Will the Fish Return?
A series of immense banks—plateaus submerged in relatively shallow ocean waters—stretches from Newfoundland to southern New England. At the southwestern end of this chain lies Georges Bank, where vast numbers of fish feed and grow. Legend has it that the first European sailors found cod so abundant that they could be scooped out of the water in baskets.
Until the last decades of this century these banks were one of the world's richest fishing grounds—until overfishing on a massive scale brought many fish populations, including cod, haddock, and halibut, to the brink of commercial extinction. Several parts of Georges Bank have been closed to commercial fishing until further notice, and scientists are monitoring the seabed to see how fast it recovers. Recent legislation that requires fisheries to protect marine habitat should provide Georges Bank some breathing room
Fishermen presume that the damage from overfishing is temporary, but the scientific outlook is far from certain. Trawling-pulling nets across the ocean bottom—is inherently destructive, and its long-term effects on marine ecosystems are unknown. In 1990 and 1991, after a fivefold increase in forty years, the annual global catch began to decline. Says Dr. Jeff Cross of the National Marine Fisheries Service, "We're just too good at catching fish."
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What's a Bank?
A bank is a huge shoal—a plateau submerged in relatively shallow ocean waters. A series of immense banks stretches from Newfoundland to southern New England on the edge of the North American continental shelf. The northernmost banks off Newfoundland and Labrador are called the Grand Banks. Georges Bank is an oval-shaped bank, 240 km long by 120 km wide, that lies at the southwestern end of this chain. It is 120 km off the coast of New England and is larger than the state of Massachusetts. Georges Bank is more than 100 m higher than the sea floor of the Gulf of Maine that lies just north of it. During the last Ice Age, when the sea was much lower, Georges Bank was part of the North American mainland.
About 11,500 years ago, the sea rose high enough to isolate the area, creating Georges Island. It was home to many large prehistoric mammals, including walruses, mastodons, and giant sloths, traces of which are sometimes found in fishing nets. They died out around 6,000 years ago, when the water level rose further to submerge the island and turn it into Georges Bank.
A prime breeding and feeding grounds for fish and shellfish, in particular cod, haddock, herring, flounder, lobster, scallops, and clam, these North American banks are one of the world’s most important fishing resources.
Why is the Fishing So Good at Georges Bank?
Georges Bank is a particularly productive continental shelf. The cold, nutrient-rich Labrador current sweeps over most of the submarine plateau, and meets the warmer Gulf stream on its eastern edge.
The mingling of the two currents, along with sunlight penetrating the shallow waters, creates an ideal environment for tiny sea creatures—phytoplankton (photosynthetic algae) and zooplankton (tiny free-floating creatures such as krill)—to flourish, attracting an entire ecosystem of marine animals. On Georges Bank, phytoplankton grow three times faster than on any other continental shelf. They feed the zooplankton, which are then eaten by the larvae of vast numbers of fish such as cod, haddock, and yellowtail flounder. Georges Bank is home to more than 100 species of fish, as well as many species of marine birds, whales, dolphins and porpoises. The combination of tides and the Labrador current create a clockwise flow around the perimeter, circulating eggs and larvae throughout the Bank.
The structural diversity of the seabed plays an important part in the abundance and distribution of different marine species. Fifteen huge canyons descend from the southern half of the Bank. Their craggy walls, out of reach of fishing gear, house many kinds of fish and shellfish. Coarse sediment, originally transported to the bank by glaciers, has been shaped by changes in sea level and the ongoing action of tidal and storm currents to form a variety of marine habitats. For instance, a rough sea bottom provides juvenile cod with protection from predators and also shelters the smaller organisms which are their optimal food sources. Strong tidal currents sweeping over gravel beds on the eastern edge of Georges Bank create ideal spawning grounds for herring, whose eggs are laid on the bottom and require clean, oxygenated water to hatch.
The Basque Secret
The first Europeans to discover these rich fishing grounds were the Basques, a fiercely independent people from northern Spain. They had salt, which they used to preserve the fish, and by the year 1000 they had established an international trade in salted cod. The Basques kept the location of their fishing grounds a secret for over 500 years, but in 1497 Giovanni Caboto, a Genovese known by the Anglicized version of his name, John Cabot, undertook a voyage for Henry VII of England. Searching for a northern spice route, Cabot instead found 1000 Basque fishing vessels, rocky shores ideal for salting and drying fish, and waters teeming with fish. A legend swiftly grew that the fish were so abundant that they could be scooped out of the water in baskets. Cabot named the place New Found Land and claimed it in the name of England. Italian explorer Giovanni da Verrazano discovered Georges Bank in the early 1500s and named it Armelline Shoals after a papal tax collector. In 1605, English colonists renamed it for St. George.
