Ocean Currents Affect Distribution of Animals in the Sea: West Coasts of North and South America

Because of wind and ocean circulation patterns, upwelling regions are usually located along the western coasts of continents, or the eastern boundaries of oceans, where "eastern boundary currents" like the California and Peru Currents flow. In these regions, upward transport of nutrients results in an abundance of phytoplankton and zooplankton. Historically, these areas have supported large fisheries, such as the sardine fisheries off the west coasts of North and South America.

Stocks of these fisheries have varied markedly over time. During the 1930s and early 1940s, Pacific sardines supported the largest fishery in the western hemisphere, leading John Steinbeck to write of the sardine fishery off Monterey Bay in his 1945 novel Cannery Row, "In the morning when the sardine fleet has made its catch, the purse-seiners waddle heavily into the bay. Silver rivers of fish pour into dock-side canneries from the boats, which slowly rise higher and higher in the water until they are empty."

Sardine catches in the U.S. peaked at more than 700,000 tons in 1936, but abruptly declined after World War II. The stock collapsed in the early 1960s and remained low for more than a decade, with a catch in 1975 of only three tons. In 1946, fish processors began canning anchovies as a substitute for the failing sardine fishery. But anchovy canning also declined in the late 1950s, the end of an era for canneries on the central California coast. Then in the late-1970s, sardine catches again began increasing off the west coasts of North and South America. The nearly simultaneous cyclic rise and fall of sardine catches in these regions may be linked to global- and decadal-scale alterations in climate patterns. U.S. GLOBEC researchers are interested in the extent to which the global El Nino climate phenomenon is responsible for changes in the large-scale distribution of marine life. El Nino begins in the Pacific along the equator every two to ten years when atmospheric pressures at the opposite sides of the Pacific Ocean change to create a shift in wind patterns. This shift allows a large pool of warm water that usually remains in the western Pacific to extend eastward as far as the South American coast. A wave of warm water is eventually created that travels eastward along the equator toward Central and South America. Surface water temperatures may rise by as much as six degrees Centigrade (ten degrees Fahrenheit). The continental shelf deflects this wave to the north and south, bringing warmer waters to coastal North and South America.

During strong El Nino years, tropical and temperate fish species like sardines and anchovies shift northward beyond their normal ranges and, in some cases, changes occur in their growth, survival, and production. In 1972, the anchovy fishery off the west coast of South America was severely impacted by an El Nino event. Anchovy stocks there were already weakened due to overfishing; the onset of an El Nino wiped out the remainder of the fishery, which has still not recovered twenty years later.