Pelagic Pair Trawl
One trawl towed in mid-water by two vessel to target pelagic fish. The height of the net in the water column can be changed by altering vessel speed and length of wire out. The nets can be very large as big as 240 metres wide and 160 metres deep but the mesh size in the mouth of the trawl are huge sometimes as big as 50 metres across.
As all pelagic trawls are towed in mid-water with no intentional contact with the seabed there should be negligible impact on the seabed environment. Very occasionally the gear may be towed very close to the seabed when there may be the odd time that the wing end weights contact the seabed but very minimal contact. the skipper is very careful not to allow his gear to get too close to the seabed as, usually, the gear does not have any ground gear to protect the bottom of the trawl.
These trawls are very species specific in operation. The large meshes in the mouth of the trawl will only herd shoaling species into the trawl. The mesh size in the cod end and extension of the trawl is set to suit the physical size of the target species. Much of the selectivity in this method of trawling is done by the skippers experience in knowing what area his target species is likely to be in through past experience, time of year and reports from other vessels in the area. Many of the pelagic species are targeted during their annual migration routes, and as each species has different migration patterns with experience the skippers know where to go for specific species at different times of the year. Also they can usually differentiate between the species by how the shoal shows up on the echo sounder and sonar screens in the wheelhouse. Occasionally some of the larger demersal species may encounter a pelagic trawl when they move up in the water column feeding on small pelagic fish. Most demersal fish that may stray into the path of a pelagic trawl will easily escape through the large meshes in the forward part of the trawl.
Very occasionally there may be a by catch of some cetaceans that enter the trawl chasing the shoals of pelagic feed fish. In some areas where certain methods of pelagic trawls were prone to this the fishing method has been banned to prevent cetacean by catch. Some fisheries have employed rigid grids to guide these large mammals out of the trawl unharmed. These devices are not popular with fishermen as they can be unwieldy to handle onboard the vessel. There are acoustic ‘pingers’ being developed to fit to trawls to prevent cetaceans entering the trawls.
Pelagic trawling is a method of towing a trawl at any point in the water column between the surface and seabed. It is generally used to target shoaling species such as mackerel, herring and sprats.
The beginnings of pelagic trawling come from demersal trawling where they tried to make a large net and tow it between two vessels (pair trawling) and lift it off the seabed. Gradually the mouth of the trawl was made bigger by the inclusion of large meshes in the mouth of the trawl. With the advancement of underwater acoustic technology by the military in the way of echo sounders and sonars and the adoption of this into the fishing industry it became possible to find shoals of pelagic fish and set the nets at the correct height with in the water column to catch these species. The introduction of lightweight curved trawl doors and the use of acoustic measurement sensors on the trawl enabled the new design pelagic trawls to be towed by one vessel as a single trawl. Nowadays much of the pelagic trawling activity, both single and pair trawl, is done by modern powerful vessels equipped with state of the art electronics to find and track the shoals of pelagic fish. However throughout the UK and EU there are many smaller scale pelagic fisheries undertaken by smaller less sophisticated vessels.
Modern pelagic trawls have a much larger opening than demersal trawls some as large as 160metres deep and 240metres wide. This is achieved by having very large meshes in the mouth or forward sections of the trawl. Pelagic trawls tend to be constructed using four panels, a top, a bottom and two side panels to enable them to achieve a much greater height than demersal trawls. The trawls can be either rectangular in cross section or square where all four panels are the same size.
Depending upon the size of trawl the individual meshes in these leading sections of the trawl can be anything from 5 metres up to 50 metres in length. The mesh size is gradually decreased as they get closer to the cod-end of the trawl. The cod-end mesh size being dictated by legislation and/or the size of the target species.
The mouth of the trawl resembles an array of light ropes that lead the shoals of target species into the body of the trawl. These large meshes are effective for the target species as they are schooling fish, i.e. when one fish becomes aware of danger (the net) the whole shoal moves clear of ‘the danger’ as one. To be effective at herding the shoals, the nets have to be designed with great care to shepherd the fish into the trawl at just the right speed, without making the shoal scatter and the fish escape through the large meshes. These large meshes allow much larger nets to be used, in effect filtering a much larger volume of water. No ground gear is needed on the bottom of pelagic trawls as the trawl does not come into contact with the seabed.
The position of the net between the surface and seabed is controlled by the speed of the vessel and the amount of trawl warp shot. In larger boats this can be monitored using electronic sensors on the headline to give a depth for both top and bottom of the net allowing the skipper to position his net is line with the shoal. On smaller vessel the skipper has to rely on his knowledge and experience to place his net at the correct depth where the target shoal of fish are.
The nets for single and pair trawling are basically the same but the bridle and towing arrangement differs.
In the pair trawl rig, the net is towed by two vessels, one towing each side of the net. As in demersal pair trawl, no doors are used, the net’s horizontal opening being set by the distance between the two vessels. This is monitored using the boat’s own radar sets or a line between the two boats. Nowadays many of the modern vessels have sophisticated electronic systems for monitoring the distance apart of the vessels and thereby the horizontal opening of the net. The net is opened vertically by the use of a chain clump on each lower wing end and floats on the headline. Many of these trawls will not have any floats on the headline, but rely on the clump weights and the water flow through the trawl to open the trawl vertically. Normally, in pelagic pair trawl, the vessels will tow the trawl on two warps from each boat, one going to the top (headline) of the net, the other to the footrope, (bottom). By slight alterations in the length of the warp to the top of the net compared to that of the lower warp, the net can be made to alter shape, and move up or down in the water column to some degree. However, the general position within the water column is controlled by the towing speed of the vessel and the amount of warp paid out.