The Cod Trade Grows
“Fishing opened up in Newfoundland with the enthusiasm of a gold rush,” writes Mark Kurlansky in Cod, his book about a fish that changed the world. By the mid-16th century, sixty per cent of all the fish eaten in Europe was cod, and that remained the case for over two hundred years. It was the European hunger for cod that built Boston and turned New England into an international commercial center by the 18th century. Catches of cod and other fish off Georges Bank were so large that the British market became saturated, so Americans expanded to other areas. One was the West Indies, where there was a demand for low-grade salted fish to feed slave laborers. This trade grew when the Gloucester schooner, a fast, two-masted vessel, shortened the sailing time between Georges Bank and the Caribbean in the early 1700s.
In the wake of the American Revolution, fishing rights were hotly disputed. In 1782, the British granted New England fishing rights on the Grand Banks, but these were rescinded after the War of 1812 and remain a source of tension between the United States and Canada to this day.
New Tools for Bigger Catches
The first sign that Georges Bank fish stocks were not inexhaustible was the near disappearance of halibut around 1850, after an intense period of overfishing. The advent of modern fishing technology in the 1900s spelled trouble for many other species.
Well into the 20th century, Georges Bank had been fished using the same tools and techniques that the first settlers had employed: small boats, propelled by sail or oars and fished with handlines, and a single baited hook (perhaps two if a spreader was used) let down with a weighted line and reeled in by hand.
In Europe, on the other hand, where competition and smaller catches provided more incentive, steam-powered trawlers—ships which drag fishing gear behind them—were in wide use by the 1880s.It was not until the 1920s that the technology crossed the ocean and a Boston trawler fleet developed. “Fish could now be pursued,” observes Kurlansky,and so they were, across ever-greater distances. The steam-powered otter trawl proceeded to decimate the Georges Bank haddock stock. Diesel power, introduced in 1928, further increased the ships’ efficiency.
The Birth of the Fish Stick
The other invention that transformed the fishing industry was the brainchild of Clarence Birdseye, the inventor of frozen foods. Birdseye moved to Gloucester in 1925 and founded the General Seafoods Company at a time when the international market for fresh fish, as opposed to cured or salted, was growing. Filleting machinery was introduced to New England in 1921. The fillets were frozen into blocks and sliced into strips, and fish sticks were shipped to a giant new market of consumers, many of whom never encountered fish in any other form.
After World War II, the advent of huge factory ships and the use of aircraft and sonar to spot schools of fish resulted in unprecedented commercial catches. Through the late ‘50s, ‘60s, and early ‘70s, fleets of factory ships from the Soviet Union, East Germany, Poland, Spain, Japan and elsewhere hauled in hundreds of millions of pounds of haddock and hake, sometimes only twenty km from shore. In an hour, a factory ship could haul in as much cod—around a hundred tons—as a typical 17th-century boat could catch in a season. Wishing to preserve its fish stocks for American fishermen, the U.S. passed the Magnuson Act in 1976. It established American jurisdiction over a 200-mile fishing limit and banned the foreign boats from U.S. waters. In 1984, under international arbitration, Canada was granted the northeast corner of Georges Bank, which lies within 200 miles of Nova Scotia, reducing New England fishing grounds. With the international factory ships gone, Canada and the U.S. missed the opportunity to restore a sustainable groundfish industry, choosing instead to exploit the resource themselves. Domestic fishing fleets expanded rapidly, and offshore commercial fisheries grew and prospered.
From Boom to Bust
At the same time, inshore stocks dwindled. Many Georges Bank fish populations declined, including cod, haddock, herring, and sea scallops. Local fishermen suspected that few fish were surviving to spawn on the Bank, a breeding ground for well over half of the most commercially valuable fish species. Although the government agencies reluctantly recognized that these stocks were declining, the fishermen’s concerns still found few listeners. The New England Council, which had been established by the Magnuson Act, was dominated by commercial fishing interests. Finally, in 1993, Canada declared a moratorium on fishing northern cod and placed strict quotas on other ground species. A 1994 National Marine Fisheries Service assessment of cod stock on Georges Bank found a drastic forty per cent decline over four years, and concluded that the fishing fleet was about twice the size that Georges Bank could sustain.
The waters had been rendered nearly devoid of the prime commercial species that had once filled them. Urgent measures were necessary. On December 7, 1994, officials closed 9600 square kilometers of fishing ground on Georges Bank. The ban was extended indefinitely in April, 1995, and still stands. A March 1997 update reported that while some stocks were beginning to grow again, groundfish were still being fished too hard to regain healthy levels. In January 1999, scientists at the National Marine Fisheries Service in Woods Hole, Massachusetts reported a continued rapid decline in cod stock. Today, fishing continues in certain areas, but it is severely regulated.
