THOUGH still an island, Britain is now surrounded by desert. New research has shown that repeated trawling has turned much of the sea bed around the UK into a barren wasteland.

Scientists using deep sea photography and painstaking analysis of hundreds of years of fishing records have discovered an underwater terrain, once rich in species such as oysters, that has now largely been denuded of life.

Their study also suggests that Britain’s coastal waters may have turned from sparkling blue towards a dirty greyer colour, partly because of the destruction of shellfish beds.

Centuries of trawler activity have exposed the sea’s muddy bottom, allowing silt and sediment to rise up into the water.

“These changes have taken place over such a long time that humans cannot see them happening,” said Callum Roberts, professor of marine biology at York University. “Fishing, especially trawling, has destroyed sea life and left us surrounded by a marine desert.”

Overfishing has become a global political problem with a recent report by the United Nations Food and Agriculture Organisation warning that 28% of the world’s fisheries stocks are at or near collapse.

Earlier this month the actors Greta Scacchi and Colin Firth joined forces with campaigners to publicise The End of the Line, a film highlighting the destruction of bluefin tuna stocks.

Roberts’s research, part of a three-year study, has traced fishing records from as far back as 800 years, when trawling began, to measure its impact on the sea bed.

Trawling — large nets being dragged along the sea floor — was initially banned around much of Europe because fishermen recognised its destructive nature.

By the 19th century, however, the advent of steam-powered fishing vessels made the potential profits irresistible, leading to restrictions on trawling being overturned.

Roberts and his colleagues have unearthed accounts of what the nets brought up from the sea bed at the time and have found them to be significantly different from today’s hauls.

They discovered that 19th-century catches included millions of shellfish, along with corals, sponges, sea anemones and other creatures that grow out of the sea bed.

Fishermen describe in the records how their catches changed once trawlers had been through an area, with hooks and nets picking up vast mats of seaweed and other debris ripped from the bottom.

“The picture that emerges is that the bottom of our seas was largely covered in thick layers of shellfish and other sea life,” said Roberts. “It also supported a huge population of fish and kept the water clean. Now it has almost all gone.”

Jason Hall-Spencer, a marine biologist at the University of Plymouth, has found that many parts of Britain’s sea bed are trawled as often as 20 times a year. He has used underwater cameras attached to nets to film their impact.

“Imagine lowering a giant net into an orchard and then towing it through to collect apples,” he said. “It would destroy the life on which the harvest depends. The damage we are doing to the sea bed affects the whole marine ecosystem. We are degrading the seas on which we depend.”

Fish stocks have suffered drastically. In 1956 the British distant-water fishing fleet brought 8.36m tons of fish back to shore. By 1997 the entire UK fleet landed only 816,000 tons, and by 2007 that had fallen to 600,000 tons.

Roberts estimates that fish populations are now a tiny fraction of their natural level. “The North Sea was so full of sharks that they even occasionally killed sailors,” he said. “Today they are all gone.”

In those rare waters that have not been trawled recently, mainly because they receive military protection, the contrast in marine life is stark.

“In Plymouth Sound, a military harbour, there are the biggest oysters in British waters and rare bivalves, such as a foot-long fanshell can thrive because of the absence of trawlers,” said Hall-Spencer.

There have been similar revivals in the few tiny areas set aside for marine reserves, such as the waters around Lundy Island in the Bristol Channel, where 2ft sponges are now common.

Such areas could become more numerous if the Marine Bill, now going through parliament, becomes law. Worldwide, however, the damage seems likely to increase.

Times recommends

• Energy-saving ideas from switched-on professor
• Rooks are as clever with tools as chimpanzees
• Burping of the lambs blows roast off menu