Potential use of a Nordmøre grid for by-catch reduction in a Portuguese bottom-trawl multispecies fishery

The bottom-trawl fishery in Portuguese continental waters is a multi-species one targeting benthic, demersal and pelagic fish, and cephalopods, where the blue whiting, Micromesistius poutassou and the longspine snipefish, Macroramphosus scolopax often constitute a major by-catch problem. The simultaneous capture of commercial and by-catch species having very different length ranges, body shapes and behavioural patterns towards the gear raises complex management issues, which cannot be addressed only by controlling the codend mesh size. A sorting system comprising a modified Nordmøre grid, 30 mm bar spacing, used alone or together with a 100 mm square-mesh window was evaluated as a by-catch reduction device, during 26 hauls carried out off the Portuguese west coast, in September 2003, on board the R/V 'Capricórnio'. Demersal and pelagic species displayed contrasting reactions towards the grid. The former (hake, Merluccius merluccius; pouting, Trisopterus luscus; dogfish, Scyliorhinus canicula) contacted the grid and were size-selected, while most of the latter (Atlantic mackerel, Scomber scombrus; horse mackerel, Trachurus trachurus; blue whiting and snipefish) were conducted along the grid and passed through its upper opening into the codend. When the square-mesh window was placed just behind the top opening of the grid, a large escapement of pelagic fish took place. Results suggest that by reducing the grid's bar spacing and the window's mesh size used in the present trials, it will still be possible to reduce the catch of undersized demersal species, such as the hake, and promote the escape of blue whiting, while retaining most of the commercially valuable species. However, it was also evident that no such system will simultaneously solve the problem of large by-catches of longspine snipefish.