An Uncertain Future
Fishermen presume that the damage from overfishing is temporary, but the scientific outlook is far from clear. Kurlansky quotes Ralph Mayo of the National Marine Fisheries Service laboratory in Woods Hole: “there is no known formula to predict how many fish—or in scientific language, what size biomass—are required to regenerate a population or how many years that might take.” In the meanwhile, species such as the skate have expanded rapidly in response to the changing species dynamics, with as yet unknown consequences for the Georges Bank ecosystem. Some have become new targets of commercial fishermen. Political pressure to loosen regulations is unending and heedless of nature’s timetable. New England fishermen grumble about boats lying idle along the New England coast. Conservationists fear that regulators will ease restrictions before populations are fully recovered. “The problem with the people out here on the headlands of North America,” observes Kurlansky, “is that they are at the wrong end of a 1,000-year fishing spree.”
On June 1, 1999, the research vessel Albatross IV set out on a ten-day cruise of the eastern Georges Bank. Built in 1962 for the National Marine Fisheries Service’s (NMFS) biological lab at Woods Hole, the Albatross IV is specially equipped to collect information on the distribution and habitat of groundfish and sea scallops in the Northwest Atlantic Ocean.
Georges Bank is the southernmost part of a chain of huge shoals that extend from Newfoundland to southern New England, on the edge of the North American continental shelf. They were one of the world’s richest fishing grounds until the last decades of this century, when overfishing on a massive scale brought many fish populations, including cod, haddock, herring, and scallops, to the brink of commercial extinction. “In the last ten years a huge amount of biomass has been removed,” says Dr. Jeff Cross, Chief of the Ecosystem Process Division of the Northeast Fisheries Science Center of the National Marine Fisheries Service. “Some areas were trawled several times a year.”
A Unique “Before and After” Opportunity
The Albatross IV cruise was designed to take advantage of a unique window of opportunity. In 1994, two areas of Georges Bank were completely closed to fishing and scallop dredging in order to give juvenile groundfish a chance to survive and grow. [link to G3-Trawling takes a Toll] In 1999, half of what had been designated Closed Area II was about to be reopened to scallop fishing. The Albatross IV cruised primarily within Closed Area II and the surrounding waters to monitor the resilience of the ocean bottom by comparing the areas closed to fishing, which had been untouched for four and a half years, with grounds outside the boundaries. Researchers designed a study which paired 48 stations in and out of the closed area.
Dr. Cross had observed that inside the protected area scallops were far larger, 7 to 12 per pound compared to 20 to 30 per pound outside. “The ones outside the area never have a chance to grow,” he explains. “This is a new area to get a lot of attention from scientists. There is good evidence that in areas where animals use rocks for shelter, it takes years for this kind of habitat to recover.” The underlying objective of the current study is to provide accurate scientific information on which to base sound management and legislation decisions. “We're trying to characterize the habitat from a geological and a biological point of view,” explains Page Valentine, a geologist with the U.S. Geological Survey. ‘When we’ve obtained some baseline information about the different kinds of habitat, their function, their use by certain species, we can provide it to managers of the fisheries. Then they can determine how the bottom is going to be utilized. Right now,” he continues, “so little is known about the function of these habitats and their distribution that it's hard for the managers to make decisions.”
Sampling the Ocean Bottom
“We’re interested in biodiversity and we want to get a handle on what goes on down there, because we don't really know. And basically the only way to do it, except for using a submarine or a remote operated vehicle, is to bring part of it up,” explains Dave Packer of the NMFS. Working on the sea bed is far more difficult than on land, and scientists are restricted in how much information they can gather while at sea. The research is designed to produce an overall portrait of the different types of bottom habitats and the organisms they support. Monitoring the sea floor is an ongoing process that ideally requires constant updating, since environmental change and fishing patterns can greatly affect the growth rates of many species, setting up a complex feedback loop.
The crew sampled the ocean bottom using three different techniques:
an otter trawl
This standard trawling gear consists of a large net pulled along the ocean bottom. The upper edge is held up by floats and the net is kept open by giant wooden or steel planks on either side, called otter doors. In this study, a 15-minute haul was made at each station, and the catch from the grab counted, weighed and measured. The scientists also looked at the fish's stomachs in order to make a direct correlation between what the fish are eating and what kind of animals actually live there. “We want to understand the food chain because a lot of what lives in and on the bottom is basically fish food for many of our managed species,” explains Packer. “A lot of the various trawls and nets that the fishermen put out have the potential to disrupt that bottom habitat. And you know, if you destroy the food web down there or disrupt the food web, you're gonna eventually be hurting the fishes that you're trying to catch.”
