In this fishing method one trawl, designed to catch pelagic fish is towed in mid-water by one vessel. The trawl is spread horizontally be a set of pelagic trawl doors. The horizontal opening is dictated by a clump weight on the lower wing ends of the net and the rigging of the bridles between the net and trawl doors.
By altering the vessel speed and / or changing the length of trawl warp between the vessel and the trawl doors, the position of the net in the water column can be altered to suit the depth where the shoal of fish are swimming at. The nets can be very large as big as 200 metres wide and 150 metres deep but the mesh size in the mouth of the trawl are huge sometimes as big as 50 metres long.
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 and trawl doors contact the seabed but this will be the exception rather than the rule. Usually the skipper is very careful not to allow his gear to get too close to the seabed because these trawls will damage easily if the come into contact with the seabed.
Pelagic trawls are very good at being 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. Any demersal fish that may stray into the path of a pelagic trawl will usually 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 fisheries where there was evidence of cetacean by catch this fishing method has been banned. This is usually confined to pelagic pair trawls rather than the single trawls. Some fisheries have trialled 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.