The wildlife of the north Pacific has been devastated by a remarkable 50-year chain reaction set off by commercial whaling, scientists claimed yesterday. Numbers of sea lions, sea otters and several species of seals have crashed in recent years, because of the mass slaughter of more than 500,000 whales in the Pacific between 1949 and 1969, the researchers believe.

As the great whales such as sperm, fin and sei whales disappeared under the harpoon, killer whales, which used to prey on them extensively, were forced to find new food sources and switched to the smaller marine mammals, causing their numbers to plunge in turn, the group of American marine biologists say.

Harbor seals declined first, followed by fur seals, then sea lions and most recently sea otters. Their alarming declines had been a mystery.

The scientists, whose theory is in The Journal of the US National Academy of Sciences, contend that a "domino effect" set off by the huge post-war whaling boom, led by Russian and Japanese whalers, is the real root cause.

If true, it is one of the most chilling examples yet of how man's large-scale interference with ecosystems can have unintended and terrible consequences elsewhere.

Along the Alaskan coast and in other parts of the north Pacific and the Bering Sea, populations of Steller's sea lions, seals and sea otters have all fallen "precipitously". Earlier theories suggested that climate change and/or industrial fishing activities were to blame by reducing the animals' fish-food.

But seabirds that prey on the same fish species have not dropped in numbers, the researchers say, and the surviving animals themselves are not undernourished. They suggest that predation by killer whales, or orcas, has driven their numbers down.

Killer whales are known to feed on great whales - their name was originally "whale killers" before being transposed - and they are also known to switch their diet opportunistically to other marine mammals. The sheer amount of great whales killed in the great 20-year Pacific whaling boom after the war must have reduced the killer whales' original food stocks enormously, the researchers say.

Modern industrial whaling took place in the North Pacific only in the late 1940s as first Japanese then Russian whalers moved from their depleted home grounds. Then the slaughter started in earnest.

"In waters within 200 nautical miles of the Aleutian islands and north coastal Gulf of Alaska alone, a minimum of 62,858 whales and an estimated 1.8m tons of whale biomass were taken between 1949 and 1969," the researchers report.

"As a measure of the magnitude of change in whale abundance in this region over this time, only 156 whales were harvested there after 1969." They add: "Altogether, at least half a million great whales were removed from the north Pacific Ocean and north Bering Sea during this period. By the mid-1970s, all great whale stocks in the North Pacific Ocean were severely diminished."

Although some species have recovered, total whales stocks are thought to be only 14 per cent of pre-exploitation levels.

The subsequent declines of the smaller marine mammals are consistent with the expectation that the killer whales would have to switch to other food sources, the researchers say. To test their theory in depth, the researchers compared the nutritional requirements of killer whales, the nutritional value of sea lions and otters, and the number of deaths required to account for observed sea lion and sea otter declines.

They found a shift of less than 1 per cent of the whales' daily calorific intake was enough to wipe out seal lion and otter populations over time.

The killer whales seem to have begun eating smaller coastal marine mammals in the 1970s. They moved from Harbor seals and fur seals, which are easier to catch and full of nutrition, to the more difficult sea lions, and eventually to the smaller sea otters.

In a further part of the domino effect, the disappearance of the sea otters led to a boom in the sea urchins on which they used to feed; and the sea urchins then overgrazed the Alaskan kelp beds along the coast. If our hypothesis is correct, either wholly or in part, commercial whaling in the north Pacific Ocean set off one of the longest and most complex ecological chain reactions ever described," the researchers report.

One of the paper's authors, Dr Jim Estes of the University of California at Santa Cruz, said yesterday: "In principle, we think that when any species is exploited to excess - be it pollock, halibut or whales - it may trigger a broad and devastating 'domino effect', and the ecosystem impacts are significant. What we've seen is that the kelp forest ecosystem in south-western Alaska went from being robust to being gone. It's staggering that it occurred over such a large area in such a short time, just a few years.

"The food web interconnectivity, that urchin explosions could be linked to whaling 50 years ago, is amazing."

© 2003 Independent Digital (UK) Ltd