Campaigners are tracking marine mammals in an effort to find out how many are being killed by fishing nets.

More than 120 dolphin and porpoise carcasses were found on the beaches of Devon and Cornwall between January and April this year.

Researchers are using Greenpeace's flagship, Rainbow Warrior to monitor the mammals.

Up to 30 dead dolphins have been recorded in a single haul, according to the Whale and Dolphin Conservation Society (WDCS).

Dolphins and porpoises are vulnerable to fishing nets

WDCS fisheries spokesman Ali Ross called on the government and the European Union to act to prevent "the terrible toll".

He said: "Understanding these animals, how many there are, and where they are is vital to be able to protect them effectively," he said.

Mid-water trawling is feared to be decimating dolphin populations.

In addition to the large number of casualties off the south west coast, about 300 dolphins were washed ashore on the French coast in January alone.

Most of the bodies showed clear signs that fishing nets were the cause of death and some carcasses had been deliberately mutilated.

Greenpeace scientific adviser Dr Doug Parr said: "We are pleased to be working with WDCS generating research information that could help protect Britain's dwindling marine mammal population."