"This is the first time that we have understood the full scale of this decline," said Dr Angus Atkinson, a marine biologist at the British Antarctic Survey (BAS).

Krill feed on algae under the ice sheet in the ocean but warmer temperatures over the last 50 years have meant there is less ice and fewer krill.

Air temperature at the Antarctic Peninsula, a key breeding ground for krill, has risen by over 2.5 degrees Centigrade in the last half century.

"The most important finding was that there was such a direct link between sea ice duration and extent and krill abundance," Atkinson said in an interview.

"There is only roughly a fifth of the krill around now that were around in the mid-70s. That is a substantial decline," he added.

Krill, which measure about 6 cm in length and swim in swarms, are important in the food chain. They feed on phytoplankton and algae and in turn are eaten by fish, squid, sea birds, whales, some seals and penguins.

"We're already seeing some effects in certain penguin species at several sites in this area where krill are declining so much," Atkinson added.

Scientists had earlier suspected that stocks of krill were dropping but the estimates were based on local surveys. The latest figures, reported in the science journal Nature, are derived from data covering 40 Antarctic summers between 1926 to 2003 that was gathered by nine countries working in Antarctica.

Atkinson and his colleagues also noted that as stocks of krill have declined, the numbers of salps, jelly-like creatures, have risen. But the species that eat salps are different from those that eat krill.

"We need to understand the mechanisms of these ecosystem interactions to be able to predict what is going to happen in the future. The key thing is the climatic change at the Antarctic Peninsula. It is this particular area that is warming up," he added.