaviour in the Bay of Biscay

Marine mammals are increasingly exposed to anthropogenic pressures, yet knowledge of their behavioural ecology remains limited, hindering effective conservation planning. Two years of continuous passive acoustic data were collected with porpoise detectors (C-POD and F-POD) in the northern Bay of Biscay (northeastern Atlantic Ocean), as part of the baseline assessment for an offshore wind farm development in the South Brittany area. Distinct temporal patterns in small cetacean presence and behaviour were observed. Dolphins exhibited dominant year-round presence with pronounced peaks in August-September and feeding concentrated during daylight hours. Harbour porpoises (Phocoena phocoena) showed consistently low activity with autumn-spring presence and variable diel feeding patterns. Porpoise feeding intensity peaked in November-December, with a diel pattern emerging exclusively at nearshore stations, where buzzes were more frequent during daytime than at night. Environmental modelling identified salinity and current speed as key drivers of dolphin activity, though explained variance remained low, highlighting the complexity of acoustic detection patterns that integrate both occurrence and behaviour. For porpoises, zooplankton biomass was the primary negative driver of activity, while bottom temperature and current speed had positive effects; however, explained variance was even lower than for dolphins, suggesting that factors not captured by the available environmental variables largely govern their distribution. Correlation between C-POD and F-POD diel profiles was high for dolphins across all seasons, while it was lower for porpoises. These findings provide a behavioural baseline for this region and directly inform Environmental Impact Assessment procedures by identifying periods of heightened cetacean activity and feeding intensity.