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Effects of Trawling and Dredging on Seafloor Habitat
1990 with the implementation of the Magnuson-Stevens Fisheries Conservation and Management Act (1977), which extended U.S. jurisdiction over fishery resources throughout the newly declared 200 mile exclusive economic zone. Large-scale domestic catcher–trawler and mothership operations arose in the waters off Alaska (Ianelli and Wespestad, 1998) and from the coast of northern California to Washington. In New England, the trawl fisheries also expanded as larger, more efficient vessels were introduced into the fleets. Currently, there are major groundfish trawl fisheries in New England and Alaska. The shrimp trawl fishery dominates in the Gulf of Mexico and in shallow waters off the coasts of the Carolinas and Georgia. Groundfish and shrimp trawl fisheries operate off the coasts of Washington, Oregon, and California.
Most dredges are rakelike devices that use bags to collect the catch. They typically remove molluscan shellfish from the seabed, but occasionally are used for crustacea, finfish, and echinoderms. Dredges take either epifauna or infauna; the design details of the gear are fauna specific. On soft bottoms, the dredge flattens the microrelief on the seabed (wave-ripples) and resuspends fine sediments. On hard rocky bottoms, the dredge scrapes off epibenthic organisms and disturbs the substrate.
In estuarine waters, dredges are used to collect clams, oysters, conch, and crabs. The oyster dredge consists of a steel frame, 0.5–2.0 m wide, with an eye and “nose” or “tongue” and a blade with teeth. The tow chain or wire and a bag for the catch are attached to the frame. The dredge is towed slowly (<1 m/s) in circles, from vessels that are 7–15 m long. Similar dredges are used to catch blue crabs in the mid-Atlantic region during the winter. Stern-rig dredge boats (averaging 15 m long) drag two dredges in tandem from a single chain warp. The dredges are equipped with long teeth (10 cm) that dig the crabs out of the bottom. The same gear is used in the Chesapeake Bay to catch whelk in summer and mussels in fall.
In the soft clam fishery, which occurs in shallow estuarine waters, the dredge head (manifold and blade) is attached to a conveyor or belt that carries the materials retained on the blade to the working deck of the vessel. These dredge vessels (generally about 15 m long) are restricted to shallow water—less than one-half the length of the escalator. Because soft clams live in shallow waters (2–6 m), this poses few limitations for the small vessel operators who use this gear.
Offshore, large dredges catch sea scallops, which inhabit a sand–gravel–cobble bottom and live on the surface of the seabed. Because scallops sense and retreat from slow-moving dredges, scallop dredges are towed at speeds of up to 2.5 m/s. The scallop dredge has a steel frame with a tongue with an eye, a blade with no teeth, and a bag (Figure 2.1). The width or mouth opening of the dredge ranges from 3.0 to 4.5 m, and dredge weight varies from 500 kg to 1000 kg. The largest scallop dredge vessels, about 60 m long drag two 4.5 m dredges, one from each side of the vessel, and they use winches and navigational electronics to maintain high efficiency. Scallop dredges disturb the seabed. Disturbance of the seabed by dredges is required to dislodge scallops for capture in the net.
Special methods are used to gently extract clams that burrow in sediments. Water jets are directed into the seabed to liquefy the sediment suspending the clams up in a sediment slurry. This blade sieves the clam– sediment slurry. The hydraulic clam dredge is used offshore to collect surf clams and ocean quahogs and inshore to collect soft clams. Hydraulic dredges are efficient, but they restructure sediments and disturb sediment biota.
Offshore vessels (>30 m) slowly tow dredges as wide as 4.5 m across the seabed. The vessels have
FIGURE 2.1 Action of a scallop and mussel dredge as it is dragged over a soft bottom seabed. The micro-relief of the seabed is smoothed as the dredge is towed over it. The gear in this figure is representative of scallop dredges used in many U.S. fisheries (DeAlteris et al., 1999; used with permission from American Fisheries Society).