SOME forms of fishing are the marine equivalent of forest clear-felling, devastating natural habitats on the seabed, say marine ecologists. At a meeting last week of the Society for Conservation Biology in Victoria, British Columbia, researchers argued that this threat to the planet鈥檚 biodiversity has been seriously neglected.
Conservationists have long been worried about declines in fish stocks caused by overfishing. But they have virtually ignored the 鈥渃ollateral damage鈥� 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 (see Figure).
Trawls and dredges leave a swath of destruction, says Leslie Watling, a marine biologist at the University of Maine鈥檚 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. 鈥淚f that kind of thing is happening on a wider scale, then we have a serious problem,鈥� says Watling.
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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鈥檚 Graduate School of Oceanography in Narragansett.
In the past, scientists could have gone to areas with boulders or rock outcrops鈥攑laces that were shunned by fishing vessels because they snagged nets. But since the mid-1980s, sophisticated 鈥渞ock-hopper鈥� 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鈥攖he equivalent of old-growth forests on land鈥攈ave 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鈥檚 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鈥檚 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鈥檚 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鈥檚 team. 鈥淚鈥檓 sure the area will recover,鈥� says Tuck.
Eleanor Dorsey, a marine biologist with the Conservation Law Foundation in Boston, believes that the damage caused by trawling depends on the extent to which the seabed is disturbed by waves and tides. The damage is worst, she argues, in deeper areas with little natural disturbance.
Watling argues that governments should prohibit trawling in especially sensitive habitats while encouraging it in the shallower, sandier waters where natural disturbance is high. This would eliminate the worst damage while still keeping trawler crews in business. But fisheries managers need to maintain a few untrawled zones in every habitat, Watling adds, so that biologists can study the effect of fishing.