Clearcutting the ocean floor
by Elliot Norse and Les Watling

Trawling is a widespread method of catching fish and invertebrates. Trawling vessels from 30 to 450 feet long (sometimes in pairs) fish by pulling large nets through the sea. Most trawling occurs on the seabed, targeting species such as groundfish, sea urchins and scallops.

The most widely used towed bottom-fishing gear is the otter trawl, whose forward motion spreads a pair of otter boards, each weighing tens to thousands of pounds, that hold the trawl mouth open. The bottom of an otter trawl mouth is a rope that can bear heavy steel weights (bobbins) which keep the trawl on the seabed. Many trawls, called roller trawls or rockhoppers, have large rubber discs or steel bobbins that ride over boulders and coral heads, which might otherwise snag the net. Some trawls are armed with tickler chains that disturb the seabed to flush shrimp or fish into the net. When filled with thousands of pounds of marine organisms, rocks, and mud, and dragged for miles across the bottom, the trawlnet itself can also disturb the seabed.

The beam trawl, held open by a steel beam instead of otter boards and typically fitted with chains, has an empty weight of up to 13 tons. Other towed gear consists of steel frames and fiber or metal chain mesh bags that plow over and through the surface layers of the seabed. Quahogs and surf clams are caught by hydraulic dredges that suck up large amounts of seabed. Corals are dragged with mobile gear called Italian bars, tangle nets, and St. Andrew's crosses.

Trawling has a long history, mainly in waters at depths from a few to hundreds of meters, but it accelerated sharply with the introduction of diesel engines in the 1920s. As more continental shelf fish stocks are overfished, trawlers have moved to depths of 6000 feet, from traditional fishing grounds such as the North Sea and Georges Bank to waters off developing nations and even the remotest oceanic seamounts of the Southern Ocean. Until the 1980s many areas were de facto refuges because their numerous obstacles or steep slopes made trawling risky. Rockhoppers, global positioning systems and fish finders allow trawlers to work in previously unfished waters. Trawls or related fishing gear are now used from subpolar to tropical waters.

Most trawling is concealed from view, so its effects have been overlooked. This is not a trivial oversight. As on land, biodiversity in the sea is profoundly threatened. The estuaries, bays, and continental shelves where most trawling occurs are among the most biologically productive - and heavily altered - marine ecosystems. Trawling has been thought to contribute to diminished fish catches, including the commercial extinction of haddock and cod on the Grand Banks in the North Atlantic. There have been protests in Europe against trawling gear since the 14th century because of its presumed effects on benthic (deep-ocean) organisms.

The sea's bed ranges from massive rocky reefs through boulders, cobbles, pebbles, sand, and mud. In general, reefs and coarser substrata are less common than muds. Reefs and coarse sediments are most prevalent on shallower parts of continental shelves where waves can remove finer sediments. Foraminiferans, coralline algae, corals, brachiopods, bryozoans, worms, and mollusks form structures of shelly calcium carbonate on rocks. Many other organisms, from sea urchins to mangroves, create solid or tubular structures on the seabed. The perception that the seabed is a featureless biological desert occurs because most people experience the seabed on sandy intertidal beaches, where waves largely eliminate long-lived structures. In contrast, the vast majority of the seabed is honeycombed with biogenic structures, and this heterogeneity is crucial to benthic ecosystems.

In general, areas of the continental shelf seabed with biogenic structures have more species diversity than areas without them. Coral reefs offer a large surface area and myriad interstices for their exceptional diversity of associated species. In the deep sea, where diversity is generally high, mudballs created by polychaete worms provide habitat for copepods; mounds made by sea cucumbers attract bivalves, amphipods, and polychaetes. As in forest ecosystems, "structural dominants" (species that dominate the gross form of the habitat) in many marine ecosystems are slow-growing and long-lived. Some sponges, for example, are believed to live 50 years or more. Pacific geoducks and Atlantic quahogs are estimated to reach up to 146 and 221 years, respectively. In the sea, no less than in forests, slow growth rates of key species make recovery from disturbance a long-term process. In areas inhabited by species adapted to disturbance, such as sandy beaches or current-swept channels between islands, trawling and similar fishing methods might approximate natural physical disturbances. But the extent of these ecosystems is very limited. Elsewhere, trawling kills seabed organisms by crushing them, by burying them under sediment, and by exposing them to predators.

