Large‐Scale Passive Acoustic Monitoring Data Shows Seasonal and Diel Diversity in Foraging Behaviour of Harbour Porpoises Within Their Distributional Range in the Northeast Atlantic

Latitudinal gradient can influence ecosystem dynamics and species distribution, yet the influence on some aspects, such as intra-species diversity, is less well understood. The large-scale distribution of harbour porpoises (<i>Phocoena phocoena</i>) in the Northeast Atlantic, indicates that ecological adaptations within the species might be greater than currently recognised. This study investigates variation in foraging behaviour using long-term passive acoustic monitoring data collected between 2009 and 2023 from Iceland, Sweden, Ireland, and France. In each area, Generalised Additive Models (GAMs) were used to investigate the influence of large-scale environmental variables (water temperature, salinity, primary production, diel phase and daylength) on foraging behaviour. Our results show variability and complexity in foraging and differences in temporal patterns between areas. In the northernmost regions, with the largest variation in daylight, foraging behaviour was related to diel phase, with primarily nocturnal foraging recorded during the year, but with predominantly diurnal foraging in Iceland during late fall. In the southernmost regions, less effect of diel phase on foraging was found. Harbour porpoises in Sweden and Iceland exhibited increased day-round foraging during calving periods, highlighting the role of reproductive energetics in behavioural adaptations, and the complexity and importance of foraging during different seasons. Our findings underscore the influence of environmental drivers in shaping foraging strategies, supporting the concept that harbour porpoises optimise these based on local conditions and prey availability. Using long-term datasets, spanning broad geographical and temporal scales, this study contributes to the wider ecological understanding of animals with extensive latitudinal distributions, highlighting intra-species variance and the need for region-specific conservation and management.