a Seabed Observation and Sampling System (SEABOSS)
This system uses two video cameras (one pointing forward and one downward), a 35mm still camera focused on the ocean floor, and an instrument called a grab sampler. As the Albatross IV drifts for 20 minutes, the apparatus “flies” above the sea floor, videotaping it in real time. At the same time, representative still photographs are taken and samples of the top few centimeters of sediment collected. The samples are used to determine grain size, and the images serve to quantify the distribution of different microhabitats and their relationship to fish. “Much of the fish that is harvested here off the East Coast are groundfish, which live on the bottom for most of their lives. So the nature of the bottom is really very important to them, as a source of food and of refuge from predators,” says Valentine. “It's really their environment, so it's really a requirement for fisheries management to know what factors are altering it, and actually forming it.”
a Smith-McIntyre a spring-loaded bottom sampler
This is a metal frame with two jaws that snap together when they hit bottom. It collects one -tenth of a square meter of bottom—mud, rocks, or sand—as well as the tiny animals, mostly invertebrates, that live in or on the bottom: worms, crustaceans, sand dollars, scallops and other mollusks. “We sift out all the sediment to leave just the animals behind, identify them, count them and weigh them, to get an idea of the actual community structure: how many animals there are and who might be eating whom, because we know the life histories of a lot of these animals,” says Packer. Different types of bottom are home to very different communities, and some are highly diverse, “especially areas that are mixtures of both sand, mud and larger boulders and rocks. They provide habitat for young fish and they provide habitat for the invertebrates upon which the fish feed,” he explains. Other instruments also measured water temperatures and salinity.
Back to the Lab
When the Albatross IV returned from its research cruise, the various samples--the images, VanVeen grab samples, marine organisms preserved in formalin, and feeding ecology data--were turned over to the NMFS laboratory in Woods Hole, Massachusetts, for analysis, while sediment core samples went to the NMFS’s Howard Laboratory in Highlands, N.J. “Probably within 6 to 9 months we will know if we see a difference between the two areas, and an indication of whether or not the change is significant,” says Cross. “It’s a snapshot, but a significant one,” says Dr. Peter Auster, science director of the National Undersea Research Center at the University of Connecticut, who also participated in the cruise.
And Back to the Ocean
The information gathered and analyzed from the June 1999 cruise will provide the scientists with one data point in time. More data points, on shorter time scales, are needed for a more complete portrait of the sea bed’s condition. Since frequent assessments aren’t logistically or financially feasible, the idea is to pick key areas and habitats and use that information as a guide for management. Within two months of Closed Area II's reopening for scallop fishing, 145 vessels had removed a total of 3.1 million pounds of scallops. That constitutes one-third of a total allowable catch of 9.4 million pounds, and the season extends to the end of the year. The NMFS has a cruise planned to the same area for June, 2000, at least six months after the scallop fleet has departed. As in 1999, the crew will assess the number of fish, the number of invertebrates, and the appearance of the habitat. “We’ll see what’s there that wasn’t and not there that was, and then we’ll combine all this information for a second data point,” says Cross.
“How quickly do these habitats recover from trawling and dredging?” asks Cross. “That’s the crux of the issue. How quickly do animals come back, does the structure of sediments return to their former structure? If recovery takes six months to a year, then that gives you one option for managing. If it takes a decade that that’s a whole different issue.” Studies have shown that communities in sandy habitats recover more quickly than those in gravel or hard-bottom habitats, where organisms are more long-lived and disturbed less often. It’s a complex equation, into which the timing and nature of fishing efforts also need to be factored. “In addition, we need to understand the linkages between sea floor habitats and economically important fishes,” says Auster. “More fundamentally, we need to view fish and fish habitat as components of regional biodiversity.”
What Lies Ahead for Georges Bank?
For the first time in 30 years, there are some signs of recovery of groundfish on Georges Bank. Growing awareness of the impact of trawling on marine ecosystems and of the need to preserve marine habitat is leading to more research and more effective legislation, such as the 1996 Sustainable Fisheries Act. One fisheries management concept is area rotation, in which certain places are closed for the amount of time it takes for stock to regenerate or the habitat to recover. However, Auster points out that, unlike in the case of crop rotation, “we’re not talking about planting anything. We’re talking about altering a natural system and assuming that during the period when we’re not altering it, the system will return to a state that’s economically productive.” Studies like the one conducted by the Albatross IV will help establish where and at what intervals area rotation might be effective.
Some areas of Georges Bank remain completely closed to fishing. On paper, however, the only reason is to give cod, haddock and yellowtail populations time to recover, “not for conservation or habitat integrity or biodiversity. Once those particular fish populations have recovered, then there is all the rationale in the world to open them up. Is that something we want to do?” asks Auster. He proposes a larger-scale priority. “If we start managing for maintenance of biodiversity--as opposed to looking at fisheries and biodiversity as two totally different objectives--then sustainable fisheries should be a natural outcome.”
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?"
- 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 sediment - whether, for example, it's composed of coarse sand or fine silt - trawling 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 extinct - that is, so few in number that it is not profitable to pursue the remaining stock - and 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.