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El Niņo's Effect on Pinnipeds
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.
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