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Reducing the short-term mortality of juvenile school prawns (Metapenaeus macleayi) discarded during trawling. e ICES Journal of Marine Science, 63: 831e839.A field experiment was carried out in southeastern Australia to assess the short-term mortality and stress incurred by juvenile school prawns (Metapenaeus macleayi) discarded from an estuarine trawler. Some 35% of the prawns died up to 72 h after being caught in a trawl, exposed to air during sorting and separation from the retained catch (as per normal commercial procedures), then discarded into replicate cages. Total mortality was partitioned into that caused by trawling (about 16% of mortalities), and by subsequent sorting and grading (about 19%). Assuming that the majority of the non-penaeid bycatch is excluded from trawls (by the use of bycatch reduction devices), the latter mortalities could be almost eliminated by sorting and separating unwanted school prawns in water-filled compartments. Emersion stress was measured as concentrations of L-lactate in the haemolymph, which were elevated for at least 40 min following capture, but similar among all trawled treatments. L-lactate levels decreased within the first 24 h post-capture, then remained constant over at least the next 48 h, and were greater than baseline levels. The potential benefits associated with subtle changes to handling practices onboard estuarine trawlers are discussed.Crown

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 (L 50 s) 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. K E Y W O R D S :Standard errors are given in parentheses. n, number of hauls used in the model; ns, non-selective for the size range encountered; -, unable to converge model.

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