The very strong 1982--1983 El Ni¿o is implicated in extraordinary northern range extensions and shifts in populations distribution of many species of marine organisms from Oregon to Alaska during 1983. Some species such as Pacific and jack mackerel maintained large populations in the Pacific Northwest during the summer of 1983, as well as the summer of 1984, a year after physical subsidence of the event. Although salmon catches were large Alaskan waters, populations off Oregon were severely reduced during this El Ni¿o period. Mortality of maturing coho was unprecedented; the sizes of both coho and chinook salmon in fisheries were at record lows, and survival of of juvenile coho that entered the ocean in 1983 and 1984 was low. Decreased reproductive success and icreased mortality of seabirds were observed during 1983 from Oregon, Washington, and Alaska and were probably related to decreased availability productivity of usual prey in the abnormally warm surface waters. The large numbers of independent observations on unusual biological events during 1983 and the coincidence of occurrences of some species with previous strong El Ni¿os infer that some changes observed in 1983 were a consequence of the strong 1982--1983 El Ni¿o. ¿ American Geophysical Union 1987