Changes in the health of northern fur seals and California sea lions appear to be associated with changes in marine mammal prey availability caused by El Niño (EN). With EN comes important changes in oceanographic conditions in California coastal waters. Upwelling, which brings cool, nutrient rich water from the depths into the surface layers, decreases during an EN event, and the mixed layer of the water column becomes much deeper. In response to these changes, marine mammal prey species move northward or deeper in the water column thereby becoming less available to foraging seals and sea lions. Pregnant and lactating females have difficulty finding adequate supplies of food to support healthy pregnancies, and females that are successful in giving birth to pups have difficulty in finding sufficient food to maintain normal milk production. Consequently, pups grow more slowly and more pups die of starvation and disease.

Following the 1983 El Niño a group of biologists met to discuss their observations of the impacts of the EN event on pinnipeds from Peru in the southern hemisphere through the Pribilof Islands in the Bering Sea. Papers on the impacts on pinniped behavior and population biology, the distribution of fish and squid and oceanography were prepared and are contained as chapters in a book entitled "Pinnipeds and El Niño", edited by Fritz Trillmich and Cathrine Ono, published in 1991 by Springer-Verlag.