Selective characteristics of a shark-excluding grid device in a Mediterranean trawl

Abstract

Galeus melastomus (blackmouth catshark) is often caught as bycatch in demersal trawls in the Mediterranean. In order to reduce bycatches of shark we tested an excluder grid with 90mm bar spacing during experimental fishing in the Tyrrhenian Sea (Western Mediterranean). Data collected made it possible to simultaneously evaluate catch losses of two commercial species: Nephrops norvegicus (Norway lobster) and Phycis blennoides (greater forkbeard). The escape outlet ahead of the grid and the codend were both mounted with a cover in order to collect escaped fish ahead of the grid and through the codend meshes. We used a structural model to estimate the contribution of the individual selective processes consisting of the excluder grid and the size selective codend. The 90mm excluder grid did not prove to be efficient in excluding G. melastomus, while it excuded more of P. blennoides. Catches of N. norvegicus were also affected by the presence of the grid, but not as much as the catches of other two species. The results obtained for the experimental grid+codend setup were then compared with the estimated selectivity for the “codend alone” setup. Furthermore, by way of explorative simulation with other grid bar spacing, we concluded that reducing the grid bar spacing to 70mm would provide better compromise between the reduction of G. melastomus as bycatch and the catch rate of P. blennoides and N. norvegicus.