To ask the Secretary of State for Environment, Food and Rural Affairs
(1) what assessment her Department has made of the effect of the (a) thickness and (b) stiffness of twine on the escape properties of a trawl net;
(2) what assessment her Department has made of the effect of the use of double twine in the construction of trawl nets on the selectivity of gear; and what the effect has been of permitting the use of different thicknesses of twine on the size of fish caught in North Sea fisheries;
(3) what the impact of lightweight high-strength fibres on the (a) selectivity properties and (b) fishing performance of trawl nets is.
The effect of the thickness of modern twines on selectivity of whitefish gears was first studied in 1993 at the FRS Marine Laboratory Aberdeen. Since then several further UK studies have been completed. Thicker twine has been found to reduce selectivity. In the case of double twine a decrease in thickness from 6mm to 4mm has an effect equivalent to increasing mesh size by 10mm. A codend made of 8mm single twine has similar selectivity to that made of 5mm double twine.
As a result of this research, legislation was introduced, initially by UK and then EU authorities, to limit the thickness of twines. It is not possible, however, to isolate the effect on stocks of this one measure limiting twine thickness as many other changes have been made to gear designs during the same period.
Mathematical models of codend geometry have shown that stiffer twine limits the expansion of the codend as the catch builds up and hence limits the openness of the codend meshes in the area where the majority of fish escape.
Nets made of lightweight, high strength twines will have lower drag for a given size. This will allow a fisherman either (a) to reduce his fuel bill and maintain the same catch or (b) to tow faster (with the same fuel bill) and possibly increase his catch per unit time or (c) to buy a larger net to increase catch at the same towing speed.