Harbor porpoise distribution and habitat use in the Northern California Current over three decades.

In the dynamic marine environment, mobile predators adapt their distribution and habitat use in alignment with regional biogeographic features that confer predictable prey resources. However, long-term changes in environmental conditions may challenge the resilience and persistence of regionally adapted populations. In this study, we investigated how densities of a marine predator, the harbor porpoise, vary relative to habitat characteristics and whether they respond to large-scale fluctuations in environmental conditions. Vessel-based visual surveys were conducted between 1992 and 2022 in the nearshore waters of the Northern California Current. We developed stock-specific density surface models to examine harbor porpoise distribution relative to environmental covariates and predict density over time. This enabled us to identify long-term, stable spatial hotspots, and evaluate inter-annual fluctuations in density relative to ocean basin-scale climate indices and annual upwelling phenology. Harbor porpoises exhibited stock-specific habitat preferences reflecting regional biogeography. Predicted densities were higher in shallow, nearshore waters, with closer proximity to capes and estuaries, and in response to intermittent upwelling. Harbor porpoise density within our nearshore study region was significantly higher during years with longer and stronger upwelling seasons, and lower during El Niño conditions. Taken together, these findings emphasize the importance of regionally guided population assessment and management, and careful consideration of the potential impacts of climate change and anthropogenic pressures on this sensitive nearshore species and the habitats they rely on in the Northern California Current.