DEEP SEA "bottom trawling" is causing untold damage to fragile reef ecosystems around the world, many of which are thousands of years old, a leading marine expert warned.

The heavy fishing nets, dragged along the seabed on large rubber rollers called "rockhoppers", smash and flatten coral outcrops that provide vital refuges for fish and other marine species, said marine biologist Dr Jason Hall-Spencer, from the University of Plymouth.

He is calling for concerted international action to ban all bottom trawling from at-risk habitats, many of which are yet to be identified.

Bottom trawling involves scouring the seabed with huge nets 60 metres wide that ride over obstacles on "rockhopper" rollers weighing as much as several hundred pounds.

The technique was originally employed to catch fish such as flounder and haddock in shallow water areas with relatively smooth floors.

But in the past 20 years, dwindling fish stocks and technological advances have led bottom trawlers to descend to ever-greater depths.

Some nets are now sunk thousands of feet below the surface, accessing sensitive seamount habitats that were previously out of reach.

Work by scientists involved in the Census of Marine Life (CoML) – a major worldwide project cataloguing life in the oceans – is now bringing to light the true extent of the destruction wrought by bottom trawling.

Dr Hall-Spencer, a leading CoML researcher, said: "Less than 1 per cent of the estimated 50,000 seamounts have ever been surveyed and our research visits have revealed pristine coral reefs and many species that are new to science.

"However, over the past five years, these surveys have also worryingly revealed that all over the world, deep sea habitats are suffering severe impacts from bottom trawling down to depths of 1,000 metres and more.

"I myself have looked at about 20 reefs, and all but one of them has been badly damaged.

"But my colleagues from around the world have seen hundreds and they all say the same thing.

"It doesn't matter what ocean you go to, these habitats are being trashed by international fishing fleets.

"What is urgently needed is a network of protected areas where any type of fishing gear that involves dragging equipment across the seabed is banned."

Dr Hall-Spencer, who spelled out his fears at the annual meeting of the American Association for the Advancement of Science in San Diego, California, described the telltale signs of bottom trawling damage.

"The most obvious thing you see is trenches ploughed through the seabed that extend for kilometres," he said.

"You can see that the coral is pulverised and broken. You get this gravel of smashed-up coral with few outcrops remaining. The fish are gone, because fish like to hide behind coral heads.

"It's more like mining than farming, since you're removing a resource and not allowing it to renew itself.

"The typical thing is for a trawler to criss-cross over an area about 33 kilometres square, and that's a very large area being ploughed up."

Many of the most threatened sites were coldwater coral reefs in temperate zones that were still in the process of being discovered and explored, he said.

Studies indicate that the total global coverage of coldwater coral reefs might equal or exceed that of more familiar warm water reefs.

Dr Hall-Spencer carried out one of the first manned submersible dives on the world's biggest coldwater reef off the north-west coast of Norway.

The Rost Reef, discovered in 2002, is a 3km-wide strip teeming with life that stretches for almost 40 kilometres (25 miles) at a depth of 450 metres.

"Unlike every other reef I've seen, this one was pristine," said Dr Hall-Spencer. "It hadn't been touched by human activity."

The Rost Reef is an important success story for conservationists, having been given the protection of a complete bottom trawling ban imposed by the Norwegian government.

Dr Hall-Spencer said similar bans existed at a number of other sites around the world, but more needed to be done.

"The good thing is that people are waking up to the problem," he added.