Codend selection in the deep-water crustacean trawl fishery in Portuguese southern waters

In this paper, the potential improvement of size-selectivity by increasing codend mesh size and changing twine type is addressed for the main commercial species exploited by crustacean trawlers in waters off the Portuguese south coast. Three cruises were conducted between autumn 1998 and summer 1999 onboard the F/V “Porto Bravo”, resulting in a total of 93 valid hauls using 3 different sizes of diamond mesh in the codend (stretched nominal sizes of 55, 70 and 80 mm) and 2 types of twine, polyamide and polyethylene. A non-linear mixed-effects model was employed to include the effect of covariates and between-haul variation in the estimation of codend selectivity parameters for the main target species in this fishery, Norway lobster, Nephrops norvegicus (L.), rose shrimp, Parapenaeus longirostris (Lucas 1846), and ‘blue and red’ shrimp, Aristeus antennatus (Risso, 1816). The chosen selectivity model found a significant (p < 0.001) linear effect of mesh size for all three target species, and a significant effect of cruise for Norway lobster (p < 0.05) and rose shrimp (p < 0.001). The cruise effect was due to a difference between the autumn 1998 cruise and the two latter cruises. This effect coincides with a change from polyamide twine in the autumn cruise to the use of polyethylene twine in the latter cruises. Other covariates, such as depth, codend-catch weight and species catch weight did not show any systematic effect on selectivity and thus were not included in the final models. The retention of a high number of undersized (or immature) individuals, while using the legal minimum mesh sizes, suggests the need for a change in the status quo fishing pattern.