Abstract
Since June 1992, when the minimum mesh size for the North Sea demersal fisheries was raised to 100 mm, there has been a trend towards the use of cod-ends made of much thicker twine. The effect of this increase in twine thickness on cod-end selectivity for haddock was investigated in June 1993 on a Scottish commercial vessel. Cod-end selectivity was measured using a cod-end cover with supporting flexible hoops. The results were analysed taking account of between haul variation in selectivity, and indicate a significant reduction in selectivity with increased twine size. The change from 3.5 mm double twine to 6 mm double twine reduced the selection factor from 2.64 to 2.38. The reduction in selectivity is thought to be due mainly to reduced mesh opening and increased twine stiffness which both inhibit escape, but behavioural effects may also be important.
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