Where the seabed consists of rocks, mobile fishing gear removes large epifaunal invertebrates (such as sponges, cnidarians, and bryozoans) and moves boulders along the bottom. This reduces habitat for myriad small species, and food for others. On pebbles, sands, and muds, homogenization of the bottom eliminates nursery habitat for commercial species and their prey, which can mean progressive decline of fisheries. On muddy bottoms, mobile gear passing over and through the upper four inches or so of the seabed smothers small invertebrates.

Not all trawling occurs on the shelf. As fisheries continue to decline, trawlers focus on "underutilized species" of the deeper continental slope and remote oceanic seamounts, such as orange roughy and grenadiers. Deep-water trawling must profoundly alter ecosystems whose species are not adapted to resisting or recovering from severe physical disturbances.

Trawling disturbs the seabed in ways that can be compared to terrestrial disturbances. It is most similar to clearcutting, but there is one great difference - forest loss is estimated at nearly 25 million acres per year worldwide, and the area trawled annually is about 150 times as great. Indeed, the UN Food and Agriculture Organization's estimate of annual worldwide forest loss, however alarming, is smaller than the combined area of Georges Bank and the Gulf of Maine that is trawled each year.

Physical disturbance of habitat is the leading cause of biological diversity loss. Until this decade, biodiversity loss in the sea was largely overlooked, with attention paid mainly to overexploitation of fisheries and pollution. Now, with growing understanding that marine biodiversity is imperiled, we have shown that the sea is experiencing physical alteration from bottom trawling and other towed fishing gear on a scale that was not previously appreciated. With the possible exception of agriculture, we doubt that any other human activity physically disturbs the biosphere to this degree. An activity that severely disturbs an area of seabed each year as large as Brazil, the Congo, and India combined must affect benthic ecosystems on both local and global scales.

Like clear-cutting, use of mobile fishing gear converts ecosystems to domination by disturbance-tolerant opportunistic species. In general, trawling undermines fisheries for species that benefit from complex benthic structure. For fishes that do not need benthic structure, some trawling is likely to increase their populations by encouraging opportunistic prey species or reducing disturbance-intolerant competitors. A terrestrial equivalent is the change in animal species when virgin forest is converted for grazing. Trawling could prevent recovery of some diminished fish stocks, such as Georges Bank and Grand Banks Atlantic cod, but it can benefit fisheries for some other species. The sea's equivalents of ancient forests are becoming cattle pastures.

Concern about fishing effects is increasing in the US. The recently reauthorized Magnuson-Stevens Fishery Conservation and Management Act requires regional Fishery Management Councils to identify Essential Fish Habitat (EFH), or "those waters and substrate necessary to fish for spawning, breeding, feeding, or growth to maturity." This law requires the National Marine Fisheries Service and the Fishery Management Councils to identify "activities with known or potential adverse effects on EFH," and allows them to regulate fishing gear or close areas to fishing.

Some management options could stem the loss of biodiversity, benefit all fishers and consumers in the longer term and minimize short-term economic harm to trawlers and dredgers:

1. Use a precautionary approach to management; the burden of proof should rest with those who would alter the sea's biodiversity and integrity. This might lead to lessened use of mobile fishing gear in structurally complex benthic ecosystems.

2. Match fishing gear to the fragility of the seabed, thus minimizing long term impacts of all types of gear. This most likely would give preference of some gear types over others in each bottom type, but would maintain species diversity and fisheries production in each.

3. Establish "no trawling zones" in a portion of all continental shelf and slope ecosystems, allowing the recovery of benthic communities. Such reserves would offset, to some extent, the loss of de facto reserves in areas that could not previously be fished with mobile gear and where commercially important fishes were more abundant. This would provide crucial information on effects of mobile gear and requirements for sustainable fisheries over the long term.

4. Educate the public about the nature of the seabed and its importance for biodiversity, including its role in supporting fisheries.

5. Ensure opportunities for sectors of society beyond fishing interests to influence policy and to hold positions of authority, as all of us are stakeholders when it comes to publicly owned marine resources.

Condensed from a longer, peer-reviewed paper; full text is online at <www.americanoceans.org> and <www.mcbi.org>.

Elliot A. Norse is president of Marine Conservation Biology Institute. Les Watling is professor of marine biology at the University of Maine. Reprinted by permission of the authors.