When people accuse environmentalists of exaggerating the damage that humans have done to the planet, sometimes its because they simply can't remember what the world once was like. None of us really can; Human memories are short.

But every once in a while some piece of news brings back that former world. The journal Nature this spring published the most comprehensive study ever conducted of the worlds fisheries. Simply put, it concluded that the worlds oceans are wrecked.

In the past 50 years, the populations of every single species of large wild fish have fallen by 90 percent or more. The sharks, tuna, marlins, swordfish, halibut and grouper that have managed to survive are, on average, one-fifth to one-half the size they were 50 years ago. In the deep oceans, where Japanese fleets use fishing lines many kilometers long, they used to catch 10 fish per 100 hooks; now they are lucky to catch one. Fifty years is not very long. Eisenhower was president; we had television; rocknroll was young; people who have not yet started to consider themselves middle-aged were being born.

LONG-TERM DAMAGE

Even then the oceans were somewhat impoverished. The schools of cod that had greeted the first Europeans in the New World -- cod 5 and 6 feet long that you could catch by dipping a basket in the sea -- were already reduced. But the damage had barely begun.

Pretty soon new technology was at work: fish-finding sonar, big factory ships that could wait offshore for months, helicopters for chasing tuna. The equipment was so good that fishermen could keep bringing in sizable catches right until the moment that the populations crashed for good. Once Canadian cod fishermen were able to efficiently locate the nurseries where the fish spawned, for instance, they were able to drag their trawls right through them. On paper everything seemed fine until 1992 when, finally, the nets came up empty.

The Canadian government imposed a moratorium on cod fishing that year, a ban thats still mostly in effect. Hundreds of communities were wiped out.

''Ten years ago we had 118 guys in our bar baseball league,'' one Canadian fishermen told me a few years ago. ``Forty-eight of them don't play anymore. They've moved away.''

But it was a case of bolting the dock door after the fish had fled. Cod populations have been cut by 99 percent, and the ecology of the ocean may have been changed so profoundly that they're never coming back.

Overall, say the authors of the Nature study, we would need to cut total ocean fish catches by 50 percent to give stocks any chance to recover. Instead, fishing pressure may actually be increasing. As big species are wiped out, the fleets go for smaller fish. Pilchard and anchovy catches are way up, in part so that they can be ground into fishmeal and fed to those farmed salmon you find in the supermarket.

FORGOTTEN BOUNTY

''We have forgotten what we used to have,'' Jeremy Jackson of the Scripps Institution of Oceanography told reporters who asked him about the Nature study. ``We had oceans full of heroic fish -- literally sea monsters. People used to harpoon 10-foot-long swordfish in rowboats. Hemingways 'Old Man and the Sea' was for real.''

So were passenger pigeons darkening the sky; so were buffalo herds shaking the plains; so were ancient forests piercing the sky. Now there are only echoes -- and even those we hardly care about. Congress, for instance, still contemplates drilling for oil in the Arctic National Wildlife Refuge, breeding grounds of one of the worlds last big caribou herds. Perhaps its a good thing our memories are so short. Perhaps we couldn't live with ourselves otherwise.

Bill McKibben is the author of 'The End of Nature'.

Copyright 2003 Knight Ridder

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