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

Selectivity experiments were carried out in the northern North Sea at the Fladen Ground and the East Ground from a Danish commercial vessel. A conventional 70 mm Nephrops codend and a similar codend with a 90 mm square mesh panel window inserted in the top panel were tested simultaneously in a twin trawl rig. Selectivity parameters and differences in catches of the target species Nephrops and the by-catch species whiting, haddock and cod were estimated. The logistic function was found to give the best description of fish selection ogives, whereas the asymmetric complementary loglog function described Nephrops selection best. The Fryer model was used to describe and compare the selection ogives for haddock, whiting, cod and Nephrops in the two 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 species. The codend catch size had a significant negatively correlated influence on l₅₀ and selection range for whiting in the window codend. There 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 reduction in catch weight of Nephrops over MLS was found for the window codend, but catches of Nephrops were low for several hauls.