THROWING dead fish back into the sea after quotas have been reached must stop because it is putting Scotland's puffins and kittiwakes at risk, according to a leading scientist.
Professor Bob Furness says the millions of tonnes of fish discarded into the North Sea each year by trawlers has artificially boosted the amount of food for scavenging seabirds, particularly great skuas.
Their numbers have rocketed, and when they
cannot find enough dead fish to eat, the hungry great skuas feast on other seabirds instead.
Prof Furness, of the University of Glasgow, believes this is a major cause of the decline in puffins and kittiwakes.
He said the aggressive great skuas, also known as bonxies, have struggled to find enough food because discards have started to fall as quotas for haddock and whiting have increased in recent years.
Prof Furness, who has spent years researching the link between great skuas and discard levels, has now called for action to be taken to put a halt to the number of dead fish being thrown into the sea.
"At the moment, the great skuas are killing such large populations of other seabirds that their numbers are under threat," he said. "I would say the levels we have are unsustainable at the moment."
In order to solve the problem, he thinks discarding must stop and populations of great skuas must be left to drop back to more "natural" levels.
"We should be putting on as much pressure to stop the discarding as soon as possible," he said. "It's an artificial boost to large, aggressive birds that switch to killing things when they are short of food."
Currently, about 6,000 pairs of great skua live in Orkney and the Shetlands alone, but Prof Furness believes the population should only be in the hundreds.
This, he said, would be similar to levels in the Faroe Islands, where there are many seabirds but few great skuas.
Just leaving the birds to decline gradually as discard levels drop will bring them back to more sustainable levels, he said.
Although the skuas will start killing more kittiwakes and puffins as they run out of food, they will also start attacking chicks from their own species, meaning the populations will soon decline.
And Prof Furness thinks some puffins and kittiwakes will be able to stay safe from attack.
"There will probably be some colonies of puffins and kittiwakes that are in places great skuas cannot easily go.
"A lot of the remaining kittiwakes in Shetland nest in colonies in caves where great skuas seem to be too big to be able to get at their nests. So perhaps those colonies will survive while great skua numbers will fall naturally to more sustainable levels."
He added that great skuas will choose not to breed as food supplies dwindle, and will disperse further afield in search of food.
A bird he ringed on Foula, Shetland, 20 years ago and saw there each year has now been spotted in Germany, following trawlers for food.
Merciless pirate of the skiesTHE great skua, also known as the bonxie, is regarded as the "merciless pirate of the skies".
About 2ft long and with a wingspan of almost 4.5ft, it will viciously attack humans who approach its nest.
It has little agility, but instead relies on brute force to attack other sea birds up to the size of the great black-backed gull.
One common technique it employs is to fly directly at a bird in mid-air, grabbing it by the wing so that it stalls and falls into the sea.
There, the great skua will physically attack it until it surrenders its catch.
In 2007, ornithologists on St Kilda noted unusual behaviour from great skuas while researching the decline of the Leach's Storm-petrel population.
Using night-vision gear they noted the great skuas using predatory behaviour towards the petrels at night.
This is regarded as a remarkable strategy for a seabird.
The great skua mainly breeds on a number of Scottish islands though a few have been spotted on the mainland, Norway and the Faroe Islands.
