Pair Trawl

In this fishing method a demersal trawl is towed by two boats simultaneously, one towing each side of the trawl and held open by the distance apart of the vessels. The design of a pair trawl differs very little from a single trawl that would be used by an individual vessel apart from being larger.

There will be some disturbance to the seabed through the contact of the sweeps and the ground gear of the net, but because pair trawl gear does not need trawl doors, there will be reduced seabed impact. However, despite the lack of trawl doors, pair trawl gear will sweep a wider area of seabed than a single trawl. Usually, pair trawl is used over sand, mud and shingle seabeds that are already subject to regular disturbance through the natural movement of tides and currents, therefore, any disturbance by the gear should have minimal long term effects. As with single boat demersal trawls, the vessels are strictly regulated through total allowable catch (TAC) and gear regulations to minimise environmental impact and by-catch. As the gear is towed on the seabed, where, in many fisheries, there tends to be a mix of fish species, there is the opportunity for a by-catch of non-target species. The capture of immature fish is minimised by use of larger cod-end mesh sizes, some due to legislation but more recently as a result of the skippers choosing to use mesh sizes larger than the minimum dictated by regulation, to generally improve the selectivity of their gear and minimise discarding.

Pair trawl has evolved mainly from a single boat trawl combined with the technology of ring netting with two boats. This is where a net is shot round a shoal of fish, with a boat at each end of the net. Instead of bringing the ends of the net together immediately, they would tow it a short distance first a form of pair trawling. In this method, a demersal trawl is towed by two boats simultaneously, one towing each side of the trawl. The design of a pair trawl differs very little from a single-trawl that would be used by an individual vessel, apart from being larger. Trawl doors are not used in this method, the horizontal opening of the trawl being maintained by the distance between the two vessels as they tow the gear. The distance apart of the two vessels can vary depending mainly upon the depth of water and the target species, but is usually in the region of 300 - 400 metres.

As there are no trawl doors, the overall drag of the gear is reduced, allowing the vessels to tow a slightly larger net without increasing their fuel consumption. Bridles and sweeps, similar to that used in a single-rig trawl, are used, with the trawl doors being replaced either by a clump weight or a length of heavy trawl wire (100 - 200 metres) to help keep the gear on the seabed. This wire on the seabed acts as an extension to the sweep in that it will help to herd the fish into the path of the trawl. Additionally, in shallow water, the vessel’s engine and propeller noise does not disturb the fish in the path of the trawl, but actually helps with the herding effect of the gear.

Vessels of all sizes operate as pair trawlers. The main criterion is that they are equipped with a trawl winch and means of hauling the gear in a similar manner to a single net trawler. The trawl is shot and hauled by one vessel in a similar manner to a single trawl. The difference comes when the net has been shot, where, instead of hooking on a set of trawl doors, one side of the trawl will be passed to the partner vessel. This is done by throwing a lightweight heaving line that is attached to one side of the trawl from one vessel to the other, enabling the partner vessel to haul across one side of the trawl and shackle it onto their trawl warp. The two vessels then start to move apart while they both pay out the required amount of trawl warp to get the gear on the seabed. The amount of warp used is dictated by the depth of water that the vessels are fishing in; it is usually between four and six times the depth of water.

As with a single trawl, the boats will tow for up to five hours at 3 - 4 knots, then both vessels will heave in their warps and edge closer together with one vessel taking both sides of the trawl to haul it aboard, as if handling a single trawl. The hauling boat will usually take the catch as well, however if there is a large haul they may split the catch between the two boats by passing the cod-end between the boats. The size of gear very much depends on the size of the vessels, but a rough guide for two 20 metre trawlers, a demersal pair trawl would open horizontally to approximately 20 - 25 metres, and vertically to between two and six metres depending on the target species and the design of trawl.

The skippers have to judge carefully what size of trawl they use, as a larger trawl will have much greater drag, resulting in greater fuel consumption, but will not necessarily return larger catches. In a similar manner to a single-trawl, pair trawls have ground gear fitted to their footrope to protect the trawl from damage. The main target species in the EU tend to be haddock, whiting, cod, etc. rather than the bottom-living species, although there will often be a by-catch of these species. Demersal pair trawl is used in many fisheries throughout the world, from small boat artisanal fisheries right up to deep sea freezer trawlers. Many EU vessels that used to operate as pair-trawlers have modified their vessels and gear to enable them to operate in a similar manner to pair seiners.