Maximising the survival of bycatch discarded from commercial estuarine fishing gears in NSW

The mortalities and contributing parameters were estimated for key species discarded during commercial gillnetting (80-mm mesh) targeting dusky flathead Platycephalus fuscus in a southeastern Australian estuary. Bycatches (1470 individuals from 16 species over 11 deployments) were assessed for their immediate mortalities onboard the gillnetter, before subsamples (570 individuals from 11 species) were discarded into cages and monitored for their short-term fate over 4 d. Appropriate controls were concurrently caged and monitored. Blood samples were taken from some live meshed-and-discarded yellowfin bream Acanthopagrus australis and luderick Girella tricuspidata and analysed for plasma cortsiol and glucose. Concomitantly angled fish were similarly sampled (to provide baseline estimates of blood physiology). The immediate mortalities of the abundant species ranged between 0 (undersize blue swimmer crab Portunus pelagicus < 6 cm carapace length) and 70% (undersize P. fuscus < 36 cm total length – TL). Covariates identified as having significant positive and negative relationships with immediate mortality included water temperature for G. tricuspidata and large tooth flounder Pseudorhombus arsius and TL for black sole Synaptura nigra, respectively. Compared to baseline estimates, mean plasma cortsiol concentrations in meshed-and-discarded G. tricuspidata and A. australis were significantly greater, and approached levels comparable to most teleosts after peak stress. Mean glucose concentrations were not concomitantly elevated, possibly reflecting limited time between stress and sampling for some individuals. Short-term mortalities remained protracted for most species and ranged from 0 (yellowfin leatherjacket Meuschenia trachylepis) to 29% (A. australis). Water temperature and TL were identified as having similar significant impacts as above on the delayed fate of A. australis and G. tricuspidata. The partitioned mortalities were combined to provide estimates of total mortality (± se) for the main species that ranged between 5.9 ± 3.3% (P. pelagicus) and 76.9 ± 7.8% (undersize P. fuscus). Overall discard mortality in this fishery could be mitigated by allowing fishers to retain a small percentage of undersize P. fuscus, restricting the deployment of nets in water temperatures > 16 – 17C, and encouraging the careful removal of catches from meshes.