These 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).
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.
(source: FAO site, 2017).