Scallop Drag
A crushed lobster left in the path of a scallop drag. Image by A. Shepard, OAR/National Undersea Research Program (NURP); University of North Carolina at Wilmington.
DB: This is Earth and Sky, with a story about scallop harvesting along the coast of Maine.
JB: Scallops are mollusks that live on the sea floor. A massive device called a drag is commonly used to harvest them in a process called dredging. A metal frame with teeth rakes the sea floor as it’s dragged by a boat, and scallops are collected into a chain-mesh bag. There’s not much known about what dredging does to sea-floor animals or habitat. Les Watling, a researcher at the University of Maine’s Darling Marine Center, decided to take a look.
Les Watling: So what we did was, we set up a couple of plots on the bottom, one that… we weren’t going to touch at all, and one that we were going to drag a scallop drag over.
DB: Watling’s crew sampled both plots before and after dredging to see what happened to the sediment community.
Les Watling: What we found… was that the scallop drag, at least in the shallow water environment like this, tends to kick up the sediment, scrape it off the surface, and throw it up into the water… By and large what the story seemed to come down to was that the animal groups that were missing from the drag site were animals that relied on high quality organic matter for their food.
JB: When sediment was churned up, organisms, like bits of algae that scallop feed on, washed right out. Thanks to the U.S. Forest Service and to the National Fish and Wildlife Foundation – supporting the conservation of native fish, wildlife, plants and their habitats. We’re Block and Byrd for Earth and Sky.
The following individual was interviewed for today’s program. Our thanks to:
Dr. Les Watling
Professor of Oceanography
Darling Marine Center
University of Maine
Walpole, Maine
The following web sites and articles provided information relevant to this script:
Linklater, Magnus. A graveyard at the bottom of the sea. The Times (London), Feb. 26, 2002.
Weiss, Kenneth. Study urges trawling ban in fragile marine habitats. Los Angeles Times, March 19, 2002, page 12.
Interview with Dr. Les Watling on the website of the PBS television series, Empty Nets, Empty Oceans.
Sparks-McConkey, P.J., and L. Watling. Effects on the ecological integrity of a soft-bottom habitat from a trawling disturbance. Hydrobiologia vol 456:73-85 (July 2001).
Auster, P.J., R.J. Malatesta, R.W. Langton, L. Watling, P.C. Valentaine, C.L.S. Donaldson, E.W. Langton, A.N. Shepard, and I.G. Babb. The impacts of mobile fishing gear on seafloor habitats in the Gulf of Maine (northwest Atlantic): Implications for conservation of fish populations. Reviews in Fisheries Science, vo. 4:185-202, 1996.
Watling, Les. And Elliott A. Norse. Disturbance of the seabed by mobil fishing gear: A comparison to forest clearcutting. Conservation Biology 12:1180-1197; 1998.
United Nations Systemwide Earthwatch – Crisis in Ocean Fisheries
Diagram of a scallop dredge (New Jersey Fishing)
Scallop Dragging (Canada’s Digital Collection)
Author’s Notes:
Scientists say a new kind of fishing gear, the rockhopper trawl, introduced in the 1980s, is turning the ocean equivalent of ancient forests into the ocean equivalent of cattle pastures. Trawling, say marine researchers Les Watling and Elliott Norse, each year severely disturbs an area of seabed as large as Brazil, the Congo, and India combined.
Once upon a time, rockstrewn stretches of ocean bottom existed as natural marine sanctuaries. Trawlers avoided them because the rocks would tear up the expensive trawl nets. But rockhopper trawls are fitted with large, rubber or metal rollers, so that the net rides up and over boulders on the bottom. These trawls are often used in water as deep as 1000 m or more. They knock over and smash everything in their path. Sponges, cold-water corals (only recently discovered in the deep ocean), and other bottom-dwelling organisms animals that, like trees in a forest, create essential structure where other animals can find shelter-are all destroyed. Deep sea creatures are long lived but grow slowly so it takes a very long time for the habitat to recover. Even if animals do recolonize an area, biodiversity is often reduced.
In March 2002, the U.S. National Academy of Sciences released a report that identified areas in the North Atlantic, along the Gulf and Pacific Coasts, and around Alaska where trawling is a cause for concern, and recommended that the federal government place new limits on bottom trawling. In the same month, U.S. Representative Joel Hefley of Colorado introduced the Ocean Habitat Protection Act to Congress. The proposed legislation would ban rockhopper trawls altogether. The states of California, Maine, and Florida already have restrictions on trawling in some locations.
Rockhopper trawls damage bottom-dwelling organisms in rocky seafloor habitat. But how about muddy or sandy seafloor habitat- is it OK to trawl there? Dr. Les Watling has been trawling impacts on sandy bottoms, and he says trawls also affect the seafloor communities here in several different ways.
For one thing, trawls mix the top layer of sediment up into the water column. When this happens, much of the organic matter that was mixed with the sediment is washed away. This is a problem for detritivores (detritus-eaters) living on or in the sediment they rely on the bits of decaying algae that sink to the sea floor for food.
Another problem is that on sandy or muddy bottoms, many animals, such as marine worms, live right in the sediment, in elaborate burrows. Trawling digs up these burrows, evicting animals from their homes. Even if they are able to burrow back in, they burn extra energy while doing so, and risk expose to predators. And many areas are trawled repeatedly.
A third problem is that, when sediment is kicked up into the water by a trawl and then sinks back to the ocean floor, the consistency of the sediment changes. Some animals just can t survive under these conditions.
Finally, the seafloor organisms are often a source of food for juvenile fish. By destroying seafloor habitat, fishermen take away food from the next generation of fish. So the fishery is no longer a renewable resource.
