Demersal otter trawl

Multi-rig otter trawl
The demersal or bottom trawl is a large, usually cone-shaped net, which is towed across the seabed. The forward part of the net – the ‘wings’ – is kept open laterally by otter boards or doors. Fish are herded between the boards and along the spreader wires or sweeps, into the mouth of the trawl where they swim until exhausted. They then drift back through the funnel of the net, along the extension or lengthening piece and into the cod-end, where they are retained.
The selectivity of trawl fisheries may be increased by the use of devices known as separator trawls. Separator trawls exploit behavioural differences between fish species and can be used, for example, to segregate cod and plaice into the lower compartment of the net, whilst haddock are taken in the upper part. The mesh size for the two compartments can be altered according to the size of the adult fish being targeted. Insertion of square mesh panels also improves selectivity of the net because square meshes, unlike the traditional diamond shape meshes, do not close when the net is towed. Discarding of immature fish may also be reduced by increasing the basic mesh size in fishing nets. Sorting grids are compulsorily fitted in nets in some prawn and shrimp fisheries to reduce bycatch of unwanted or non-target species, including small prawns and shrimp.

Depending on the depth of water fished and the way in which the gear is constructed and rigged, trawling may be used to catch different species. Trawls can be towed by one vessel using otter boards, as in bottom-trawling, or by two vessels, each towing one warp, as in pair-trawling. Or more than one trawl can be towed simultaneously as in multi-rig trawling.
Multi-rigs are used widely for the capture of panaeid shrimps in tropical waters and more recently for Nephrops (langoustines or Dublin Bay prawns) and deep-water prawns in temperate waters. The speed at which the net is towed is important, varying with the swimming speed of the target species from about 1.5 to 5 knots for fast swimming fish.