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Trawl nets are towed by either one (single trawl) or two (paired trawl) vessels. Trawl nets are funnel-shaped, with sides extending forward to form wings which guide fish into the mouth of the net and down into the cod-end. The mesh size in the cod-end, as well as a range of other devices, can regulate the size and species captured to some degree.
While being towed, the mouth of the net is kept open by beams, trawl doors (shearing devices used to spread the net horizontally) or by the distance between the two towing vessels (when pair trawling). Fully mechanised winches, power blocks and net drums are used to move, store, shoot and haul the gear. Trawling takes place from continental shelf waters (<200m) out to deep sea fisheries (waters >200m deep).
Beam trawls target demersal fish species on the seabed and are towed in very close contact with the sea floor. The mouth of a beam trawl is held open by a rigid framework to ensure the net maintains its shape and effectiveness, despite any change in towing speed. Horizontally, the net is held open by a heavy steel beam which is supported by steel beam heads at each end. These beam heads have wide ‘shoes’ which slide over the seabed. Most commercial beam trawlers tow two beam trawls attached to long booms projecting from each side of the vessel. Lighter beam trawling gear can employ ‘ticklers’ which disturb fish on the seabed, causing them to rise and be caught in the net. Beam trawls can generally be towed faster than bottom trawls and are used only in continental shelf waters (<200m depth).
Depending on the substrate, beam trawling can have a detrimental impact on sea floor habitats. There are also issues relating to bycatch of non-target species and under-sized target specimens, although advances in design can mitigate bycatch in some fisheries. Historically, within British waters, unregulated beam trawling has contributed to significant declines in some demersal shark and large skate species - with the Angelshark (Squatina squatina), Common Skate (Dipturus batis complex) and White Skate (Rostroraja alba) now listed as Critically Endangered.
Like beam trawls, bottom trawls target fish species found on the seabed and are also towed in very close contact with the sea floor. Rather than using a beam, bottom trawls use trawl doors to keep the mouth of the net open. As well as being heavy enough to keep the gear on the sea floor, the trawl doors also kick up large clouds of sediment which ‘herd’ fish into the net. Along the bottom of the net is the ground-gear which is in constant contact with the sea floor and protects the fragile netting from damage. Ground-gear comes in different forms, depending on the substrate on which the trawl is being towed: on soft sand and mud a heavy chain is used, whereas on rougher ground heavier rubber discs and wheels are used.
Like beam trawls, bottom trawling can also have a detrimental impact on sea floor habitats and experiences the same issues relating to bycatch of non-target species and under-sized target specimens.
Mid-water (pelagic) trawling
Mid-water trawls target pelagic fish such as herring, sardines and mackerel as they shoal in the water column; this method of trawling has little or no contact with the seabed. Mid-water trawl nets are generally much larger than those used for bottom and beam trawling - and can be 160m deep and 240m wide. The horizontal opening of the net is maintained either by trawl doors (single boat) or by the distance between the two boats towing the net (pair trawling). Large vessels are capable of catching several hundred tons of fish in one haul - with the fish then usually pumped aboard. As with beam and bottom trawling, there are issues relating to bycatch of non-target species and under-sized target specimens, although advances in design can mitigate bycatch in some fisheries.
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