HARBOUR porpoises are starving to death in the North Sea as a result of rising water temperatures, scientists have revealed.

Climate change has resulted in a dramatic decline in the numbers of sandeels - a major part of the staple diet of the porpoises.

Marine scientists have recorded a significant rise in the percentage of porpoise deaths due to malnutrition. They are also becoming increasingly concerned about the impact of the declining sandeel populations on other species such as the bottle-nosed dolphin and the minke whale, believing this could jeopardise the future of Scotland's booming whale-watching sector.

The potential crisis was highlighted yesterday in a study by a team of scientists from Aberdeen University and the Scottish Agricultural College in Inverness, published in the Royal Society journal Biology Letters.

Previous reports have already revealed that seabird populations around Scotland's coast have been seriously hit by the decline in sandeel numbers.

But, unlike seabirds that only eat sandeels, it had always been assumed that harbour porpoises and other cetaceans would simply switch to eating other fish species when sandeel numbers fell, without suffering any ill-effects. The study, however, suggests that this is not the case.

Sandeels are anchovy-like fish which spend most of their lives buried in the sand before emerging for a few months in the spring when they become a vital food source.

Separate studies have found the number of sandeels living to adulthood falls during warmer winters, when they grow at too fast a rate to be supported by the available food.

The percentage of stranded harbour porpoises on the North Sea coast of Scotland found to have died as result of malnutrition has risen from 5 per cent to 33 per cent in the past six years.

Colin MacLeod, the scientist at Aberdeen University who led the research, said: "The problem is that climate change is not like bycatch or chemical pollution that can be solved at a local or regional level, it's a global issue that is affecting porpoises at a local level. This was not an effect of climate change we expected for harbour porpoises.

"It makes you wonder how many more hidden impacts of climate change there are for whales and dolphins that we simply did not expect to occur and so haven't taken into account when deciding on suitable conservation strategies."

Mr MacLeod said there were an estimated 350,000 harbour porpoises in the waters around the UK, with 120,000 in the northern North Sea.

Scientists, examining the stomach contents of stranded porpoises from the east coast of Scotland since 1992, have already confirmed that the cetaceans rely heavily on sandeels for food.

But the researchers have now discovered a dramatic change in the cause of death for recent strandings.

Mr MacLeod said that of about 90 animals found stranded on the east coast of Scotland in the late 1990s, only 5 per cent had died of starvation. However, seven of the 21 animals found in the same area between 2002-3 had died from malnutrition.

"These are small numbers but we have shown using statistical analysis that this is a real effect," he said. "There is an increase in the number of harbour porpoises who are dying of starvation but we haven't done the research yet to find out whether that is having a knock-on effect and reducing the overall population. But, certainly, the decline in sandeels cannot be good for population."

He said: "It is a worrying change. Harbour porpoises eat lots of other fish - haddock, whiting and the occasional cod, mackerel or herring. But it seems that, particularly in the spring in the Scottish North Sea area, sandeels are very, very important to them."

Mr MacLeod added: "If, as predicted, the waters of the North Sea continue to warm, the numbers of sandeels are expected to continue to decline."

Peter Ftevick, the science director with the Hebridean Whale and Dolphin Trust, said: "Evidence on the west coast suggests sandeel populations are down. The increase in the number of stranded animals which starved to death is quite dramatic, but how that extrapolates into the population is a little hard to say."