Killer whales in seas off the coast of Alaska are reported to be eating the world's smallest sea mammal, the sea otter, which they previously tended to ignore.

Killer whales can eat up to five a day

Writing in the journal, Science, the researchers say this is because their usual prey, sea lions and harbour seals, are dying out through over-fishing and altered fish migration patterns caused by global warming.

The researchers say the effect on sea otters is devastating: one killer whale can eat nearly 2,000 of them in a year.

The population of sea otters through large areas of western Alaska has dropped by about 25% a year since the phenomenon was first noticed in 1991, according to the report.

Threat to ecosystem

The killer whales' new eating habits are beginning to have a deeper - and potentially more dangerous impact on the environment - as their effects begin taking their toll on other levels of the food chain.

Sea urchins feed on kelp

James Estes, a marine ecologist at the University of California, Santa Cruz, who led the study said: "A whole number of species are affected by what the sea otters do. Things that are rare can impact the ecosystem quite a bit."

Crucially, sea otters control the population of sea urchin, which strips kelp forests that many marine species need to survive.

With the number of sea otters dwindling, sea urchins have begun to flourish, in turn causing kelp forests of the western Alaska coastal ecosystem to disappear.

"I do think this is a huge conservation issue," said Mr Estes.

"The whole concept is to maintain an ecosystem and that is not being done in this one."