Trawling Takes a Toll

What is Trawling? 

Trawling is a widespread method of catching marine fishes and invertebrates in which ships pull funnel-shaped nets through the sea. Although industrialized fishing is just one of the human activities that threaten the oceans, it is among the most serious. And of all fishing methods, "trawling is the worst for marine ecology by many orders of magnitude," says Dr. Les Watling, Professor of Oceanography at the University of Maine and an expert on benthic, or bottom-dwelling, organisms. Most trawling occurs along the sea bed, targeting such species as groundfish, shrimp, and scallops, and disturbing or destroying a great number of other species in the process. Noted oceanographer Sylvia Earle describes bottom trawling as "the subsea equivalent of collecting the entire farm when the goal is to bring in a bushel of apples." Says Watling, "It simply sweeps up everything at the bottom, destroying a diversity of life that has existed for as long as the earth has." Because we know so little about benthic biodiversity to begin with, some species are almost certainly being lost without our knowledge.

Out of Sight, Out of Mind, Out of Stock

Over the past several decades, people have become increasingly aware that the destruction of tropical forests is causing great biodiversity loss. "It is difficult to imagine that another severe human disturbance of even greater extent could occur almost unnoticed by scientists, the media, and political leaders. But there is one: fishing on the seabed with towed gear such as trawls and dredges," write Watling and Elliott A. Norse of the Marine Conservation Biology Institute in a controversial paper published in December 1998. In heavily trawled areas, the ocean floor becomes a flattened wasteland. Along with destroying habitat, trawling literally empties the sea. Any creature larger than the holes in the net is caught in its vast reach.

"In the minds' eye, people can easily visualize coral reef or sea grass fishes. But when you start talking about animals like cod and flounder, the mental pictures tend to be of the fish on the deck of a boat or on a dinner plate," observes Dr. Peter Auster, science director of the National Undersea Research Center at the University of Connecticut. "The public is more apathetic about areas they're not familiar with, like the sea floor in general." Because what happens under the surface of the sea is unseen and difficult to study, the impact of trawling on marine ecosystems has been largely overlooked. Yet, write Watling and Norse, "with the possible exception of agriculture, we doubt that any other human activity physically disturbs the biosphere to this degree."

The Ocean Floor is A Crucial Marine Habitat

Although to the naked eye the ocean floor may look flat and dull, it is actually a complex, three-dimensional habitat in which rich, biologically diverse communities thrive. These communities are shaped by the rocks, sand, or mud which make up the seabed; by living organisms that provide structure, such as seaweeds, sponges and mollusks; and by biological activities such as digging and burrowing by clams, worms, and sea anemones, which create spaces within and oxygenate the seafloor sediment. Many organisms that contribute to this structural complexity are scarce, slow-growing, and long-lived, and do not quickly replace themselves if removed or killed. Most are too small to be easily seen, orders of magnitude smaller than comparable trees and terrestrial plants, "but they serve the same functions that forests and meadows do on land," as Auster points out, thereby providing habitat for a wide range of ocean life. Even structures only a few centimeters high are important to marine biodiversity, as they may be heavily used by many different organisms.

Trawling Reduces Species Diversity and Abundance

Trawling profoundly disturbs this structure and hence the composition of the seabed. It affects species directly by:

  • permanently removing fish, and other animal and plant species on which they feed, from the ocean bottom.
  • killing species outright. The mobile gear crushes, buries, or exposes organisms.

 Trawling also indirectly influences species diversity. Says Dave Packer, a research fishery biologist with the National Marine Fisheries Service, who is assessing the effect of trawling on a section of Georges Bank [Link to G3-What Does the Seafloor Say?], "Sometimes you can kill the animals outright, and sometimes you just disrupt the habitat enough so the animals can't grow or reproduce very well."

Trawling Reduces Habitat Complexity

Because trawling leaves the seabed flattened and homogenous, it eliminates important nursery areas and refuges for groundfish (such as cod) and shellfish. This can make it harder for juveniles of commercially harvested species to escape predators, so fewer survive to replenish the next generation. "Some of these juvenile fish need little crevices in gravel or cobbled areas to hide in to survive their first months of life. Drag a trawl across there or plough the gravel under and you lose a lot of shelter," observes Dr. Jeff Cross, Chief of the Ecosystem Process Division of the Northeast Fisheries Science Center of the National Marine Fisheries Service. "If you destroy habitat and it takes years and years to recover, what effect does that have on the subsequent populations of fish?"

