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Trawls and dredges are nets towed at various depths to catch fish or shellfish. Trawl nets, which can be as large as a football field, are either dragged along the sea floor or midway between the floor and the surface. Trawlers catch fish such as pollock, cod, flounder and shrimp. Bottom trawling can result in high levels of bycatch. Dredging involves dragging a heavy frame with an attached mesh bag along the sea floor to catch animals living on or in the mud or sand; catches include scallops, clams and oysters. Dredging can damage the sea floor by scraping the bottom and also often results in significant bycatch.