Overfishing has subtle and far-reaching side effects
In the 1960s, Peru's anchovy fisheries outproduced such famous grounds as the North Sea, the Mid-Atlantic Bight, or the Bering Sea tenfold. Yet, by the early 1970s, overfishing had sent the Peruvian abundance into sharp decline.
Subsequent research, whose findings have been collated and analyzed by John L. Walsh of the US Brookhaven National Laboratory, now suggests that the effects of that overfishing havebeen both subtle and far reaching. It has not been simply a matter of depleting the stock of anchovies. An ecological system has been upset in a fundamental way so that, at the very base of the food chain, excessive amounts of nutrient now are being lost.
The Peruvian fisheries are the result partly of what oceanographers call upwelling. The pattern of winds and currents along that coast forces surface water away from the shore, to be replaced by water welling up from below. This upwelling water is rich in nutrients that have dissolved from detritus drifting down from the sunlit uppeer zone where marine life thrives. It is a recycling of nutrients that fertilizes the growth of microscopic plants which are the base of the marine food chain.
This upwelling-sustained system produced an abundance of varied life. Fish, especially anchovies, fed on the tiny floating plants and animals that swarmed in those waters. Sea birds, in turn, fed on the fish. They formed large colonies that, among other things, produced the famous guano deposits on offshore islands that have been a rich source of fertilizer for farmers.
Only when the wind-and-current regime that produced the upwelling failed was this prolific system in trouble. Such wind and current shifts, called El Nino, were accompanied by massive fish die-offs.
Then in the late 1950s, North American fishing companies discovered the Peruvian cornucopia and, in cooperation with Peru, began to exploit it aggressively. This led to the overfishing that has seriously depleted the resource.
It also has changed the nutrient recyclying budget. Walsh shows that carbon, a key element in measuring biological production, is being lost in excessive amounts. More of the element is being carried down to deeper waters instead of reentering the food chain of the upper sunlit zone. Some carbon is buried in bottom sediments. Some seems to be carried away by deep currents. Anchoviews have declined. Sardines and deep living fish have increased. But they don't begin to make up for the anchovy loss.
Thus overfishing has changed the ecological system in a fundamental way. More carbon is being lost, less is being recycled to produce fish. Whether or not this change will reverse itself as fishing pressure is reduced remains to be seen. But the point has already been made that careless use of marine fisheries can have d etrimental effects that are not immediately obvious to the fishermen.