Should I stay or should I go? Modelling year-round habitat suitability and drivers of residency for fin whales in the California Current

Aim: Understanding the spatial ecology of endangered species is crucial to predicting habitat use at scales relevant to conservation and management. Here, we aim to model the influence of biophysical conditions on habitat suitability for endangered fin whales Balaenoptera physalus, with a view to informing management in a heavily impacted ocean region. Location: We satellite-tracked the movements of 67 fin whales through the California Current System (CCS), a dynamic eastern boundary upwelling ecosystem in the Northeast Pacific. Methods: We use a multi-scale modelling framework to elucidate biophysical influences on habitat suitability for fin whales in the CCS. Using generalized additive mixed models, we quantify the influence of a suite of remotely sensed variables on broad-scale patterns of occupancy and present the first year-round, high-resolution predictions of seasonal habitat suitability. Further, we model the influence of contemporaneous biophysical conditions on individual-level residence times in high-use habitat. Results: We present evidence of year-round habitat suitability in the southern California Current System, robust to interannual variability, establishing that North Pacific fin whales do not follow the canonical baleen whale migration model. Within the high-use habitat in the Southern California Bight (SCB), individual-level residency in localized areas (n = 16 for > 30 days; n = 4 for > 6 months) was associated with warm, shallow, nearshore waters (> 18 degrees C, < 500 m), with cool waters (14-15 degrees C) occurring over complex seafloor topographies and with convergent (sub) mesoscale structures at the surface. Main Conclusions: Biophysical conditions in the southern CCS generate productive foraging habitats that can support the fin whale population year-round and allow for extended periods of residency in localized areas. High-use habitats for fin whales are colocated with areas of intense human use, including international shipping routes and a major naval training range. Seasonal habitat suitability maps presented here could inform the management of anthropogenic threats to endangered baleen whales in this globally significant biodiversity hotspot.