Impacts of El Niño on Fish Distribution
from NOAA Fisheries
El Niño events are associated with physical and biological changes in our oceans that affect fish distribution. Among the variations in oceanographic features that are observed following an El Niño/Southern Oscillation (ENSO) event are changes in sea-surface temperatures, changes in the vertical, thermal structure of the ocean (particularly in coastal regions), and altered coastal and upwelling currents. These changes can directly affect the species composition and abundance of fishes. In the northern hemisphere, El Niño events typically result in observations of tropical, warm water species moving north (thereby extending their range). Cold water species move north or into deeper water (thereby restricting their range). Surface-oriented, schooling fish often disperse and move into deeper waters. Fishes that remain in an affected region experience reduced growth, reproduction, and survival.
Observations during the ongoing 1997 El Niño event are generally consistent with what has been recorded during previous events along the coastal regions of California, Oregon, and Washington, and in the Gulf of Alaska and the Bering Sea. Recent reports include the capture near San Francisco of the equatorial game fish mahi mahi, and swordfish, striped marlin, and blue marlin have been observed off California and along the Washington coast. Movement of yellowtail, Pacific bonito, and albacore into northerly waters has also been observed. Their onshore movement has been particularly surprising as these fish are typically found 100 miles offshore. Other members of the tuna family, such as the Pacific and jack mackerel, have also moved north. Whitebait smelt, thresher shark, finescale triggerfish, spotted cuskeel, Pacific saury, common dolphinfish, white seabass, fantail ragfish, halfmoon, ocean sunfish, barracuda, California tonguefish, and California lizardfish all have been reported in areas where they are not typically observed.
A major consequence of an El Niño is the loss of commercially important species where they traditionally occur. A notable example is the movement of the market squid to cooler waters to the north, away from established fisheries in California. This phenomenon is also true for many rockfish species that move from nearshore areas to deeper or more northerly and cooler waters. Pacific whiting likewise shift northward from their spawning and feeding areas off California, Oregon, and Washington to the more temperate latitudes centered off Vancouver Island.
Pacific salmon are also affected by El Niño induced changes to their marine habitat. Increased mortalities and reduced growth have been noted in Pacific salmon populations off Oregon and Washington after previous El Niño events. With respect to the current El Niño, the loss of millions of adult sockeye salmon in the Bering Sea is being attributed to a nearly 10°C increase in summer water temperatures which occurred while the sockeye salmon were migrating back to their natal streams. Changes in the migration patterns of adult sockeye salmon have also been recorded for Fraser River (Canada) stocks. During an El Niño event, sockeye salmon use a more northerly approach, entering the Fraser River from the eastern side of Vancouver Island rather than using the more traditional entry through the more southerly Strait of Juan de Fuca.
The ocean is not a uniform and constant environment. El Niño events provide an excellent opportunity to observe and evaluate how changes in microhabitats influence fish distribution. Identifying changes in fish distribution will increase our understanding of the habitat requirements of important marine resources and will help us better manage our fisheries in the future.