Ecosystem Decline Tied to Whaling

Commercial whaling half a century ago may have triggered the collapse of one of Earth’s richest ocean ecosystems, setting in motion a chain reaction that has harmed sea mammals and kelp forests in the North Pacific and Bering Sea, according to a scientific study published Monday.

Scientists for years have been debating the cause of massive and abrupt ecological changes surrounding Alaska’s Aleutian Islands, where the Pacific meets the Bering Sea.

A team of scientists, led by Alan Springer of the University of Alaska’s Institute of Marine Science in Fairbanks, now theorizes that the ecosystem collapse has its roots in the late 1940s, when Japanese and Russian whalers began using modern hunting techniques. At least half a million bowhead, sperm, humpback and other large whales were harvested in the North Pacific before commercial whaling ended in the 1970s.

Springer, an oceanographer, believes that pods of killer whales used to hunt the giant whales, and when whaling crews reduced their populations, the killer whales were forced to turn to other prey. They moved down Alaska’s aquatic food web, devouring seals, then sea lions and then sea otter populations, which, in turn, led to dramatic shifts in life along the ocean floor.

“If our hypothesis is correct, either wholly or in significant part, commercial whaling in the North Pacific Ocean set off one of the longest and most complex ecological chain reactions ever described,” the team reported in the Proceedings of the National Academy of Sciences.

The Aleutian Islands ecosystem is entirely different today than a few decades ago. Fur and harbor seals began disappearing in the 1970s, followed by Steller sea lions in the 1980s, then sea otters in the 1990s. As the otters disappeared, the sea urchins that they fed on proliferated, wiping out thick kelp forests that used to stretch 20 feet high.

The region had been a stronghold for marine mammals, and the scope and pace of the declines caught marine experts by surprise. Killer whales, the ocean’s top predators, are the only mammals still thriving there.

James Estes, a coauthor with the U.S. Geological Survey in Santa Cruz, has studied the Aleutians’ sea otter population for more than 30 years, and was shocked in the early 1990s as their numbers dropped abruptly and kelp forests disappeared.

Scientists had thought that killer whale predation was responsible for the disappearing otters but that commercial fishing and a warming climactic condition called a regime shift had triggered declines of the other sea mammals. The authors now suggest that while the other factors probably play some role, over-harvesting of whales was the driving force.

Humans, they reported, are the ultimate predators -- their exploitation of ocean life capable of altering entire ecosystems for decades, perhaps centuries.

“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,” Estes said.

Other scientists say the new theory is logical but virtually impossible to prove.

Robert Paine, a University of Washington scientist who specializes in ocean species interactions, called it “a plausible and unifying explanation for the sequential collapse.”

In the report, Springer and Estes called their theory “speculative” because there is no direct evidence that killer whales caused the seal and sea lion declines. But Estes has documented killer whale attacks on otters -- which seemed extremely uncharacteristic for the voracious predators because otters have little caloric value.

The study determined that a shift in diet among fewer than 1% of the region’s estimated 3,900 killer whales would have been enough to drive the declines.

Historical accounts also provide some evidence that killer whales used to eat the big whales, because early whalers referred to them as “whale killers.” A lack of carcasses also supports the predation theory.

The new killer whale diet is not sustainable. The big whales provided sixtyfold more biomass than the combined total for seals, sea lions and otters, the report says.

Steller sea lions, which can weigh more than a ton, are nearly extinct. Their protection under the Endangered Species Act has led to controversial federal restrictions on Alaskan pollock fishing, the largest, most profitable fishery in the world. Some scientists and fishing groups have been dubious that commercial fishing is to blame for the sea lion decline because fish are abundant.

If the new theory is correct, there is little that the Endangered Species Act can do to restore the ecosystem to its previous condition.

“The message,” said Springer, “is that overfishing and massive extraction can lead to food web impacts that are unexpected and unintended.”