A Fishing Industry Guide to Offshore Operations
(Courtesy of Scottish Executive)http://www.scotland.gov.uk/library3/fisheries/figoo-04.asp

PELAGIC OR MIDWATER FISHING METHODS

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Pelagic gears are used principally in the capture of shoaling species such as herring, mackerel, scad, blue whiting and sprats which may be found close to the surface, in mid-water or just off the bottom.

Of particular note are the mackerel fishery throughout UK waters but especially off the South West of England and to the north and west of the Scottish Islands, the North Sea and Irish Sea herring fishery, the blue whiting fishery off North West Scotland and in the Northern North Sea and, the west coast horse-mackerel fishery. Pelagic gear is also used for a number of small seasonal fisheries such as that for hake in the Clyde and North Channel, the bass fishery in the English Channel, certain cod and haddock fisheries in the Irish Sea and various targeted North Sea fisheries.

Single Boat Pelagic Trawl
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Like demersal trawls single boat pelagic trawls are towed using a wire rig (Figure 10) which incorporates otterboards (a) to provide horizontal spreading forces. Both net and doors are much larger than their demersal equivalents, but of much lighter construction as neither is designed for bottom contact.

The otterboards, usually of high aspect ratio and aerofoil section, are towed by the vessel using two steelwire warps (b), and connected to the net by four-wire bridles (c).      Unlike demersal trawls pelagic trawls are towed on the headline, and solid steel or lead weights (d) provide the vertical forces required to open the net in a downwards direction. As these weights are slung on the lowest point of the gear they will inevitably make contact with the    bottom when fishing hard down, even although both doors and net may not. Pelagic trawls are towed at the appropriate level in the water column to intercept target shoals with gear depth    being controlled by altering towing speed and/or warp length. Electronic equipment such as sonar, net and catch monitors have greatly improved the precision of this method of fishing.     Normal towing speed is in the region of 3.75 knots, but may be increased to as high as 5 knots when fishing mackerel. Pelagic trawlers can be found in all NE Atlantic waters, both on and off the continental shelf.

Typical (and maximum) values for above components: - Otterboard (a) size and weight - 11 square metres x 1500 kilograms (16 square metres x 2500 kilograms); Trawl warp (b) wire     diameter - 32mm (38mm); Bridle (c) wire diameter- 26mm (30mm); Weight (d) - 1.1 tonnes (5 tonnes).

Pelagic Pair Trawl
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As is the case with demersal pair trawlers, the absence of otterboards allows the two vessels to deploy a much bigger net than would otherwise have been possible to tow at an effective speed. Target species and fishing techniques are similar to those set out above, with the           additional requirement that both vessels have comparable towing power available and must at all times act in concert. The main difference in gear rigging is that two towing warps (a) are deployed from each vessel, one to the upper bridles, the other to the sinker weight (b) ahead of the footrope (Figure 2). The lower bridles (c) connect the weight to each side of the footrope.  

Towing speeds are high, normally 4 knots but up to 5.5 knots on occasion. Smaller vessels (<20m) fish for herring, mackerel and sprats in coastal waters but larger vessels search for shoals far offshore.
Typical (and maximum) values for components: - Trawl warp (a) wire diameter - 26mm (34mm); Bridle (c) wire diameter - 26mm (28mm); Weight (b) - 1.1 tons (5 tons).

Courtesy of Scottish Executive  http://www.scotland.gov.uk/library3/fisheries/figoo-04.asp



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