Movements and residency of fin whales ( Balaenoptera physalus ) in the California Current System

Fin whales ( Balaenoptera physalus ) along the western United States are managed as a single stock whose range overlaps with the California Current System (CCS). We used sighting histories of 932 individual fin whales photographed in the CCS from 1987 to 2018 to investigate movements and residency patterns within and among latitudinal regions. While 167 whales (18%) were sighted in multiple years, only 4 were documented in both the Northern and Southern CCS, with a boundary at 38.5 °N. A permutation test of annual recaptures suggested movements among latitudinal regions of the CCS occurred significantly less than expected if whales moved freely within current stock boundaries. Fifteen whales were sighted in 6–10 different years on an average of 30 different days (range: 8–101 days), all in the heavily sampled Southern California Bight (SCB). There, we used lagged identification rates (LIRs) to assess whether the probability of re-sighting an individual over time differed from random values for the region overall, within and beyond 25 km of the mainland, and by season. Our results suggest that the SCB is used seasonally by whales from the larger CCS stock but is also home to a smaller, year-round resident subpopulation. This latter group increasingly uses the nearshore waters of the SCB, where they are exposed to significantly elevated levels of anthropogenic activity.