As the seas around the UK warm due to global climate change, new evidence suggests that increasing numbers of Europe’s smallest cetacean - the harbour porpoise - are starving to death in the North Sea around Scotland.
While the porpoises themselves don't mind warmer waters, their preferred prey, the sandeel, doesn't and is becoming less abundant as a result. Researchers say this is bad news for porpoises, because fewer sandeels means less to eat and a higher risk of starvation.
Harbour porpoises are the most common species of cetacean - the collective name for whales, dolphins and porpoises - in the shallow coastal seas around the UK. However, their abundance is affected by humans in a number of ways including bycatch in fisheries, pollution and noise in the ocean.
Now a study conducted by a team of scientists from the University of Aberdeen and the Scottish Agricultural College in Inverness, and published this week in the Royal Society journal Biology Letters, has found that porpoises are also affected by climate change, and in an unexpected way.
Research on seabirds has shown that there is a strong link between breeding success, survival and the abundance of the small fish they eat called sandeels. It is also known that sandeels are less abundant in warmer waters.
However, unlike seabirds that only eat sandeels, it has always been assumed that harbour porpoises, and other cetaceans, are more generalist and would simply switch to eating other fish species when sandeel numbers fell, without suffering any ill effects. Therefore, it has always been thought that porpoises would not be affected by climate change in the same way as seabirds. This latest research suggests that this is not the case.
By examining the stomach contents of stranded porpoises from the east coast of Scotland collected each spring between 1992 and 2001, researchers from the University of Aberdeen found that at this time of year, harbour porpoises rely heavily on sandeels for food.
But when they looked at stomachs from animals that died in the springs of 2002 and 2003, they found they contained many fewer sandeels and much less food overall. To work out whether this was affecting the porpoises' well-being, this information was compared to the results of necropsies on stranded porpoises conducted by staff from the Scottish Agricultural College in Inverness. The results were dramatic: while only 5% of stranded porpoises had died due to starvation in the late 1990s, 33% of stranded porpoises had starved to death in the springs of 2002 and 2003.
Researchers say this is bad news. If, as predicted by climate change models, the waters of the North Sea continue to warm, the numbers of sandeels are expected to continue to decline. This means that as waters warm there will be fewer sandeels for harbour porpoises each spring and more animals starving to death.
Colin MacLeod, the scientist at the University of Aberdeen who led the research, said: "Harbour porpoises are already affected by humans in a number of ways, 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 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."
The scientists say the implications of their research spread well beyond harbour porpoises in the North Sea.
Dr MacLeod, Research and Teaching Fellow within the University's School of Biological Sciences, added: "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 to those already affecting porpoises in Scotland.
"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 sandeels. This makes the already difficult task of conserving the many threatened populations and species of whales, dolphins and porpoises even more so. 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."