Catchability and selectivity of juvenile snapper (Pagrus auratus, Sparidae) and western butterfish (Pentapodus vitta, Nemipteridae) from prawn trawling in a large marine embayment in Western Australia

Allegations of growth overfishing of the already depleted snapper (Pagrus auratus) stock in Denham Sound, Shark Bay, Western Australia, due to prawn trawling, and concerns for by-catch species generally, led to the investigation of size-related catchability and cod-end selectivity of juvenile snapper and western butterfish (Pentapodus vitta). The cod-end selectivity of prawn trawling was investigated by fitting a fine-mesh cod-end cover to commercial trawl nets to measure the size of fish passing through the cod-end. A Leslie-Delury depletion experiment involving trawling the same area over four consecutive nights was used to determine the catchability coefficients of snapper and butterfish. The size of snapper and butterfish caught on the trawlable grounds ranged from 3 to 18 and 5 to 20 cm fork length (FL), respectively. Ontogenetic changes in habitat preference meant that snapper were susceptible to commercial trawling only between approximately 9-17 months of age. Retention of snapper and butterfish in the trawl nets increased with increasing length, with the deeper-bodied snapper retained at smaller lengths. Catchability coefficients also increased with fish length. A single trawl pass was estimated to remove up to 57% and 52% of juvenile snapper and butterfish, respectively. The rate of natural mortality (M) for juvenile snapper was estimated to be high; mean M = 2.58 (92% year-1). Catchability of butterfish was low prior to their sexual maturation and trawling is assessed not to be a threat to this species. To reduce the impact of prawn trawling on snapper recruitment in Denham Sound, restricting trawling from areas where juvenile snapper are in high abundance, combined with fish escape panels, would be preferable to increasing mesh size in commercial trawl gear