The use of new fish-trawling gear is doing incalculable damage to the seabed, destroying essential habitat for marine life.
The economic and social consequences of overfishing, along with the indiscriminate killing of other marine animals and the loss of coastal habitats, have stimulated media coverage of problems in the oceans. Attention to marine habitat destruction tends to focus on wetland loss, agricultural runoff, dams, and other onshore activities that are visible and easily photographed. In tropical regions, fishing with coral reef-destroying dynamite or cyanide has been in the news, the latter making it to the front page of the New York Times.
Yet a little-known but pervasive kind of fishing ravages far more marine habitats than any of these more noticeable activities. Bottom trawls-large bag-shaped nets towed over the sea floor-account for more of the world's catch of fish, shrimp, squid, and other marine animals than any other fishing method. But trawling also disturbs the sea floor more than any other human activity, with increasingly devastating consequences for the world's fish populations.
Trawl nets can be pulled either through mid-water (for catching fish such as herring) or along the bottom with a weighted net (for cod, flounder, or shrimp). In the latter method, a pair of heavy planers called "doors" or a rigid steel beam keeps the mouth of the net stretched open as the boat tows it along, and a weighted line or chain across the bottom of the net's mouth keeps it on the seabed. Often this "tickler" chain frightens fish or shrimp into rising off the sea bottom; they then fall back into the moving net. Scallopers employ a modified trawl called a dredge, which is a chain bag that plows through the bottom, straining sediment through the mesh while retaining scallops and some other animals.
Until just a few years ago, trawlers were unable to work on rough bottom habitats or those strewn with rubble or boulders without risking hanging up and losing their nets and gear. For animal and plant communities that live on the sea bottom, these areas were thus de facto sanctuaries. Nowadays, every kind of seabed-silt, sand, clay, gravel, cobble, boulder, rock reef, worm reef, mussel bed, seagrass flat, sponge bottom, or coral reef-is vulnerable to trawling. For fishing rough terrain or areas with coral heads, trawlers have since the mid-1980s employed "rockhopper" nets equipped with heavy wheels that roll over obstructions. In addition to the biological problems rockhoppers create, this fishing gear also displaces commercial hook-and-line and trap fishers who formerly worked such sites without degrading the habitat. Wherever they fish and whatever they are catching, bottom trawls churn the upper few inches of the seabed, gouging the bottom and dislodging rocks, shells, and other structures and the creatures that live there.
Ravaging the seabed
Much of the world's seabed is encrusted and honeycombed with structures built by living things. Trawls crush, kill, expose to enemies, and remove these sources of nourishment and hiding places, making life difficult and dangerous for young fish and lowering the quality of the habitat and its ability to produce abundant fish populations.
Bottom trawling is akin to harvesting corn with bulldozers that scoop up topsoil and cornstalks along with the ears. Trawling commonly affects the top two inches of sediment, which are the habitat of most of the animals that provide shelter and food for the fish, shrimp, and other animals that humans eat. At one Gulf of Maine site that was surveyed before trawling and again after rockhopper gear was used, researchers noted profound changes. Trawling had eliminated much of the mud surface of the site, along with extensive colonies of sponges and other surface-growing organisms. Rocks and boulders had been moved and overturned.
It may be hard to get excited about vanished sponges and overturned rocks. But for the fishing industry-like that in New England, which has lost thousands of jobs and hundreds of millions of dollars in recent years and is suffering the resultant social consequences-habitat changes caused by fishing gear are significant. The simplification of habitat caused by trawling makes the young fish of commercially important species more vulnerable to natural predation. In lab studies of the effects of bottom type on fish predation, the presence of cobbles, as opposed to open sand or gravel-pebble bottoms, extended the time it took for a predatory fish to capture a young cod and allowed more juvenile cod to escape predation.
But virtually the entire Gulf of Maine is raked by nets annually, and New England's celebrated Georges Bank, the once-premier and now-exhausted fishing ground, is swept three to four times per year. Parts of the North Sea are hit seven times, and along Australia's Queensland coast, shrimp trawlers plow along the bottom up to eight times annually. A single pass kills 5 to 20 percent of the seafloor animals, so a year's shrimping can wholly deplete the bottom communities.
More data needed
Considering how commonplace trawling has become in the world's seas, researchers have completed astonishingly few studies. For example, virtually nothing is known about shrimp trawling's effects on the Gulf of Mexico's seabed, although this is one of the world's most heavily trawled areas. The effects on fish populations and the fishing industry, although probably significant, have been difficult to quantify because there are few unaltered reference sites. But the studies available suggest that the large increases in bottom fishing from the 1960s through the early 1990s are likely to have reduced the productivity of seafloor habitats substantially, exacerbating depletion from overfishing.
Peter Auster and his colleagues at the University of Connecticut's National Undersea Research Center have found that recent levels of fishing effort on the continental shelves by trawl and dredge gear "may have had profound impacts on the early life history in general, and survivorship in particular, of a variety of species." At three New England sites, which scientists have studied either within and adjacent to areas closed to bottom trawls or before and after initial impact, trawls significantly reduced cover for juvenile fishes and the bottom community. In northwestern Australia, the proportion of high-value snappers, emperors, and groupers-species that congregate around sponge and soft-coral communities-dropped from about 60 percent of the catch before trawling to 15 percent thereafter, whereas less valuable fish associated with sand bottoms became more abundant.
