THE harbour porpoise has plied the North Sea in search of fish for thousands of years. But so, too, has man, using vast trawler nets in which large numbers of porpoises are accidentally entangled and dragged to their death.

The accidental entrapment of porpoises, known as bye-catch, has now become a cause for concern for many organisations, among them the RSPCA, whose Scientific Officer, Laila Sadler, says modern non-selective fishing practices kill thousands of porpoises every year in European waters, as well as sharks, sea birds, turtles, whales and dolphins.

She says: "Dolphins and porpoises die agonising deaths as they desperately try to escape the nets."

Similar concerns are shared by Ali Ross, the fisheries policy officer for the Whale and Dolphin Conservation Society, which estimates that three per cent of North Sea porpoises perish in nets each year.

Ms Ross says the main problem are the large static nets which reach down to the seabed.

Because porpoises hunt along the bottom, they are looking down and tend not to see the nets until it is too late.

Ms Ross says the problem was at its peak due to the high level of fishing in the mid-1990s, with up to 8,000 porpoises a year lost in the North Sea, 7,000 of them claimed by the Danish fleet, the rest by the UK fleet, an annual loss of four per cent.

With the decline of fishing that number has dropped to three per cent but Ms Ross says: "Bye-catch is still a major problem for porpoises. Even with the decline in fishing, porpoises are still being lost at an unsustainable and unacceptable level."

Jonathan Gibson, an ecologist with Industry Nature Conservation Association (INCA) in Billingham, Teesside, says the North Sea porpoise population was 279,000, of which up to 5,000 a year were bye-catch.

He says: "There are concerns about the level of bye-catch, because the figures suggest that it is simply not sustainable for the porpoise population."

And Northumberland Wildlife Trust conservation officer Kevin O'Hara says bye-catch was becoming an increasing worry in the North Sea as the warming waters push fish north in search of cooler waters

He says: "Bye-catch has been limited in the North-East but as the fishing fleets move further north, fishing technology will migrate with them and that could create a problem for creatures like porpoises and dolphins. Bye-catch is a big problem for these creatures because it can maim and kill them."

The RSPCA and other organisations have welcomed efforts by the British Government to tackle the problem, through researching the use of nets with larger meshes and campaigning for the use of pingers, which emit a short sound at regular intervals to deter animals like harbour porpoises from approaching the nets.

EU regulations which came into force this summer require fishermen to use pingers, following representations from the British Government, but pingers are not the complete answer.

A spokesman for Defra says: "We are not sitting back on this one. There are problems with pingers, their batteries can run low, they can become detached from nets and there is an argument that they deter creatures like porpoises from important feeding grounds. We are conducting more research into other solutions, including changes in fishing gear."

Barry Deas, chief executive of the National Federation of Fisherman's Organisations (NFFO), says: "We recognise there is an issue but it is not a blanket problem, it happens in specific conditions. That is not say that we as an industry can ignore it.

"The problem with pingers is that they are expensive and we feel we have not had Government help with them. In addition, fishing gear is handled very roughly and pingers can have a very short life."

The NFFO is currently taking part in research by the Sea Mammal Research Unit, at St Andrews University in Scotland, and Mr Deas says: " We need to establish total porpoise populations and what impact we are having."

If there seems widespread agreement that bye-catch is a problem, no such universal agreement exists when it comes to continued rumours of dark deeds out at sea.

The issue came into sharp focus earlier this year when more than 30 porpoise corpses were washed up along the North-East coast between Blyth, in Northumberland, and Lincolnshire, including two at Redcar, in east Cleveland.

Some appeared to bear signs of clubbing, at least two others looked like they had been shot in the head.

Establishing what really happened is difficult but Simon Drayton, of the Humber Coastguard and a member of British Divers Marine Life Rescue, which helps stranded porpoises, believes some were deliberately harmed.

He says: "Some deaths are as a result of natural causes but some are the result of bye-catch. It is illegal to kill porpoises but, in some cases, if they get too near to the boat, some fishermen do their utmost to shoot or spear them."

The policeman who investigated the incident, Sergeant Chris Hine, a wildlife liaison officer at Bridlington, on Humberside, says: "There was a significant number washed up which appeared to have these injuries but I was unable to identify an offender. There was a large school of thought in the scientific community that they might have been caused by dolphin attacks."

Kevin O'Hara, at Northumberland Wildlife Trust, confirmed that there were reports of stranded porpoises bearing shooting injuries but no firm evidence to point to the perpetrator.

The RSPCA believes the porpoises may have collided with large ocean vessels and been washed in, the bullet-like injuries caused by gulls pecking at the corpses.

And Mr Deas, of the NFFO, says that whereas seals would take from static nets, porpoises tended not to. He says: "I am not sure under what circumstances fishermen would feel the need to harm porpoises."

What does seem certain is that the loss of porpoises through whatever means is a growing concern.

Jonathan Gibson, of INCA, believes they should be more valued. "You do see porpoise off the North-East coast and also up the Tees as far as the barrage, and six were recently seen off Hartlepool, which is a fantastic thing for tourism," he says.