Mitigating the bycatch of giant cuttlefish Sepia apama and blue swimmer crabs Portunus armatus in an Australian penaeid-trawl fishery

Trawlers in Spencer Gulf, Australia, target western king prawns Melicertus latisulcatus, with nearly all other species discarded; two of the latter have evoked significant, although quite different, concerns. The first is the giant cuttlefish Sepia apama, which has undergone a drastic decline in numbers in Spencer Gulf in recent years, and is listed as 'Near Threatened' by the International Union for the Conservation of Nature. S. apama are susceptible to trawling during their annual spawning migration through Spencer Gulf. The other bycatch problem involves large quantities of blue swimmer crabs Portunus armatus, which currently are separated inside the trawl using a large-meshed liner. Due to the additional handling required and the physical damage caused by the crabs' exoskeletons on the soft-bodied M. latisulcatus and Sepia spp., P. armatus would ideally be able to escape of their own accord during towing. This study examined the utility of mechanical-separating bycatch reduction devices (BRDs) for reducing unwanted bycatches of Sepia spp. and P. armatus. We compared (against conventional codends) the smallest and largest practical sizes of Nordmore-grids (the latter to maximise sorting area), with correspondingly large and low grid angles. The large Nordmore-grid significantly reduced Sepia spp., P. armatus and total bycatch (by 33-50%), but had no effect on catches of M. latisulcatus. Whilst additional research is required, a modified Nordmore-grid should help to resolve the bycatch of P. armatus and S. apama in this fishery, with minimal commercial impacts.