Optimising codend configuration in a multispecies demersal trawl fishery

The relative efficiencies and selectivities of conventional and modified codends were examined in a demersal trawl fishery targeting several species, including eastern king prawns, Penaeus plebejus (Hess), whiting, Sillago spp. and cephalopods. The modifications to codends included: (i) reducing the circumference; (ii) increasing the mesh size in conventional diamond-mesh designs; and (iii) orientating meshes on the bar so that they were square shaped. The codends were tested against a fine-meshed control in paired comparisons onboard three commercial trawlers. The conventional codend comprised 41-mm diamond-shaped mesh attached to an anterior extension section at a ratio of 150 to 100 meshes and was demonstrated to be non-selective for the targeted species. Reducing codend circumference to 100 meshes and increasing the size of mesh to 45 mm both improved selection for eastern king prawns, but the lateral mesh openings were estimated to be insufficient to allow juveniles of the other key species to escape. By contrast, codends made from 35- and 41-mm mesh hung on the bar improved the size selection for eastern king prawns and selected stout whiting, Sillago robusta (Stead) (the smallest commercialsized fish) across narrow selection ranges and at 50% sizes of retention (Ls) that were closely correlated to the transverse morphology of fish and the maximum mesh opening. With the exception of a reduction in catches of octopus, Octopus spp., by the 41-square codend, there were no other impacts on commercial catches by the squaremesh designs. It was concluded that diamond-mesh codends are inappropriate for use throughout this multispecies fishery and that a modified design comprising at least 35-mm mesh hung on the bar is required to minimise the fishing mortality of unwanted sizes of the key target species. The results are discussed in terms of the utility of these types of modifications for closely regulating selection in penaeid-shrimp trawls.

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