Specifically, trawling:

  • removes and reduces biogenic structures (those produced by organisms) such as shell aggregates, pits, tunnels, tubes, and reefs, as well as the species that create them. Mobile gear collapses the burrows and breaks the tubes that shelter small invertebrates, exposing them to predators. Some of these structures also stabilize or oxygenate bottom sediment.
  • reduces seabed formations, such as ripples and sandwaves, that provide shelter from currents.
  • overturns and displaces rocks, stones, and pebbles. This reduces cover and predator protection for crevice dwellers and breaks off organisms like tubeworms or sponges that provide further structure and shelter. Moreover, groundfish such as herring attach their eggs to pebbles, which are detached and damaged by trawling.
Trawling Disturbs Bottom Sediment and Geochemical Cycling

Marine bottom-dwelling organisms provide a number of extremely important ecosystem services, such as decomposing organic matter and storing and recycling carbon. Biogeochemical flows are therefore altered when trawling and dredging equipment churns up bottom sediment and resuspends it in the water column. Nutrient dynamics are further influenced because, as Dave Packer explains, "When the gear upsets the bottom or tears up the bottom . . . you're affecting the food chain, because many of these animals that live in the sediments or on the sediments are basically fish food."

Depending on location and the type of bottom sedimentwhether, for example, it's composed of coarse sand or fine silttrawling can result in a variety of negative effects:

  • The water may become increasingly turbid (murky). This diminishes photosynthesis in the water column in shallow waters, makes it harder for species that hunt by sight to find food, and causes filter-feeding animals (those that strain their food from the water) to expend energy sucking in particles, such as sand, which have no nutritional value.
  • Toxins can be exposed and circulated. For instance, harbor floors often store heavy pollution loads from coastal runoff. When they are dredged, the contaminants are dispersed to other ocean habitats.
  • Food availability or quality for bottom-feeders can diminish. The uppermost layers of sediment swept up by trawls typically contain the highest-quality food particles and nutrients. These nutrients and organic particles get re-suspended in the water column, where they can be oxidized, thus resettling to the bottom as lower-quality food.
  • This resuspended organic matter can also affect regional nutrient cycling, introducing sedimentary nitrogen, silica, etc. into the water column. Greater quantities of these nutrients can cause phytoplankton blooms. Furthermore, significant amounts of carbon that had previously been locked away in seabed sediment are sometimes released back into the water column, and possibly to the atmosphere. If this were to happen on a large and frequent enough scale, it could contribute to climate warming.
  • By displacing accumulated top layers of organic sediment, trawling can expose barren clay or rock, which is unlikely to be recolonized by local species. Furthermore, the displaced sediment can resettle on very different areas of the seafloor where the organisms carried within the sediment can't survive.
  • When it resettles, sediment can bury and suffocate organisms on the seafloor.
Trawling Alters Community Composition

Trawling reshuffles bottom-dwelling communities at many levels. Although trawled areas may be quickly recolonized, the organisms that appear there may not be the same ones that originally made up the community. For instance, increased murkiness of the water column may cause a shift from species that hunt by sight to those that locate prey by sound or touch, or from filter-feeders to deposit-feeders. Often, short-lived, rapidly-reproducing creatures (such as nematode worms) move in, tending to replace larger, longer-lived organisms (such as sponges or shellfish) that take longer to propagate and reestablish themselves. However, such effects can often be temporary, reversing themselves if the cause of disturbance is removed.

Typically, reducing structural complexity results in an increased abundance of opportunistic, more adaptable species that benefit from disturbance, at the expense of a richer variety of species and more fragile organisms. The loss of a dominant species can dramatically alter community structure and food chains, benefiting some species and harming others. Whether these changes are short-term or long-term depends on how much time the community is given to recover. Enduring changes are most likely to occur in communities where natural disturbance is rare, or in which long-lived and fragile species are key players.

The activities that are most liable to have unexpected long-term effects are the direct removal of mass quantities of fish and other species or the significant destruction of intricate habitat structure, like that provided by sponges or corals. When older, more fertile individuals are captured, spawning stocks are depleted and the ability of the species to sustain its numbers is reduced. When the habitat of a commercial species is damaged or destroyed, more juveniles are preyed upon, and numbers diminish. In the eastern Atlantic, for example, trawling has impoverished haddock, cod, and flounder populations, and skate and dogfish populations have vastly increased in the resulting vacuum [link to G1-Sorry Story of George's Bank]. As long as intensive trawling continues in this area, the species composition is unlikely ever to revert to its prior state. Even if the area were left completely undisturbed, full recovery would probably take decades.

