The Better Scallop Dredge

by Andrea Cohen, MIT Sea Grant

Testing the scallop dredge in the Irish Sea, off of the Isle of Man.
- Photo by Cliff Goudey, MIT Sea Grant

Cliff Goudey's version of the better mousetrap is the better scallop dredge. The director of MIT Sea Grant's Center for Fisheries Engineering Research is keen on building a better dredge-even though he's the first to admit that current dredges do a fine job of catching scallops, with a $300 million industry in New Bedford alone.

What current dredges don't do, says Goudey, is take into consideration unintended consequences, such as damaging bottom habitat-- a concern since the 1986 reauthorization of the Magnuson-Stevens Act introduced the issue of essential fish habitat. "By taking a rational approach and looking at how dredges actually work rather than by guessing," says Goudey, "we might be able to get better performance from them from the standpoint of catch rates, longevity and even initial cost and at the same time reduce the negative consequences of towing a dredge."

The standard dredge used to harvest scallops consists of a heavy steel towing frame and a chain bag that drags along the sea floor behind the frame. The dredge includes a cutting bar, which has little effect on a perfectly level bottom. However, on a more typical sea bottom with sand waves or humps and valleys, the cutting bar levels the bottom so that the chain bag can scoop up scallops in its path. However, along with the scallops, says Goudey, other organisms living on and buried just below the surface, things biologists call "emergent epifauna," can get caught or damaged.

Is there a way to catch scallops without leveling the bottom in front of the dredge? Goudey figured that would require disturbing or lifting the scallops, in preparation for the chain bag, without physically contacting the ground. The best option for that, he decided, was exploiting hydrodynamic effects through the insertion of flow control devices. So using MIT's towing tank, Goudey experimented with devices of different shapes and sizes to see how they affected scallop shells placed on the bottom of the tank. The most promising results from those tests were implemented in a prototype dredge for fishing trials.

"We built a small dredge fitted with four 11-inch hollow hemispheres mounted at 30-degree angle of attack and positioned close to the seabed. The shapes were mounted on pivots so that if they hit something they could deflect up out of the way," says Goudey. The hemispheres "produce a downward directed jet of water that seems to have a profound effect on scallops when they're hit by it," he explains. The dredge also rides on wheels that replace the skid plates of a normal dredge. "Essentially the scallops get caught up in the large vortices that trail this shape, and they start spinning up in the water high enough so that they'd still be suspended in the water when the chain bag came by. This is exactly the reaction that we were looking for," he states.

In field tests on Stellwagen Bank, the newfangled scallop dredge caught 50% to 60% of a normal catch. "We believe that with a little adjusting of the sweep chain that catch rate could become competitive," says Goudey. He also notes that cameras mounted on the dredge indicated flow patterns seen in the ocean as the dredge was dragged along the bottom were essentially the same as those observed in the tow tank.

A talk Goudey gave at the October 2006 International Council for the Exploration of the Seas (ICES) prompted an invitation from Sam Shepherd and Michel Kaiser of the University of Wales in Bangor, Ireland to try the dredge out on the Isle of Man. So in April, Goudey shipped the dredge across the Atlantic, and then followed along for field tests funded by Ireland's Department of Agriculture, Fisheries and Forestry.

In those trials, the researchers used the dredge aboard a research vessel and a commercial scallop trawler, both with the participation of local fishermen. The dredge was particularly successful in catching queen scallops (called queenies). A lower than expected catch of larger types of scallops suggested that some simple modifications may make the dredge more effective. Additionally, the dredge caused far less damage to scallops than conventional gear, which also suggests an associated decrease in bycatch mortality. Shepherd and Kaiser hope to employ a version of the gear as part of a developing management strategy for Manx/Irish sea scallop and queenie fisheries.

"The idea of developing gear that would both benefit fisheries and the essential fish habitat that supports the resource is always a laudable goal, but not necessarily something that people jump aboard," acknowledges Goudey. His hope is that the research trials will demonstrate that improved fishing gear is good for fishermen and good for preserving the sea floor. He also acknowledges that reducing the habitat impact of the dredge frame still leaves the question about the chain bag being dragged behind it.

"It's an obvious question," says Goudey. "But we have to attack one problem at a time. If we can prove that scallops can be efficiently harvested with a bottom-sparing dredge frame, then the design of the chain bag can be examined to reduce its impact. I already have some ideas."