Fishing dredge

Construction

The dredge is usually constructed from a heavy steel frame in the form of a scoop. The frame is covered with chain mesh which is open on the front, which is towed. The chain mesh functions as a net.

Dredges may or may not have teeth along the bottom bar of the frame. In Europe, early dredges had teeth, called tynes, at the bottom. These teeth raked or ploughed the sand and mud, digging up buried clams. This design was improved by using spring-loaded teeth that reduced bottom snagging, and so could be used over rough ground. The New Bedford (USA) dredge does not have teeth.

Dredge nets have a coarse mesh in order to let organisms smaller than the target species through. The net catches the larger organisms: in the case of scallop dredging that includes the scallops' predators, such as whelks, starfish and octopus.

In some cases, several dredges are attached to a wheeled rigid axle in groups of three or four. A number of these dredges can be towed from a heavy spreading bar, usually one from each side of the vessel. The length of the bar and number of dredges towed depends on the power of the vessel and the room on the side deck for working the dredges. The number might be three on each side of a small 10 m boat up to 20 on each side for a 30 m vessel with 1500 hp. The great weight and strength of the gear can disturb the ground it is towed over, overturning rocks and dislodging and crushing organisms in its path.

There is still a challenge for inventors to produce a more gentle scallop dredge.

Types of dredgers

File:Mussel dredgers at Carlingford harbour - geograph.org.uk - 174327.jpg|Mussel dredgers File:N.58, Pascin - Nieuwpoort.jpg|Fishing vessel equipped with a benthic dredge, leaving the port of Nieuwpoort File:Oyster boats, Carrick Roads - geograph.org.uk - 625045.jpg|Oyster boats of the Truro oyster fleet. This fishery is the last in the world to work by sail alone

Dredging history

File:FMIB 34138 Scallop dredge.jpeg|Scallop dredge, 1889 File:PSM V06 D022 Vessels dredging for oysters.jpg|Vessels dredging for oysters, c. 1875 File:Oyster dredging boat, Ballina NSW (Northern Star (Lismore), 29 Sep 1926, p 13).jpg|Rare photograph of an oyster dredging boat, used in Australian estuaries (Ballina, NSW, 1926)

Dredging art

File:Twachtman John Oyster Boats North River.jpg|Oyster Boats North River, by John Henry Twachtman (1853–1902) File:Oyster Sloop, Cos Cob c1902 Childe Hassam.jpg|Oyster Sloop, Cos Cob, by Childe Hassam (c.1902) File:Oysterdredging1.jpg|Dredging For Oysters, by Alexander Rummler. WPA mural. File:Hemy Ch N@Einheimische in Falmouth.JPG| Oyster dredging, by Charles Napier Hemy (1841–1917)

Notes

References

• National Research Council (US) (2002) Effects of Trawling and Dredging on Seafloor Habitat. Committee on Ecosystem Effects of Fishing.
• Hall-Spencer & Moore PG (2000) Scallop dredging has profound, long-term impacts on maerl beds. ICES Journal of Marine Science. 57, 1407-1415.

References

1. 0-632-05608-8. Page 14
2. [http://www.seafish.org/upload/b2b/file/r__d/DREDGES_9.pdf Dredges] {{webarchive. link. (2009-03-26 seafood.org. Retrieved 11 February 2009.)
3. [[Massachusetts Institute of Technology. MIT]] (2007) [https://www.sciencedaily.com/releases/2007/07/070731125746.htm ''Kinder, gentler scallop dredge invented.''] Retrieved April 13, 2008.