Trawlers devastate seabed ~ marine ecologists ignored.

Destruction follows in trawlers' wake

SOME forms of fishing are the marine equivalent of forest clear-felling, devastating natural habitats on the seabed, say marine ecologists. At a meeting of the Society for Conservation Biology in Victoria, British Columbia, researchers argued that this threat to the planet's biodiversity has been seriously neglected.

Conservationists have long been worried about declines in fish stocks caused by overfishing. But they have virtually ignored the "collateral damage" caused by trawling and dredging, in which vessels drag large nets or scoops across the ocean floor to gather prawns, scallops and fish such as cod and haddock. These are among the most popular fishing techniques in the shallow seas of continental shelves. In some waters, fishing is so intensive that each hectare of seabed is on average trawled in its entirety seven times per year .

Trawls and dredges leave a swath of destruction, says Leslie Watling, a marine biologist at the University of Maine's Darling Marine Center in Walpole. Several small studies have shown that nets destroy sponges and other invertebrates that live attached to the ocean floor. Nets can roll boulders over, exposing animals sheltering underneath and crushing creatures on the seabed. Trawls and dredges also churn up sediment, killing delicate burrowing creatures. "If that kind of thing is happening on a wider scale, then we have a serious problem," says Watling.

So far, scientists have few data on how serious or widespread the damage is. One major problem is that it is difficult to find an area of ocean floor on a continental shelf that has not already been trawled, says Jeremy Collie of the University of Rhode Island's Graduate School of Oceanography in Narragansett.

In the past, scientists could have gone to areas with boulders or rock outcrops-places that were shunned by fishing vessels because they snagged nets. But since the mid-1980s, sophisticated "rock-hopper" gear, consisting of sonar sounders and machinery to raise nets over obstacles, has allowed trawlers to guide their nets into even the tightest spots. As a result, undisturbed areas of seabed-the equivalent of old-growth forests on land-have almost completely vanished in heavily fished waters.

However, Collie and a handful of other researchers have found a few such pristine places and begun to make the critical comparisons. Their preliminary results, presented at the Society for Conservation Biology meeting, suggest that trawls and dredges can cause major changes in some seabed ecosystems.

On George's Bank off the coast of New England, Collie and his colleagues have monitored an area that has been closed to all fishing since late 1994 to allow depleted stocks of cod and other commercial fish to recover. In the year and a half after the ban came into force, the number of animals longer than 5 millimetres living in or on the bottom sediment increased fourfold, while species biodiversity doubled. A nearby area that is dredged for scallops showed only a slight increase in numbers and a decrease in diversity.

The extent of the damage caused by trawling and dredging seems to vary from area to area, however. In Scotland's Firth of Clyde, for example, the presence of a submarine base in Gare Loch has precluded any fishing since the 1960s. Ian Tuck of the Scottish Office Fisheries Research Service's Marine Laboratory in Aberdeen trawled part of this area one day per month for 15 months. He found that a few species of small polychaete worm increased their numbers, while other mud-dwelling worms became less common. This effect was still detectable some 18 months after Tuck stopped trawling, but was nothing like as dramatic as that seen by Collie's team. "I'm sure the area will recover," says Tuck