Association of foraging Steller sea lions with persistent prey hot spots in southeast Alaska

Understanding how air-breathing marine vertebrates find and utilize prey provides insight into their foraging mechanisms and ultimately their population productivity and trends. Utilization depends on their ability to locate areas where productive foraging conditions exist. We quantified the abundance of forage fish in southeast Alaska during acoustic surveys between October and April to improve our understanding of Steller sea lion Eumetopias jubatus foraging behavior. Energy densities (millions kJ km(-2)) of forage fish were orders of magnitude greater between November and February due to the presence of large schools of Pacific herring Clupea pallasi. Herring schools were highly aggregated, although the location of these aggregations shifted southward from November to April. Thus, a productive foraging area in one month did not necessarily equate to a productive area in the next month. However, by surveying on successive days and weeks, we found that herring aggregations persisted at shorter time scales. When the study area was partitioned into 1 x 1 km blocks, the day-to-day abundance of prey within a block was highly correlated with prey abundance the following day (correlation coefficient, r = 0.75, p < 0.001) and with prey abundance for the following week (r = 0.55, p < 0.001). More importantly, the persistence of these prey hot spots was an important characteristic in determining whether foraging sea lions utilized them. The odds of observing a foraging sea lion were about 1 in 3 for locations where prey hot spots were persistent. The persistence of these hot spots allowed predators to predict their locations and concentrate search efforts accordingly.