Scientists have reported a surge in the number of dead whales, dolphins and porpoises washing up on Cornish shores after lethal encounters with nets and other fishing gear.

They found that since 2000 the numbers of cetaceans found dead has increased fivefold with up to 250 being found on Cornish beaches each year.

This is likely to be only a tiny fraction of the total deaths since tides carry most carcasses onto French shores or out to sea.

The figures mean that Britain’s cetaceans, all supposedly under legal protection, are being killed far faster than anyone had realised, at least in the southwest.

Dr Brendan Godley, a marine biologist at Exeter University, who did the study, said: “Many people were shocked by the recent dolphin strandings in Cornwall but the important message is that strandings have increased in recent years and that the majority are attributable to fisheries.”

Godley analysed records of cetacean strandings around Cornwall and the Isles of Scilly from 1911 to 2006.

He found an increase in strandings, to about 50 a year, from the early 1980s, with common dolphins and harbour porpoises being the worst affected.

From 2000, however, there was an even greater surge and numbers since then have ranged from 100 to 250 a year.

He also found that, since 1990, at least 61% of incidents in Cornwall were due to cetaceans being caught in nets. Blame for this increase is being pinned on French, Spanish and Irish pair trawlers who use giant nets strung between two boats.

The report will strengthen campaigns being waged by conservationists who want tough restrictions on the types of fishing allowed in waters with cetacean populations.

Willie Mackenzie, of Green-peace UK, said: “Cetaceans are supposed to be fully protected but you have this odd situation where if you kill the animal accidentally through ‘by-catch’ then it’s no-one’s fault.”

Mackenzie said fishermen often tried to sink dolphin carcasses to avoid discovery. “We have found dead dolphins that were still warm and which had their stomachs slit open so they would sink.”

Mark Simmonds, from the Whale and Dolphin Conservation Society, said: “Thousands of animals are probably dying each year.”

The political reaction to dolphin deaths has been glacial. Over the past 15 years, the government’s main response has been to commission research.

In 2004, one of the worst years for dolphin deaths, the government banned UK fishermen from a 12-mile zone off Cornwall – European Union rules prevented them banning foreign boats.

Bill Wiggin, the shadow minister for agriculture and fisheries, said the upcoming Marine Bill would not improve matters. “A French or Spanish trawler could steam through a protected zone,” he said. “It’s heartbreaking.”

Barrie Deas, chief executive of the National Federation of Fishermen’s Organisations, said: “Dolphins are an iconic species and no one likes to see them being killed but it’s a question of proportion. Why don’t you stop motor cars because of the death of hedgehogs?” he said.

The species being hit

Last year about 268 harbour porpoises were found dead around Britain’s coasts – about half of all the reported cetacean deaths.

The real total is probably far higher, even though the UK population of harbour porpoises is estimated at just 36,000 and the species is legally protected. Tidal currents sweep most carcasses onto French coasts or out to sea.

The short-beaked common dolphin is another victim of commercial fishing. It is found mainly around the warmer waters of Britain’s southwest coasts, where 92 deaths were reported last year. Studies show that up to 77% of these deaths followed entanglement in nets.

Five species of true dolphin are found in British waters, with the bottle-nosed and common dolphins seen most frequently; the killer whale, from the wider dolphin family, is also found. More than a dozen species of whale, including pilot and sperm whales, are often sighted, mostly further offshore.

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