Selectivity experiments with escape windows in the North Sea Nephrops (Nephrops norvegicus) trawl fishery

Selectivity experiments were carded out in the northern North Sea at the Fl aden Ground and the East Ground from a Danish commercial vessel. A conventi onal 70 mm Nephrops codend and a similar codend with a 90 mm square mesh pa nel window inserted in the top panel were tested simultaneously in a twin t rawl rig. Selectivity parameters and differences in catches of the target s pecies Nephrops and the by-catch species whiting, haddock and cod were esti mated. The logistic function was found to give the best description of fish selection ogives, whereas the asymmetric complementary loglog function des cribed Nephrops selection best. The Fryer model was used to describe and co mpare the selection ogives for haddock, whiting, cod and Nephrops in the tw o codends. Due to low catches of cod and Nephrops only four out of 11 hauls could be used for modelling the selection of these two species. Selection factor and selection range were larger for the window codend for all fish s pecies. The codend catch size had a significant negatively correlated influ ence on iota(50) and selection range for whiting in the window codend. Ther e were large significant reductions in numbers of cod and haddock under the Danish MLS with the window codend. Catches of small cod, however, were low , Very few whiting under MLS were retained in either codend but there was a large significant reduction in individuals over MLS with the window codend . A large proportion of the Nephrops in the codends were under MLS. A reduc tion in catch weight of Nephrops over MLS was found for the window codend, but catches of Nephrops were low for several hauls. (C) 1999 Elsevier Scien ce B.V. All rights reserved.