Most of the information comes from spending hours and hours sifting through seal faeces

Keith Reid, BAS

The study, published in the Proceedings of the Royal Society on Wednesday, says the animals did well in the 1980s but found the going tougher as demand for krill began to draw level with supply in the 1990s.

The research has implications for the management of krill stocks, Mr Reid told BBC News Online.

"Rather than looking at managing krill just on its own, we have to look at how it's linked into the components of the ecosystem," he said.

Scientific advice

"We are one of the national bodies which provide scientific advice to the people who govern fishery and it's up to them to use that information to ensure that any fishing quotas set for krill explicitly include the needs of other predator species - commercial fish species or predators such as sea birds, penguins or seals."

Gentoo penguins switch to fish when there are fewer krill

Some species will find alternatives when they fail to get enough krill.

"Macaroni penguins take other small crustaceans and they seem to maintain their breeding success.

"Gentoo penguins sometimes have a bit more difficulty. They can switch to feeding on fish species but they tend not to be able to maintain the level of breeding success they would have if they were feeding 100% on krill," Mr Reid said.

Long-term study

It is believed that krill stocks are affected by the slow but steady rise in air temperature over the Antarctic Peninsula during the last 50 years but it is very difficult to work out whether the changes are due to natural cycles or human activity.

Krill: At the centre of the Antarctic food web

"You've got increased evidence that the lines of krill supply and demand are converging," Mr Reid said.

"It might be that that in itself is just a long cyclical process. The only way to determine that is to continue monitoring".

But for those who have to work out how best to conserve krill stocks, the cause of the changes may not matter.

"Even if they are cyclical, they are cyclical over such a long time-series that for the purposes of managing resources, they are long-term trends," Mr Reid said.