Population Abundance, Pup Mortality, and Copulation Frequency in the California Sea Lion in Relation to the 1983 El Niño on San Nicolas Island

Large-scale oceanographic events such as EN can be expected to affect pinnipeds in various ways, one of which is the number that return to their island breeding areas during a year of environmental perturbation. Altered food supplies may lead to changes in the distribution of animals throughout their range, in the amount of time spent at sea on feeding forays from the breeding areas, in the fecundity of females and of males, and in the mortality of animals at sea and on land. Each of these variations can lead to changes in the population size and composition observed on the rookeries. By examining in detail the population of one island (San Nicolas Island) we will attempt to clarify not only the net changes in sea lion numbers that occurred during EN, but by what mechanisms (mortality, migration, copulation frequency, etc.) these changes are brought about.