Walruses: their stomachs may not tell the whole story.

Diet-based estimates of the health and environmental impact of the heavily hunted Pacific walrus could be skewed. Conservationists peering at stomach contents as a proxy for population health, do not get an accurate picture of walrus diet because these one-tonne, three-metre long mammals digest different parts of their dinner at different rates, a new study suggests1.

Gay Sheffield of the Alaska Department of Fish and Game and her colleagues at the University of Alaska simulated walrus digestion in the laboratory by mixing worms, snails and clams commonly eaten by walruses in a warm vat of dilute hydrochloric acid and digestive enzymes. Six hours later the worms were long gone and about half the clams remained.

Chemical analysis of stomach contents might give researchers a better idea of their provenance, Sheffield's team suggest. In addition, researchers monitoring other marine mammals such as seals and whales often draw on other biological indicators such as teeth and reproductive organ size to gauge population health.

"This study isn't going to change what we know about the walrus diet," says Joel Garlich-Miller, a biologist with the US Fish and Wildlife Service that monitors walruses being hunted throughout Alaska. "But we might want to re-interpret some of our data about diet composition and volume." For example, if scarce food supplies force walruses to eat different types of prey such as fish and seals, stomach contents might not reveal this shift in feeding behaviour.

An estimated 200,000 to 300,000 Pacific walruses live on the ice packs of the Bering and Chukchi Seas between Russia and Alaska. With each one consuming thousands of kilograms of shellfish and worms a year, they are believed to have a "profound effect on the ecology of the Bering Sea," explains Garlich-Miller. The new results hint that walruses could be consuming as little as a quarter of the number of clams previously estimated.

These huge mammals are heavily exploited by Alaskan and Russian native subsistence farmers at an estimated rate of at least 10,000 a year. And population declines have been observed in other Bering Strait species such as sea lions and marine birds. So conservationists are eager to know whether the walrus population is stable or fluctuating.

Unfortunately walrus populations are "notoriously difficult to track," explains Brendan Kelly, one of Sheffield's team. The practice of counting the animals from aeroplanes was discontinued because of government cutbacks and because animals under water and irregularly spread out over a large area were difficult to spot.