The Bycatch Problem

By its nature, trawling is highly nonselective. All species larger than the mesh size can be swept up, regardless of whether they were the targeted species. Although regulations mandate minimum mesh size, once the back wall of the net is lined with catch, little of any size escapes. This unintentionally snared catch is called bycatch or incidental take, and it involves all different kinds of marine life: species with no commercial value, fish that are undersized, fish that are over quota, even large vertebrates. Bycatch also removes large numbers of juvenile fish before they have spawned for the first time. Millions of these undesired organisms are tossed back overboard, maimed or dying, making them easy prey. Often they are already dead when discarded ("bykill").

In some fisheries, those targeting shrimp in particular, the bycatch exceeds the target catch because of the extremely small mesh size of the nets. For every pound of shrimp caught in the Gulf of Mexico, eight to nine pounds of "trash fish" like rays, eel, and flounder are mangled and discarded. This doesn't count the tons of plants and animals, such as starfish, sand dollars, seaweed, and coral, that are not even considered worth reporting as bycatch. While marine mammals, turtles, and seabirds get the most attention, the removal of all the smaller creatures probably has a much greater effect on ocean ecosystems.

Bycatch is not a problem of trawling alone. Every fisherman inadvertently catches species other than the intended one. Data on bycatch is much more limited than on target species, but it is clear that bycatch constitutes a major threat to marine biodiversity. Large-scale drift nets or gill nets, which can stretch for up to 40 miles, catch fish which become entangled at the gills and strangle.

Still in use by a number of countries, although now legal only if less than 2 kilometers long, drift nets indiscriminately catch innumerable marine animals, including dolphins, seals, sharks, sea birds, and sea turtles. If it becomes detached from buoys, a drift net can "fish" on its own for years, especially if made of monofilament, which is virtually indestructible. These near-invisible "ghost nets" snare and drown untold numbers of animals yearly. Giant ocean fishes such as tunas, sharks, swordfish and marlins are especially hard hit by nonselective equipment like driftnets and drift longlines, drifting lines of baited hooks many miles long. Birds which dive after the bait on longlines become entangled and drown; an estimated 44,000 albatrosses are killed annually in the Southern Ocean by longline vessels fishing for tuna. Spanish longliners catch an estimated 20,000 loggerhead turtles every year, of which an estimated 4000 die after being returned to the sea with the hooks still in their mouths.

Specialized devices, such as "turtle excluders," have worked well to reduce bycatch in some fisheries. Some trawl nets are designed to permit certain non-target species to escape. However, in almost all cases, technical fixes benefit only a few of the species at risk. For example, acoustic pingers warn mammals away from nets but do not protect turtles, sharks, rays or birds. Bycatch solutions require a combination of approaches and the incentive to employ them. Reducing bycatch is a key part of the 1996 Sustainable Fisheries Act.

How Trawling Gear Works

Bottom trawling has been practiced on a small scale for centuries. Shrimp were caught in the English Channel with a net pulled from shore at low tide by horses. Even with such simple technologies, some recognized the harm trawling could do. The first law against trawls that "rooted up and swept away the seaweeds which served to shelter the fish" was passed in Flanders in 1499, and Holland, France, and England soon followed suit. For 300 years New England fishermen caught bottom-dwelling fish using longlines, lines set with 200 to 1000 baited hooks. Sail-powered bottom draggers, which pull nets just above the ocean's floor, only began trawling the North Sea in the 1830s. Once ships had engine power, these draggers increased in number all over the world.

Today, the most common and most destructive kind of trawling gear is called an otter trawl, which drags a 150- to 200-foot-long net shaped like a wind sock across the sea floor. The upper side is held up by floats and the net is kept open by giant planks on either side, called otter boards or doors. Technical advances have made trawling ever more efficient. In the mid-1980s, thousands of trawlers installed "rockhoppers," large rollers that enable vessels to drag close to a very rough bottom without damaging the net. These turned otter trawls into the undersea equivalent of all-terrain vehicles, and vast areas of once-untouched habitat became accessible.

