Terry R. Spraker1, DVM, PhD; Francis Gulland2, DVM, PhD; Robert DeLong3, PhD
El Niño is a disruption of the ocean and atmospheric systems that affects weather worldwide. In non-El Niño years trade winds in the Pacific blow towards the west. These winds pile up warm surface water (about 0.5 m) in the western Pacific. The waters in the western Pacific are up to 8°C higher than the eastern Pacific along the South American coast. The upwelling of cold water off the South American coast is associated with nutrient-rich water, supporting high levels of productivity at all levels of the food chain. During El Niño, the trade winds relax in the central and western Pacific leading to a depression of the thermocline in the eastern Pacific and an elevation of the thermocline in the west. During the El Niño of 1982–1983 the thermocline (level where water is below 17°C) dropped from 50-150 m. This reduces the efficiency of upwelling to the surface and cuts off the supply of nutrient rich water to the euphotic zone. The results are a rise in sea surface temperature and a decline in primary productivity, the latter of which adversely affects higher trophic levels of the food chain.
The impact of an El Niño on marine mammals depends on its intensity and, therefore, can be minimal to severe. We have had an opportunity to necropsy California sea lion pups from San Miguel Island during two different El Niños, the first in the summers of 1991 and 1992 and again in January of 1998. Many of the young pups that were found dead and examined in the summers of 1991 and 1992 were emaciated and had severe hookworm and lice infections. Most of the pups found dead in January of 1998 were emaciated and had minimal hookworm infections. El Niño of 1998 had a tremendous impact on rehabilitation centers along the California coast. For example, approximately 500 animals are brought into the Marine Mammal Center, Sausalito, California in an “average year;” however, during the El Niño years the number of animals brought into the Center doubles. Also, the number of specific species varies. For example, in an “average year” the Center receives approximately four to five northern fur seals and this year alone the Center has received 45 fur seals. Conditions found in these animals brought to the rehabilitation center included emaciation, increased parasitism, lungworm pneumonias, penetrating gastric ulcers with localized peritonitis, and gun shot.