Chain Reaction Impacts Many Species

Overfishing of great whales lined to population collapse of seals, sea lions and sea otters in North Pacific.

A new theory put forward by Alan Springer of the University of Alaska maintains that overfishing of whales in the North Pacific triggered one of the longest and most complex ecological chain reactions ever described. It began in the open oceans 50 years ago, leading to the decimation of Alaska's populations of harbor seals, fur seals, sea lions and sea otters.

The theory is that the decimation of baleen and sperm whale populations by overfishing removed a major source of food for killer whales. This may have forced some killer whales to look down the food web, preying on other marine mammals which in turn had devastating impacts on marine ecosystems.

"The lightening rod issue in Alaska is the decline of Steller sea lions," says Springer. "$100 million has been spent in the last three years to study Stellers because they are so intimately connected with species of commercial interest. But Stellers aren't the only species of marine mammals in collapse up here."

Harbor seals declined first, followed by fur seals, then sea lions and most recently sea otters. It started with the capture of hundreds of thousands of great whales from the North Pacific Ocean from 1946 to 1979. Springer and his team of researchers argue that this removal of prey forced some killer whales to seek alternative sources of food.

Beginning with harbor seals whose populations collapsed early 70s to early 80s, then fur seals from the mid 70s to mid 80s, followed by sea lions whose populations nose dived from the late 70s to 90s, and finally sea otters, with declining numbers from the 1990s to today, the killer whales targeted populations of small, coastal marine mammals. The researchers surmise that killer whales may have preferred harbor seals and fur seals to sea lions because of the higher nutritional value of harbor seals and because seals are less aggressive and easier to catch.

As their preferred prey became relatively scarce, some killer whales expanded their diet to include the calorically least desirable mammals - the sea otters - with rippling ecosystem effects. By the late 1990s, low numbers of sea otters allowed an explosion of sea urchins and decimation of the kelp forests due to the sea urchins' over grazing.

The researchers say the lesson and warning in all this is that when any species is exploited to excess - be it pollock, halibut or whales - it may trigger a broad and devastating 'domino effect' with significant ecosystem impacts.