S.G.S.O.
Dredging
Dredging is when fishermen drag a heavy frame with an attached mesh bag called a dredge along the seaflorr to catch bottom-living fish fish such as scallops, clams, oysters, and other shellfish that live on the seaflorr or burrow into mud or sand. Some dredges have metal "teeth along the base of the frame that act like rakes, scooping up shellfish as it moves along. The dredge is controlled by the boat that is pulling it across the ocean floor. The bottom of the ocean is a significant habitat and dredges cause a lot of damage when they are dragged along gravel and rocky bottoms. They can break coral reefs and even kill non-resourceful shellfish and even small fish. Their is a huge bycatch from dredgind and most likely the marine life are unable to survive under the weight of the heavy bag. Dredges also smooth out sandy and muddy habitats, removing a variety of habitats and plant and animal life. Dredging is very similar to twaling in that both drag very heavy fishing material through the ocean floor destroying many habitats.


Fish dredging is also known as scallop dredging and oyster dredging. The dredge is made up of a heavy steel frame in the form of a scoop with a chain mesh net. Early dredges were created with teeth, called tynes that acted like a rake. This design was improved by using spring-loaded teeth that reduced bottom snagging and so it could be used over rough ground such as gravel. Dredging nets have coarse mesh in order to let smaller organisms in which are their target species. The net also catches larger marine animals such as whelks, starfish, and octopus which could cause harm and death to these animals. Dredging is not always destructive. The dredge ploughs the sea bottom to a depth of a few inches, but this layer of the sea bottom is often turned over anyway by fast burrowing organisms like worms, and is periodically disturbed by storms. Scallop beds can be exploited in sustainable ways, providing sufficient unexploited areas remain for male and female scallops to mix spawn. Still, scallop dredging tends to result in scallops containing grit, and can damage the seabed if done carelessly, so these days scallop dredging is often replaced by suba diving.


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http://en.wikipedia.org/wiki/Fishing_dredge
http://www.montereybayaquarium.org/cr/cr_seafoodwatch/sfw_gear.aspx

"Maintenance Dredging and the Environment"

"Maintenance dredging refers to the routine removal of accumulated sediment from channel beds to maintain the design depths of navigation channels, harbours, marinas, boat launches, and port facilities. As defined by the corps of Engineers - Baltimore, routine maintenance dredging is conducted regularly for navigational purposes (at least once every five years) and typically does not include any expansion of the previously dredged area. Dredging is usually conducted by mechanical methods such as clam buckets or draglines for deep draft channels and by hydraulic dredges for shallow draft channels. The main characteristics of maintenance dredging projects are;

  • variable quantities of material;
  • soft, uncompacted soil;
  • contaminant content possible;
  • thin layers of material;
  • occurring in navigation channels and harbors;
  • Repetitive activity
Since maintenance dredging occurs mainly in artificially deepened navigation areas, the dredging activity is, in itself, not necessarily damaging to the natural environment. The main potential for environmental impact is from the disposal of the dredged material and by the increasing quantities of suspended sediments during the dredging process (possibly inducing dispersion of contaminants). Suspended sediment problems can, however, be readily controlled by careful choice of dredging equipment and procedures as discussed later. These problems are compounded by the need to repeat maintenance dredging regularly, since siltation is a never- ending story. The contaminant content of the material to be dredged can also have an important bearing on the environmental impact. Many of our cities have for a long time allowed their sewage and industrial waste to spill out into navigation channels, while the silt on the bottom of our rivers has over the years soaked up numerous contaminants that have entered the water stream. Dredging can spread the particles to which the contaminants are attached and increase the speed with which they spread. The extent of this phenomenon depends upon the type of dredging equipment used and the attentiveness of those engaged in the dredging work. From a fisheries perspective, the largest threat to fish habitat from these activities is the increased amount of suspended sediments introduced to the water column during the dredging process."
http://www.nab.usace.army.mil/whatwedo/dredging.htm