Some trawls are equipped with "tickler chains" which stir up the bottom, creating noise and dust and flushing fish up into the nets. Other gear, including oyster, scallop and crab dredges, consists of steel frames and chain or fiber bags that scrape the bottom. Hydraulic dredges liquefy and suck up large amounts of sea bed. All bottom trawling is inherently destructive.

Not Just the Gear but the Scale

Advanced technology has altered not only the gear but the level of fishing as well. Colossal factory ships came into use during World War II. As big as football fields, they pull trawl nets large enough to swallow twelve jumbo jets. These ships may also set out up to 80 miles of submerged longlines, or 40-mile drift nets. Pair fishing, now outlawed in some places, suspends a huge trawl between two factory ships, which alternate trawling and processing operations so that fishing can continue 24 hours a day. Fleets, too, have grown, assisted by generous subsidies. "One of the shocking realities of the global fishing industry is that about a third of all of the economic revenue from fisheries is actually provided by government subsidy. It's not coming from the sea, it's being poured into building bigger and bigger fishing fleets, when we're already at twice the capacity necessary," Stiassny points out. "Fish and other ocean products are the only significant global food source that we still actually hunt, and we hunt them on a scale unimaginable to past generations." More than a million large-scale fishing vessels, twice as many as in 1970, now comb the oceans of the world.

Just as on land, the resilience of an ecosystem is affected by the intensity of any disruption. Evidence indicates that the greater the disturbance, the fewer species a habitat can sustain afterwards. Many key bottom-dwelling species propagate seasonally and/or across small distances, so disturbed patches may sit barren for some time before they show signs of recolonization. When disturbances are both severe and extensive, recovery times can be very long.

Not Just the Scale But the Frequency

The severity of trawling's effects also depends on how often an area is dredged or trawled. When the interval between visits is shorter than the time it takes ecosystems to recover, only the most resistant or resilient species are likely to be present as adults when the region is trawled again. Few species can withstand continual disturbance or habitat destruction. Shrimp fisheries in the North Sea of Cortez sweep the entire trawling grounds several times a year. Off the coast of Denmark, some sea beds are trawled almost once a month. A 1992 report from the New Zealand Ministry of Agriculture & Fisheries reported that "the greater the frequency of gear impact on an area, the greater the likelihood of permanent change."

Not Just the Frequency but the Location

Although they constitute less than 8% of the area of the sea, the continental shelves, bays and estuaries which are among the most biologically productive ecosystems on earth are also the most heavily trawled. Offshore, prime fishing grounds lie where continental shelves extend far into the ocean, like the Grand Banks off the New England and Canadian coast, or where currents concentrate nutrients, like the waters off Peru. Fishing in the open ocean is generally far less productive, unless boats manage to locate species that hunt in schools.

Developing technology has now allowed bottom trawling, traditionally confined to shallow coastal seas, to extend into waters up to two kilometers deep.

Unlike shallow areas in tidal zones, whose species are used to being constantly buffeted and disrupted by wave action, these deeper ecosystems are much less resilient to this level of disturbance. Using equipment like rockhoppers, fishing boats can now trawl on any the rockiest of bottoms. Using sonar, global positioning systems and airplanes to locate schools, fishing boats can now descend upon fish anywhere in the world. Tens of thousands of trawlers are gouging ocean bottoms across the globe, from small Gulf Coast shrimpers in tropical waters to factory boats vacuuming subpolar seas. "The changes in ocean fish composition that we're bringing about will actually have shock waves throughout the marine ecosystem," explains Stiassny. "We really can change whole communities, wiping out some species, reducing other species to just a shadow of their former populations, and some of them will never recover again."

Reaching the Bottom of the Barrel

In 1989, the worldwide catch peaked: around 86 million tons of fish and shellfish were taken from the sea. In 1990 and 1991, after a fivefold increase in forty years, the annual take began to decline before more or less stagnating. It has become painfully apparent that the centuries-long increase was owed only to ever-more-efficient fishing methods, which allowed us to expand into new fishing grounds and switch to different species as favored stocks crashed. But many once-abundant fish are now considered commercially extinctthat is, so few in number that it is not profitable to pursue the remaining stockand many of the world's pre-eminent fishing grounds have been largely exhausted. "We're just too good at catching fish," says Jeff Cross simply. Unless fisheries become better managed, further increases in annual marine catch are unlikely. Meanwhile the human population and the demand for fish continue to grow.