Different types of boat dredges
Source: Fish Catching Methods of the World, III ed. p.235.
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|OverviewThese are gears consisting of a mouth frame to which a holding bag constructed of metal rings or meshes is attached. Boat dredges are principally of two variants: dredges that scrape the surface of the bottom and dredges that penetrate the sea bottom to a depth of 30 cm or more to harvest macro-infauna. Some surface dredges include rakes or teeths to penetrate the top layer of substrate and capture animals recessed into the sea bed. Infaunal dredges can be further separated into those that penetrate the substrate by mechanical force (i.e. long teeth) and those that use water jets to fluidize the sediment (hydraulic dredges).Handling EquipmentMost dredges are heavy and require winches and sometimes cranes for handling. Some dredges are mechanized for transporting the catch by pumps or conveyor belts to the deck for sorting.Vessel Overview Dredgers.Fish OperationThe gears are dragged over the bottom, and frequently the gear is heaved and emitted. They scrape up molluscs or shellfish from the sea bed.Target SpeciesDredges are employed to catch a variety of bottom dwelling animals, like mussels, oysters, scallops, clams, sea cucumbers, etc.Water Area OverviewAll over the world.Gear EnvironmentDredges are operated mainly in coastal areas, but sometimes also in deeper waters on the continental shelf, as well as in inland waters.ImpactsSpecies The effect of dredging on the bottom may include 1) bringing stones to the surface, 2) sediment compaction and chemical changes, 3) damage to reef and similar structures, 4) non-catch mortalities and 5) increased vulnerability to predation. The physical effect diminish with time, depending on the level of natural disturbance, influenced by exposure to prevailing weather conditions and tidal strength, depth and sediment type. The degree of dredge effect will be influenced by a number of factors, including: the dredge type, the width and weight, sediment type, number of dredges operated, methods of fishing and whether any form of deflector or rakes are used.|