By Jonathan Amos
BBC News Online science staff
Scientists believe they are closer to understanding why the populations of Steller sea lions and other mammals have collapsed in the north Pacific.
The sea lions used to number hundreds of thousands
The sea lions have seen their numbers fall by more than 80% since the 1970s.
The North Pacific Universities Marine Mammal Consortium says the animals appear to have switched to eating fish with a very much lower fat content.
This means the sea lions cannot build up sufficient blubber to survive and breed in their cold environment.
"In 1977, there was knife-edge change - it cut right through the ecosystem," the consortium's director Dr Andrew Trites told the BBC.
"It not only affected the seals and sea lions, it also affected the shrimps, the crabs, the salmon - a whole suite of species flipped overnight in their abundances."
Steller sea lions are found from Japan through Russia, Alaska, British Columbia and down to California.
Their core population, which is around the Gulf of Alaska and the Aleutian Islands, has declined to about 30,000 individuals.
Quite why sea lions should switch from eating nutritionally superior fish, such as herring, to "junk food" of mainly pollock and flatfish is a mystery.
Scientists have been using animals at the Vancouver Aquarium to test this "nutritional stress hypothesis".
Some of the sea lions have been fed different diets to get an insight into their food needs, growth and metabolism. Some animals are even being trained for open ocean trials to understand how wild sea lions expend energy when they forage for fish.
"What we will be doing is measuring the amount of oxygen they breathe in before a dive; getting them to go down to certain depths for particular durations and then measuring the amount of oxygen they breathe afterwards," said Dr Gordon Hastie, from the University of British Columbia.
"From this we will get an idea of how much it would cost sea lions in the wild to dive to depths to catch fish."
A number of factors may have caused the switch in diet. Overexploitation could have impacted fish stocks; global warming may have played a role, too. But entirely natural phenomena that affect the ocean climate could also be responsible for changing the quantities of available fish.
"So many changes that have occurred in the world to the abundances of animals are related to human activities," said Dr Trites.
"This is a case where humans may not be involved - but we need absolute proof.
"We need to approach this in many different ways. One is to work with captive animals, another is to do our studies and observations in the wild. We can also work in the labs using models to reconstruct the ecosystems."