Trawl cod-end mesh selectivity for some fishes of North-Western Australia

Abstract

A cod-end mesh selectivity experiment was conducted using the covered cod-end technique on the continental shelf of northern Australia. Cod-end (stretched) mesh sizes of 40, 51, 55, 70 and 100 mm were tested with a 30-mm-mesh cover bag. Mesh selection parameters were estimated for 28 species by fitting a logistic curve to the size-specific escapement data, and a gross selection rate was calculated for an additional 27 species. Some species (notably Lutjanus spp., Pristipomoides spp. and Plectorhynchus pictus) were largely retained by all cod-end mesh sizes tested, while others (notably Loligo spp.) largely escaped all the cod-ends tested. The common assumption that the selection factor does not alter with mesh size was accepted for Nemipterus nematopus, Parupeneus pleurospilus and Lutjanus vittus, but rejected for Saurida undosquamis. Dependence of the selection factor upon mesh size for S. undosquamis was due to a non-zero intercept in the relationship between fish size at 50% escapement and mesh size. The general implications of dependence of selection factor on mesh size are discussed.