The effect on juvenile haddock by-catch of changing cod-end twine thickness in EU trawl fisheries

The selectivity of North Sea haddock (Melanogrammus aeglefinus) was measure d for 100 mrt mesh trawl cod-ends made of three different twines: 4 mm doub le twine used commercially until the introduction of thicker twines during the 1990's; 6 mm double twine and 8 mm single twine proposed by the Europea n Commission as maximum thicknesses starting in January 2000. A significant difference in selectivity was found between the 4 mm double twine and the two thicker, stiffer twines. The 50% retention length was increased by almo st 4 cm when using the thinner twine. Models of the selectivity of these co d-ends have been developed, taking into account small mesh size differences and using catch size as a variable and sea state and twine size as categor ical variables. The benefits of using a thinner twine cod-end are illustrated by showing th e effect on the catch of juvenile and marketable fish from a typical North Sea population of haddock. The potential increases in survival of the 2 + a nd 3 + year classes of such a haddock stock are estimated to be 17% and 12% respectively.