In temperate areas, biological structures are much more subtle than the spectacular coral reefs of the tropics. A variety of animals, including the young of commercially important fish, mollusks, and crustaceans, rely on cover afforded by shells piled in the troughs of shallow sand ridges caused by storm wave action, depressions created by crabs and lobsters, and the havens provided by worm burrows, amphipod tubes, anemones, sea cucumbers, and small mosslike organisms such as bryozoans and sponges.
Some of these associations are specific: postlarval silver hake gather in the cover of amphipod tubes, young redfish associate with cerrianid tubes, and small squid and scup shelter in depressions made by skates. Newly settled juvenile cod defend territories around a shelter. Studies off Nova Scotia indicate that the survival of juvenile cod is higher in more complex habitats, which offer more shelter from predators. In another study, the density of small shrimp was 13 per square meter outside trawl drag paths and zero in a scallop dredge path.
A general misperception is that small invertebrate marine bottom dwellers are highly fecund and reproduce by means of drifting larvae that can recolonize large areas quickly. In truth, key creatures of the bottom community can disperse over only short distances. Offspring must find suitable habitat in the immediate vicinity of their parents or perish. The seafloor structures that juvenile fish rely on are often small in scale and are easily dispersed or eliminated by bottom trawls. Not only is the cover obliterated, but the organisms that create it are often killed or scattered by the trawls.
Les Watling of the University of Maine (who has studied the effect of mobile fishing gear in situ) and Marine Conservation Biology Institute director Elliott Norse have shown that trawling is analogous to strip mining or clear-cutting-except that trawling affects territories that are larger by orders of magnitude. An area equal to that of all the world's continental shelves is hit by trawls every 24 months, a rate of habitat alteration variously calculated at between 15 and 150 times that of global deforestation through clear-cutting.
A multinational group of scientists at a workshop Norse convened in 1996
at the University of Maine concluded that bottom trawling is the most important human source of physical disturbance on the world's continental shelves. Indeed, so few of the shelves are unscarred by trawling that studies comparing trawled and untrawled areas are often difficult to design. The lack of research contributes to the lack of awareness, and this could be one reason why trawling is permitted even in U.S. national marine sanctuaries.
Trawling is not uniformly bad for all species or all bottom habitats. In fact, just as a few species do better in clear-cuts, some marine species do better in trawled than in undisturbed habitats. A flatfish called the dab, for instance, benefits because trawling eliminates its predators and competitors and the trawls' wakes provide lots of food.
But most species are not helped by trawling, and marine communities can be seriously damaged, sometimes for many decades. Communities that live in shallow sandy habitats subject to storms or natural traumas such as ice scouring tend to be resilient and resist physical disturbances. But deeper communities that seldom experience natural disturbances are more vulnerable and less equipped to recover quickly from trawling. In Watling and Norse's global review of studies covering various habitats and depths, none showed general increases in species after bottom trawling, one showed that some species increased while others decreased, and four indicated little significant change. But 18 showed serious negative effects, and many of these were done in relatively shallow areas, which generally tend to be more resilient than deeper areas.
Comparing the damage caused by bottom trawling to the clear-cutting of forests is not unreasonable in light of the fact that some bottom organisms providing food or shelter may require extended undisturbed periods to recover. Sponges on New England's sea floor can be 50 years old. Watling has said that if trawling stopped today, some areas could recover substantially within months, but certain bottom communities may need as much as a century.
Reducing the damage
Humanity's focus on extracting food from the oceans has effectively blinded fishery managers to the nourishment and shelter that these fish themselves require. If attention were paid instead to conservation of the living diversity on the seabed, fisheries would benefit automatically because the ecosystem's productivity potential and inherent output and service capacity would remain high. Actions that would simultaneously safeguard the fishing industry as well as the seabed need to be taken now. These measures would include:
- No-take replenishment zones where fishing is prohibited. This would help create healthy habitats supplying adjacent areas with catchable fish. Such designations are increasingly common around the world, particularly in certain areas of the tropics, and benefits often appear within a few years. In New England, fish populations are still very low, but they are increasing in areas that the regional fishery management councils and National Marine Fisheries Service have temporarily closed to fishing after the collapse of cod and other important fish populations. The agencies should make some of these closings permanent to permit the areas' replenishment and allow research on their recovery rates.
- Fixed-gear-only zones where trawls and other mobile gear are banned in favor of stationary fishing gear, such as traps or hooks and lines, that doesn't destroy habitat. New Zealand and Australia have closed areas to bottom trawls. So have some U.S. states, although these closures are usually attempts to protect fish in especially vulnerable areas or to reduce conflicts between trawls and other fishers, not to protect habitat. Temporary closures in federal waters, such as those in New England, should in some cases be made permanent for trawls but opened to relatively benign stationary gear. What gear is permitted should depend on bottom type, with mobile gear allowed more on shallow sandy bottoms that are relatively resistant to disturbance but barred from harder, higher-relief, and deeper bottoms where trawler damage is much more serious.
- Incentives for development of fishing gear that does not degrade the very habitat on which the fishing communities ultimately depend. Fish and fisheries have been hurt by perverse subsidies that have encouraged overfishing, overcapacity of fishing boats, and degradation of habitat and marine ecosystems. Intelligently designed financial incentives for encouraging new and more benign technology could tap the inherent inventiveness of fishers in constructive ways.
Carl Safina is director of the National Audubon Society's Living Oceans Program in Islip, New York, and the author of Song for the Blue Ocean: Encounters Along the World's Coasts and Beneath the Seas (Henry Holt and Company, 1997).