Oceanic islands as seasonal aggregation sites: restricted residency and wide-ranging movements in short-finned pilot whales

Oceanic islands can serve as predictable, high-value habitat patches in otherwise resource-sparse pelagic environments, potentially favouring different residency strategies for highly mobile, deep-diving cetaceans. By modifying local circulation and seasonal stratification, islands may influence when and where prey becomes accessible across depth layers. However, the processes linking these dynamics to individual movements and population-level aggregation in deep-diving predators remain poorly understood. We employed biotelemetry and movement models that explicitly account for location uncertainty and temporal irregularity to investigate the spatial and temporal dynamics of short-finned pilot whales ( Globicephala macrorhynchus ) in the Eastern North Atlantic. Sixteen biologgers deployed off Madeira Island between 2018 and 2024, transmitting data for periods ranging from 13 to 180 days (mean = 85), revealed movements across an area of 1,170,790 km 2 over a cumulative 1,362 tracking days. Using a multiple-imputation hidden Markov model, we identified two distinct behavioural states: area-restricted search (ARS), which is concentrated around islands, and travelling, which occurs during offshore exploration. We then explored the relation between these states and respective transitions to two dynamic environmental covariates, sea surface temperature and mixed layer depth, with the latter showing the strongest effect on state transition probabilities. Whales were more likely to remain in or switch to ARS under shallow, stratified conditions and to switch to travelling under deeper, mixed conditions; although with inter-individual variation. The autocorrelated kernel density estimation indicated seasonal residency around Madeira (mean 53 consecutive days; max 120), producing a constrained population range (95% UD 1,450 km²; 50% UD 193 km 2 ), centred approximately at 10 km offshore. Integrating telemetry with long-term photo-identification showed that individuals spanning resident-to-visitor fidelity classes congregated within the same seasonal residency range, consistent with condition-dependent movement tactics. These findings show how seasonal ocean structure can shape transitions between movement states in a highly mobile marine predator and are consistent with a broad partial-migration framework in which residency and wider-ranging movement coexist within a population.