Throughout the Gulf of Alaska and probably the Bering Sea, too, the balance of prey and predator has been upended, a transformation so extreme that it's being called a "regime shift." Waters once brimming with seals, otters and king crab are now dominated by sharks, pollock and urchins. Virtually no creature remains untouched.

Piece by piece, over the last three years, scientists have started to solve the puzzle. Clues point toward something - almost imperceptible - that happened in the ocean in 1977. But the answers are more disturbing than satisfying, more elusive than conclusive. It seems the ocean's chain of life is actually a fragile, silken web. If just one strand is removed, the whole thing unravels. And it may never be whole again.

In the 1980s, as many as 100,000 otters inhabited the islands. Today, only about 6,000 remain, according to aerial surveys. Between 1992 and 2000, the population dropped by 70 percent, a rate of decline that researchers say is unprecedented for any mammal population in the world.

In 1995, when they began to notice the signs of a population decline, marine biologist Tim Tinker and Estes, who specialize in otter behavior and population biology, at first looked for signs of disease, famine or reproductive troubles. They found none.

For a couple of years, as the decline steepened, they were baffled. If thousands of otters had died, where were the bodies?

Then it dawned on Tinker: Perhaps the animals were being eaten.

By killer whales.

Otters are so small and killer whales so voracious that four whales could have eaten 40,000 otters in five years.