Selectivity of Norway lobster Nephrops norvegicus (L.) in the northwestern Mediterranean

The selectivity of trawl mesh sizes of 38,42, 45, 52. and 60 mm on Norway lobster (Nephrops norvegicus) in the Western Mediterranean was analyzed. An outside codend liner and replications of hauls were used to fit logistic curves to the percentage retention for each size class. The results were analyzed separately by haul and for all hauls combined for each mesh size using linear regression analysis and the maximum likelihood method which yielded uninflected selectivity curves for Nephrops norvegicus. The results indicated that the 38-mm mesh captured practically all size classes. The 42 and 45-mm meshes also captured sizes whose L50 value was less than the size at first maturity. The 52-mm mesh yielded better results, which was nearly the size at first maturity. The 60mm mesh permitted escapement of all first-spawning individuals, though the curve was too flat to be effective. The results of the different fits were discussed and related to such biological aspects as the sex ratio, size frequency and size at first maturity. Based on the behaviour of the selectivity curves in the Norway lobster fishery in the Western Mediterranean, regulation of mesh size would not appear to be an optimum means of fisheries management.