Productivity

Upwelling occurs both along coastlines and in midocean when a collision of deep ocean currents brings cold water rich in nutrients to the surface. These upwellings support blooms of pytoplankton, which in turn produce zooplankton and support many of the world's main fisheries. If the upwelling fails then fisheries in the area fail.

In the 1960s the Peruvian anchoveta fishery was the world's largest fishery. The anchoveta population was greatly reduced шт 1972, when warm water drifted over the cold Humboldt Current, as part of a 50 year cycle, lowering the depth of the thermocline. The upwelling stopped and phytoplankton production plummeted, as did the anchoveta population, and millions of seabirds, dependant on the anchoveta, died. Since the mid 1980s, the upwelling has resumed, and the Peruvian anchoveta catch levels have returned to the 1960s levels.

Off Japan, the collision of the Oyashio Current with the Kuroshio Current produces nutrient-rich upwellings. Cyclic changes in these currents resulted in a decline in the sardine sardinops melanosticta populations. Fisheries catches fell from 5 million tonnes in 1988 to 280 thousand tonnes in 1998. As a further consequence, Pacific bluefin tuna stopped moving into the region to feed.

Ocean currents can shape how fish are distributed, both concentrating and dispersing them. Adjacent ocean currents can define distinct, if shifting, boundaries. These boundaries can even be visible, but usually their presence is marked by rapid changes in salinity, temperature and turbitity.

For example, in the Asian northern Pacific, albacore are confined between two current systems. The northern boundary is determined by the cold North Pacific Current and the southern boundary is determined by the North Equatorial Current. To complicate things, their distribution is further modified within the area defined by the two current systems by another current, the Kuroshio Current, whose flows fluctuate seasonally.

Epipelagic fish often spawn in an area where the eggs and larvae drift downstream into suitable feeding areas, and eventually drift into adult feeding areas.

Islands and banks can interact with currents and upwellings in a manner that results in areas of high ocean productivity. Large eddies can form downcurrent or downwind from islands, concentrating plankton. Banks and reefs can intercept deep currents that upwell.

Are the following statesment true or false:

Ocean currents brings water rich in nutrients to the surface.

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