Effect of El Niño on the Southern California Bight

El Niño is a warmwater phenomenon in the Pacific Ocean that can have a great effect on the distribution and production of marine mammals and fishes. In the Southern California Bight ecosystem, reduced availability of prey species such as chub mackerel resulted in reduced reproductive success of California sea lions. Along the west coast of North America, the particularly strong 1982-1983 El Niño was associated with unusually warm sea surface temperatures (+4°C off California), high coastal sea levels, and strong northward currents along the California and Oregon coasts in the summer of 1983 (Bailey and Incze 1985; Pearcy and Schoener 1987). These changes altered the distribution of chub mackerel, including the areas where they spawned and the distribution and survival of their eggs and larvae. A 1983-1984 decrease of chub mackerel in the diet of California sea lions in the California Channel Islands was likely caused by the northward displacement of the fish out of the bight in response to ocean warming (Brodeur and Pearcy 1986). Chub mackerel were uncharacteristically abundant off the Washington and Oregon coasts during that period; it was the second most common species collected in purse seines off the Oregon coast in 1983-1984, whereas in most other years it did not rank in the top ten species collected (Pearcy and Schoener 1987).

The 1982-1983 El Niño resulted in decreased prey availability for female sea lions (Fig. 1) in the Southern California Bight. This reduced food availability meant that females had to spend more time and energy foraging for food, resulting in lowered reproductive success and pup growth. Thus, there were large declines in both pup numbers and weights at Channel Island rookeries in 1983 and 1984. Mortality of adults and juveniles was also higher than normal. Although the California sea lion population had increased about 5% annually between 1971 and 1981, their numbers declined 30%-70% at all rookeries in the Channel Islands in 1983 (DeLong et al. 1991). However, births declined the least (a drop of 30%) at San Miguel (Fig. 2), the most northern island, where sea surface temperatures were not as warm.