Gray whale migration patterns through the Southern California Bight from multi-year visual and acoustic monitoring

Sightings and acoustic recordings from eastern North Pacific gray whales in the Southern California Bight were analyzed for interannual changes and compared with concurrent environmental measurements during 7 migration seasons (2008-2009 to 2014-2015). Acoustic call counts recorded on an offshore hydrophone were highly variable from year to year. Assuming an average calling rate of 7.5 calls whale(-1) d(-1), the estimated number of whales migrating by this hydrophone would be <10% of the population within 20 km of the offshore hydrophone in most years. In contrast, the estimated number of gray whales migrating off Santa Barbara and Los Angeles based on visual surveys grew at a greater rate (11% yr(-1) and 26% yr(-1), respectively) than the population size growth rate (5 % yr(-1)). Over the studied migration seasons it seems an increasing proportion of the population was using the nearshore migration corridor in the Southern California Bight, especially near Los Angeles. This trend could increase the negative anthropogenic impact on this species. Although several large-scale climatic events occurred between 2008 and 2015, neither water temperature in the Southern California Bight nor sea ice timing in the gray whale Arctic feeding area improved generalized additive models of gray whale nearshore sightings or offshore acoustic presence. Over these times, the gray whale migration timing appears to be driven more by their biological clock and instinct than by the extrinsic factors accounted for in the present analysis. Future work should test if other factors influence the gray whale migration over longer timescales.