The Effects of El Niño Conditions on Marine Mammal Strandings in Central California
Physical and Biological Sciences
Ecology & Evolutionary Biology Department
Marine mammal species thrive in Monterey Bay due to the success of the lower trophic levels and the abundance of available nutrients provided by coastal upwelling. During El Niño events, the thermocline is deepened, the upwelling process is disrupted and primary productivity is reduced. We tested the hypothesis that oceanographic changes and decreased primary productivity associated with El Niño events would result in increased numbers of stranded marine mammals along the central California coast. In stranding records collected from 1975-2013 across 11 El Niño events, the number of total strandings and of pinniped strandings were significantly higher during El Niño events than in normal conditions or La Niña events (p<0.0001). In contrast, the number of cetacean strandings could not be correlated with El Niño events using stranding network data (p=0.880), suggesting that pinnipeds are more likely to strand than cetaceans during El Niño events, possibly due to their nearshore distributions. Independent animals were found to strand far more often than dependent animals (p<0.0001), and, interestingly, rare species strandings were negatively correlated with El Niño conditions (p=0.0052). As marine mammals are sentinel species, these trends provide insight into changes in habitat utilization of marine predators off the Central California coast when prey availability and distribution are modified. This work also highlights the importance of long-term, consistent data collection by local chapters of the National Marine Mammal Stranding Network because such datasets are required to begin to understand complex mechanisms linking oceanographic conditions and population trends in marine predators.
