When farmers or ranchers want to clear an area of unwanted brush, small trees, or other vegetation, they sometimes attach a heavy chain between a pair of tractors or bulldozers. Dragged over the ground, the chain pretty much eliminates everything - plants, ground-nesting birds, and burrowing animals.
That, in essence, is what many scientists say happens to the ocean floor when fishing vessels engage in bottom trawling or dredging. Large quantities of seafood are gathered up in weighted nets (often dragged along between two boats), but in the process millions of square miles of marine habitat are damaged or destroyed when heavy equipment scours the seabed.
Reports published this week in the journal Conservation Biology document the extent and impact of what is being called the underwater equivalent of forest clearcutting.
"When I make a dive ... in a research submersible, I can tell immediately whether an area has been trawled for fish or dredged for scallops," says Les Watling, professor of oceanography at the University of Maine. "After trawling, the sponges and mussels, the tube-dwelling worms and amphipod crustaceans that live in undisturbed areas are almost all gone. Boulders formerly covered with marine animals are almost lifeless from being rolled around by nets or dredges. And the mud has deep scars...."
"It doesn't take a marine biologist to realize that these fishing methods are terrible for marine animals," adds Dr. Watling, author of one of the reports published this week. "Nothing humans do to the sea has more physical impact than bottom trawling."
'Fisheries are crashing'
Elliott Norse, president of the Marine Conservation Biology Institute in Redmond, Wash., estimates there are 89,000 trawling vessels around the world, and their largely unregulated operations cover an area twice the size of the contiguous 48 states every year. Dr. Norse, who has written extensively on forest-logging practices, says "trawling is amazingly like clearcutting."
The result, says Norse, is long-term damage to plant and animal habitat that harms the biodiversity essential to maintaining a healthy marine environment. "No wonder so many fisheries are crashing," he says.
Nearly 70 percent of the world's marine fish species are overfished or fished to the limit of sustainability, according to the United Nations Food and Agriculture Organization. In US waters, the National Marine Fisheries Service (NMFS) has found that 96 of 279 fish species surveyed have been overfished or nearly so. Still, NMFS officials this week cautioned that there are many factors other than trawling behind the decline in fisheries.
For its part, the fishing industry dismisses charges that trawling is as environmentally harmful as critics say it is.
David Nitchman, spokesman for the National Fisheries Institute (the largest United States seafood trade organization), called this week's published reports a "public-relations gambit."
"The analogy [to clearcutting] is catchy, but there's no scientific basis to it," says Mr. Nitchman.
"Our industry is working very hard to make sure that we keep our incidental impact to fish habitat to a minimum," adds Nitchman, whose organization represent about 1,000 businesses ranging from fishing-boat owners to restaurants. "We're always trying to increase the efficiency of fishing technology, and a big part of that is to protect the habitat so that fishermen can use the resource to support themselves and their families."
But marine experts say advancing technology may be part of the problem as well.
Global positioning systems and depth sounders have made it easier to find commercially valuable groundfish, mussels, and shrimp. New kinds of nets, weighting devices, and "rockhoppers" that can roll along the bottom now allow fishing on rough bottoms (including coral reefs) that once were impossible to fish.
"I've seen [seabed] bottoms once covered with gardens of corals, sponges, mussels, and worms turned into barren wastelands," says Peter Auster, science director of the North Atlantic National Undersea Research Center at the University of Connecticut in Storrs. "And when just one tow of a scallop dredge does that, imagine what happens when a spot is hit several times a year, as many areas are."
Limiting trawling in Alaska
Federal officials this week ordered stricter limits on trawling for pollock in Alaskan waters, where Steller sea lions (which feed on pollock) are endangered. Trawling has been banned in the Oculina Bank off the coast of Florida in order to protect coral and other marine life. In its first annual report, the Centre for Environment, Fisheries, and Aquaculture Science (part of the British Agriculture and Fisheries Ministry) last month warned that trawling in British waters has had significant impact on seabed plants and animals.
The Conservation Law Foundation, based in Boston, recently published 28 papers by scientists and fishermen documenting the impact of fishing gear on the sea floor.
This week's findings in Conservation Biology should heighten the debate over fishing methods around the world.