Size selectivity for four fish species of the deep groundfish assemblage off the Portuguese southwest coast: evidence of mesh size, mesh configuration and cod end catch effects

Abstract

The effects were evaluated of an increase in cod end mesh size from 65 to 70 and 80 mm and change of mesh configuration from 65 mm diamond to 65 mm square mesh on the size selectivity of hake ( Merluccius merluccius), horse mackerel ( Trachurus trachurus), four-spot megrim ( Lepidorhombus boscii) and blue whiting ( Micromesistius poutassou) captured on the upper continental slope off the Portuguese southwest coast, at depths from approximately 200–400 m. A number of individual hauls were analysed in the diamond mesh cod ends, for hake and blue whiting, and in the square mesh cod end, for horse mackerel, and mean selection curves were estimated for these cod ends taking into account between-haul variation. For the four-spot megrim, the selectivity estimates were based on pooled data for all the cod ends tested. Selectivity models are proposed for hake and blue whiting in which positive effects of both the increase in mesh size and in cod end catch were estimated for L 50, while SR was only affected by mesh size. For horse mackerel a positive effect of cod end catch was estimated for SR in the square mesh cod end. The selectivity was greatly affected by the change in mesh configuration for all species with the exception of the four-spot megrim, for which only mesh size was found to affect selectivity.