The effect of mesh configuration and codend circumference on selectivity in the Mediterranean trawl Nephrops fishery

Abstract

The Pomo pit area is both the main Adriatic nursery area for European hake ( Merluccius merluccius) and Norway lobster ( Nephrops norvegicus) and an important trawling ground for the Adriatic trawl fleet. The current paper is not only restricted to the escapement of Norway lobster but also takes note of the associated white fish community (European hake, blue whiting Micromesistius poutassou and poor-cod Trisopterus minutus capelanus) which makes an important contribution to the commercial catch of trawlers. This study was undertaken to analyse the effect on trawl selectivity of inserting first a 40 mm square-mesh codend and second of increasing the circumference in the diamond-mesh codend. A traditional 40 mm diamond-mesh codend, typical of the commercial trawl fisheries, was selected as a reference codend for the comparative analysis. The square-mesh codend was always more selective than the diamond-mesh codends, while the influence of codend circumference on diamond-mesh selectivity was less evident. In addition to mesh configuration and the number of meshes around the codend, the selectivity data were modelled by estimating the individual contribution of catch size and season, but there was no evidence of a coherent effect on the selectivity of these two parameters. This study confirms that all the four investigated species make the best use of the square-mesh opening, either because of their body shape or because the form of the mesh facilitates forcing of the net. Results seem to suggest that for the main target species, Norway lobster, square-mesh codend would protect specimens under the 20 mm of MLS (carapace length). Square-mesh also improves the L50 of European hake, but it seems not to be sufficient to avoid the catch of specimens under MLS of 20 cm as fixed by the EC Reg. Nr. 1967/2006. In conclusion, enforcement of installation of square-mesh codends in Mediterranean trawl Nephrops fishing could contribute to decreasing the capture of individuals from the particular nursery area of Pomo pit.