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There have been five strong El Niño (EN) events in the past 25 years while biologists from the National Marine Mammal Laboratory have been studying pinnipeds at San Miguel Island, California. The EN of 1987 had no detectable impact on seals and sea lions on San Miguel, but the EN 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 EN was the most powerful EN 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 resighting 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. Although more difficult to demonstrate scientifically, it appeared that juvenile and adult female mortality also increased during that EN. 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 EN 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 EN 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 EN.


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