The porpoise had blood coming out of the blowhole and slits on its body
A father and son have had an ordinary fishing trip disturbed by the discovery of a dead porpoise - which they believe was beaten to death by dolphins.
Donny Hay was mackerel fishing near Lunan Bay in Angus with his 10-year-old son Tom and local farmer Duncan Gray when they made the grisly find.
Scientists cannot explain why usually gentle dolphins become killers of porpoises and young dolphins.
However, they suggest competition for food and space could be a factor.
Mr Hay, from Edinburgh, told the BBC Scotland news website that Tom was the first to discover the dead animal.
"He spotted what he thought was a baby dolphin floating about 30 yards away in the sea," he said.
"We went closer to it and were able to pull it out the water and realised it was a porpoise and it was about 4ft long and weighed about 50lbs.
"It had obviously just recently died - we put our hand in its mouth and you could feel the blood was still warm and it had some blood coming out of the blowhole and you could see lung froth, so it obviously had a problem with its lungs.
"We looked a bit closer and it had been bashed on both sides quite hard, which had created little splits on its side - it looked like it had just been beaten up in the sea."
The family later buried the porpoise in a field near Carnoustie.
Since 1992, the Scottish Agricultural College has carried out 635 post-mortem examinations of harbour porpoises.
In total, 181 were found to have been killed by bottlenose dolphins.
The phenomena is particularly common in the Moray Firth, Lunan Bay and the Firth of Forth.
Bob Reid, Scottish strandings co-ordinator at the college, told the BBC Scotland news website that the reason for the murders was still a mystery.
He said: "Some people have suggested that this could be adult males killing young dolphins to bring the females back into breeding condition again. So they're killing the progeny of another male so that they then have the opportunity to fertilise the females themselves.
"That might lead to the killing of harbour porpoises as a practice infanticide - they're killing the animals because they're about the same size as young bottlenose dolphins.
"Food competition is another one, space competition is another, but it may just be a wholly natural phenomenon that they do it because they can."
Mr Reid urged anyone who finds a dead porpoise, dolphin or whale to report it so the carcasses can be examined.