Increasing numbers of harbour porpoises are starving to death in the North Sea around Scotland because of climate change, it has been claimed.
Experts say harbour porpoises face more risk of starvation
Evidence suggests warmer waters are affecting supplies of sand eels, which are the preferred prey of porpoises.
Experts say the mammals are not switching diet, so risk starvation.
The study has been conducted by a team of scientists from Aberdeen University and the Scottish Agricultural College (SAC) in Inverness.
The porpoises themselves do not mind warmer waters, but the sand eel is becoming less abundant.
Porpoises are affected by humans in a number of ways including by-catch in fisheries, pollution and noise in the ocean.
The new study, published this week in the Royal Society journal Biology Letters, has found the animal could also be affected by climate change.
Research on seabirds has shown there is a strong link between breeding success, survival and the abundance of sand eels.
However, unlike seabirds that only eat sand eels, it had been assumed that harbour porpoises would simply switch to eating other fish species when sand eel numbers fell, without suffering any ill effects.
By examining the stomach contents of stranded porpoises from the east coast of Scotland collected each spring between 1992 and 2001, researchers found that at this time of year, harbour porpoises rely heavily on sand eels for food.
When they looked at stomachs from animals that died in the springs of 2002 and 2003, they found they contained many fewer sand eels and much less food overall.
To work out whether this was affecting the porpoises' well-being, the information was compared to the results on stranded porpoises conducted by staff from SAC in Inverness.
Scientists say the implications spread beyond the North Sea
The results showed that while only 5% of stranded porpoises had died because of starvation in the late 1990s, 33% of stranded porpoises had starved to death in the springs of 2002 and 2003.
Researchers say it is bad news as if, as predicted, the waters of the North Sea continue to warm, the numbers of sand eels are expected to continue to decline.
Colin MacLeod, the scientist at Aberdeen University who led the research, said: "Harbour porpoises are already affected by humans, and this latest research shows that we have yet another thing to worry about when trying to conserve them.
"The problem is that climate change is not like by-catch 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."
The scientists say the implications of their research spread well beyond harbour porpoises in the North Sea.
Dr MacLeod explained: "Climate change is altering sea temperatures throughout the world and so many species of whales, dolphins and porpoises are being exposed to similar changes in local temperatures.
"However, in many cases we do not know what prey the animals are eating or whether these prey are as sensitive to changes in water temperature as sand eels.
"This makes the already difficult task of conserving the many threatened populations and species of whales, dolphins and porpoises even more so.
He added: "It also makes it very clear that to save the whales, you also need to save the fish, squid and other animals they rely on for food."