El Niño is a meteorological and oceanographic phenomenon, which occurs at irregular intervals in the eastern tropical Pacific. Its most obvious characteristic is the warming of surface waters, which causes enormous disturbances of the marine environment. A severe Niño may also affect continental systems worldwide. This book gathers in a comprehensive way the information available on the effects of the exceptionally strong 1982-83 Niño on a group of marine mammals, the pinnipeds. It presents an analysis of the effects of environmental stress on the populations of top predators. Data and interpretations are based on a most unusual collection of long-term studies of pinniped population dynamics, behavior and ecology which spanned the El Niño event. The responses of pinniped populations to the El Niño disturbance of the marine ecosystem also has important implications for the management and conservation of marine mammal populations.