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Collapsing Populations of Marine Mammals—the North Pacific's Whaling Legacy?

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graph showing population data 
from 1950 to present for the North Pacific Ocean and southern Bering Sea, showing sequential declines in the populations of great whales, harbor seals, sea otters, sea lions, and fur seals
Above: Sequential collapse of marine mammals in the North Pacific Ocean and southern Bering Sea.

Male killer whale
Above: Male killer whale in Prince William Sound, Alaska. Photograph by Janice Waite, National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Alaska Fisheries Science Center, National Marine Mammal Laboratory (NMML).

Sleeping harbor seal
Above: Sleeping harbor seal. Photograph from NMML.

Sea otter
Above: Sea otter. Photograph from the U.S. Fish and Wildlife Service.

Steller sea-lion adult male
Above: Steller sea-lion adult male, yawning. Photograph by Rolf Ream, NMML, 1993, St. Paul Island.

Northern fur-seal pups
Above: Northern fur-seal pups. Photograph by Rolf Ream, NMML.

The rapid removal of at least half a million great whales from the North Pacific Ocean by intensive industrial whaling more than 50 years ago may have unleashed a complex ecological chain reaction that has since rippled resoundingly from ocean to coastal ecosystems, according to a team of eight scientists, including Jim Estes, a U.S. Geological Survey (USGS) research ecologist and adjunct professor at the University of California, Santa Cruz.

The team's paper on this subject, which prompted articles in newspapers around the country, was published in October in the Proceedings of the National Academy of Sciences.

The scientists believe that when great whales became scarce, their foremost natural predators, killer whales, turned to other marine mammals as primary sources of food, causing sequential declines in southwest Alaska during the 1960s and 1970s of first the harbor seals, followed by northern fur seals, Steller sea lions, and finally, in the 1990s, sea otters, as killer whales "fished down" the food web.

"During three decades of research in the Bering Sea and North Pacific Ocean, I watched other scientists struggle to understand the precipitous population declines of northern fur seals, harbor seals, and Steller sea lions, never imagining that my area of research—sea otters and kelp forests—might be affected by these changes," said Estes.

It was the decades of sea-otter research by Estes and colleagues that ultimately shed light on the pinniped declines. In about 1990, the Aleutian sea-otter population Estes studied plummeted, from an estimated 55,000-100,000 individuals in the 1980s to 6,000 individuals in 2000.

"By the late 1990s, sea otters occurred at such a low density throughout the archipelago that sea urchins were overgrazing the kelp forest," said Estes.

The absence of dead otters, as well as realizing that neither malnutrition nor disease could explain the declines—the remaining animals were healthy—led the scientists to consider predation as the cause of the deaths.

Their conclusions in 1998 pointed to an oceanic problem that drove killer whales to switch from other prey to sea otters. They calculated that a killer whale on a steady diet of sea otters could consume as many as 1,825 otters in a year, and that as few as four whales on an exclusive sea-otter diet could have caused the documented sea-otter declines that occurred.

The sea-otter decline led Estes and other scientists in this new study to wonder whether increased killer-whale predation might also explain the precipitous declines of northern fur seals, harbor seals, and Steller sea lions, and so they searched the oceans for the ultimate cause.

When modern industrial whaling arrived in the North Pacific in the late 1940s, several species of great whales had already been depleted in the region 50 to 100 years earlier. The new whaling fleets, from Japan and the Soviet Union, equipped with maritime technology developed during World War II, intensively sought fin whales, sei whales, and sperm whales, species that had not been taken in large numbers until after the war. By the mid-1970s, all of the great whales of the North Pacific were severely depleted.

Past beliefs regarding the abrupt collapses of seal and sea-lion populations in the 1960s and 1970s attributed the declines to limited or changing foods, stemming from climate change and competition with regional fisheries.

Looking back at these events, however, this team of scientists found that their whale hypothesis was consistent with information on the abundance, diet, and foraging behavior of both predators and prey, as well as with feasibility analyses they conducted based on demographic and energetic modeling.

The scientists found that very small changes—as little as 1 percent of the total caloric intake—in killer-whale foraging behavior could account for both sea-otter and sea-lion declines.

The stunning magnitude of the caloric void that would have been left in the food chain by this whaling period has also strengthened the scientists' conviction about the origins of the chain of ecological events.

When the great whales were abundant, their biomass may have been 60 times the combined biomass of all of the seals, sea lions, and sea otters. The great whales would have been able to sustain vastly more killer whales than could populations of pinnipeds and sea otters, Estes said.

The full citation of the recent article is:

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Congressional Briefing on California Sea Otter Research
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What's Wrong with the California Sea Otter?
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Related Web Sites
Sequential megafaunal collapse in the North Pacific Ocean; an ongoing legacy of industrial whaling?
Proceedings of the National Academy of Sciences
Press Release - Collapse Of Seals, Sea Lions and Sea Otters in North Pacific Triggered by Overfishing of Great Whales
SeaWeb - multimedia public education project
Sea Otter Research
U.S. Geological Survey (USGS)

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