[Editors] MIT aims for kinder, gentler scallop dredge

[Elizabeth Thomson]

MIT News Office
Massachusetts Institute of Technology
Room 11-400
77 Massachusetts Avenue
Cambridge, MA  02139-4307
Phone: 617-253-2700
http://web.mit.edu/newsoffice/www

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MIT aims for kinder, gentler scallop dredge
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For Immediate Release
TUESDAY, JULY 31, 2007
Contact: Elizabeth A. Thomson, MIT News Office
Phone: 617-258-5402
Email: thomson at mit.edu

PHOTO AVAILABLE

STORY ONLINE AT: http://web.mit.edu/newsoffice/2007/scallop-0731.html


CAMBRIDGE, Mass.--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 wants to build a better dredge-even though he's the first to  
admit that current dredges do a fine job of catching the creatures.

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.

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.  
But along with the scallops, says Goudey, other organisms living on  
and buried just below the surface 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 to use jets of  
water. So Goudey experimented with devices of different shapes and  
sizes to see how they affected scallop shells placed on the bottom of  
MIT's towing tank. The most promising results were implemented in a  
prototype dredge.

“We built a small dredge fitted with four 11-inch hollow hemispheres  
positioned close to the seabed and 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. Goudey notes that most mobile creatures near the dredge can  
escape from its path. “While a conventional dredge impacts subsurface  
organisms, this one does not,” he said.

“Essentially the scallops...start spinning up in the water high  
enough so that they're still suspended in the water when the chain  
bag comes by.”

In field tests on Stellwagen Bank off the Massachusetts coastline,  
the newfangled scallop dredge caught 50- to 60 percent of a normal  
catch. “We believe that with a little adjusting...that catch rate  
could become competitive,” says Goudey.

A talk Goudey gave prompted an invitation from the University of  
Wales in Ireland to try the dredge out off the Isle of Man. So in  
April, Goudey shipped the dredge across the Atlantic, then followed  
along for field tests.

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. 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. As a result, Ireland may  
employ a version of the gear as part of a developing management  
strategy for scallop fisheries.

--END--

Written by Andrea Cohen, MIT Sea Grant