Cailfornia Sea Lion | Northern Fur Seal

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 Niņo. With El Niņo 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 El Niņo 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.

There have been five strong El Niņo (El Niņo) events in the past 25 years while biologists from the National Marine Mammal Laboratory have been studying pinnipeds at San Miguel Island, California. The El Niņo of 1972, 1983, 1992, and 1997 had dramatic impacts on one or more of the populations' vital rates of pup births, pup growth and survival. The 1983 El Niņo was the most powerful El Niņo event yet recorded in California. On San Miguel Island in 1983, numbers of northern fur seal pups born declined 60% from the year before in a population which had been experiencing a 20% annual increase in pup births for the preceding 10 years. Fur seal pup growth declined, and based upon re-sighting of tagged pups, it appeared that no pups from the 1983 cohort survived. Similar impacts were seen on California sea lions on all rookery islands in the Channel Islands where pup production declined 30% - 71% in 1983. For California sea lions it required 6 years and for northern fur seals 8 years until the number of pups born again reached levels equal to those observed in 1982. We interpret this as evidence that adult and juvenile female mortality increased during 1983 and 1984. During the pupping and breeding season from June to September 1997, before oceanographers could agree there was an El Niņo in California waters, northern fur seals and California sea lions at San Miguel Island began to show signs of nutritional stress. Pup mortality increased dramatically for both species, and it is possible that few pups born in 1997 will survive the first year. At the time of this writing in mid-December 1997, it is unclear whether warm conditions associated with this El Niņo will continue to intensify, remain the same, or dissipate before the 1998 fur seal and sea lion breeding season begins. Should it intensify or remain the same, it is probable that impacts on fur seal and sea lion populations in the California Channel Islands will be as great or greater than those seen during the 1983 El Niņo.

CALIFORNIA SEA LION

Image courtesy of NOAA.

Approximately 25 percent of the 23 000 pups born in June, 1997 at San Miguel Island have died because of the strong El Niņo.  In a typical year, out of every three pups, one dies in their first year.   This is referred to as infant mortality.  However, during the 1992 El Niņo, out of every three pup, two died in their first year, thus increasing infant mortality by almost 33 percent. Nevertheless, because of the execellent care-takers and better prediction methods, pup numbers have almost tripled since 1975.

Data courtesy of NOAA.

NORTHERN FUR SEAL

Image courtesy of NOAA.

During the 1983 El Niņo, approximately 75 percent of the 2 000 pups born died in less that a year.  During that incredibly strong El Niņo year, pup birth rate decreased by almost 60 percent.

Data courtesy of NOAA.