Source: FAO-Fish.Tech.Pap.222, p. 26
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|OverviewA beam trawl consists of a cone-shaped body ending in a bag or codend, which retains the catch. In these trawls the horizontal opening of the net is provided by a beam, made of wood or metal, which is up to 12 m long. The vertically opening is provided by two hoop-like trawl mostly made from steel. No hydrodynamic forces are needed to keep a beam trawl open.heads/shoesPlan|Accessory EquipmentWhile fishing for flatfish the beam trawl is often equipped with tickler chains to disturb the fish from the seabed. For operations on very rough fishing grounds they can be equipped with chain matrices. Chain matrices are rigged between the beam and the groundrope and prevent boulders/stones from being caught by the trawl. Shrimp beam trawls are not so heavy and have smaller mesh sizes. A bobbin of groundrope with rubber bobbins keeps the shrimp beam trawl in contact with the bottom and gives flatfish the opportunity to escape.Handling EquipmentBeam trawl is normally towed on outriggers, one trawl on each side.Vessel Overview Beam trawlers are in most cases specialized medium size vessels, equipped with powerful engines arranged with large outriggers tow two parallel beam trawls.Fish OperationClose bottom contact is necessary for successful operation. To avoid bycatch of most juvenile fishes selectivity devices are assembled (sieve nets, sorting grids, escape holes). While targeting flatfish the beam trawls are towed with up to seven knots, therefore the gear is very heavy; the largest gears weight up to 10 tonnes. The towing speed for shrimp is between 2.5 and 3 knots.Target SpeciesBeam trawls are used mainly for flatfish and shrimp fishing.Water Area OverviewThis technique is very successful used in many parts of the world for catching shrimp and flatfish normally in shallower depth than 100 meters.Gear EnvironmentBeam trawls are towed with very close bottom contact.ImpactsSpecies Conclusions from some recent research on the physical impact of beam trawls on the sea bed are: 1) the penetration depth of a beam trawl depends on sediment characteristics and varies between 1 cm and 8 cm; 2) the pressure force exerted on the sea floor is strongly related to towing speed and the warp length to depth ratio; 3) beam trawls leave detectable marks on the seabed. The duration that the beam trawl marks remain visible depends on the upper sediment layer and on the hydrographic conditions. On the seabed consisting of medium to coarse sand, tracks have been observed to remain visible for up to 6 days. On sediments with mainly finer particles a corresponding figure of 37 hours was observed. The major impact of beam trawl on species is capture and removal from the ecosystem of small sized organisms (juveniles) and non-target species, which frequently are discarded at sea. Sieve netting and selection grids together with larger codend mesh sizes are used to mitigate this problem, particularly in shrimp trawl fisheries. Square mesh panels are used with some success to reduce capture of non targets species in flatfish fisheries.
Principal features of a beam trawl