Multi-scale habitat modelling reveals contractions of harbour porpoise distribution within the northeast Atlantic

In the northeast Atlantic, large-scale changes in marine ecosystems are resulting in an ecological regime shift characterised by the degradation and loss of habitats, alterations to the food web, and ultimately, the redistribution and loss of species. Given the complexity and scale of these environmental changes, there is a critical need to reliably detect and monitor changes in species' distribution. Here, we used a multi-year and multi-season dataset to model harbour porpoise (Phocoena phocoena) summer and winter distribution in the Irish Atlantic, using a multi-scale modelling approach to identify the best spatio-temporal scales of oceanographic variables. Porpoise sightings data were collected during dedicated line-transect aerial surveys from 2015 to 2017 and 2021-2023. Binary generalised additive models were used to assess the relationships between porpoise presence and oceanographic variables at different spatial (5, 20 and 40 km) and temporal (daily, monthly, and average across survey period) scales. Porpoises were present in coastal waters around Ireland across years and seasons. Rising sea surface temperature was negatively associated with porpoise presence, and models predicted a decline in occurrence probability for the summer and winter seasons after 2016, alongside a contraction in distribution to the Irish Sea, which remained a key habitat throughout the study. As their range contracts, porpoises are likely to be exposed to increased human-induced stressors, highlighting the need for targeted conservation measures. This study demonstrates the value of distribution datasets covering multiple years and different seasons for assessing the responses of large vertebrate species to environmental change.