DAO prepress abstract  -  doi: 10.3354/dao01904

ABSTRACT: Two experiments were done in an Australian estuary to quantify the mortalities and contributing factors for key species discarded during 8 and 9 deployments of commercial beach (or shore) seines and gillnets, respectively. In both experiments, bycatches (2347 individuals comprising 16 species) were handled according to conventional practices and assessed for immediate mortalities before live samples of selected species were discarded into replicate cages along with appropriate controls, and monitored for short-term mortalities (≤ 10 d). All of the seined or gilled organisms were alive prior to discarding. During experiment 1, 20% of caged seined-and-discarded surf bream Acanthopagrus australis (n = 290) were dead after 5 d, with most mortalities occurring between the second and fifth d. In experiment 2, 42 and 11% of gilled-and-discarded A. australis (n = 161) and lesser salmon catfish Arius graeffei (n = 67), respectively died during a 10-d monitoring period, but mostly within the first 5 d. There were no deaths to any controls for these fish. Mixed-effects logistic models revealed that the mortality of A. australis discarded from both gears was significantly (p < 0.01) and negatively correlated with their total length, while A. graeffei had a significantly (p < 0.05) greater (5 fold) probability of dying when jellyfish Catostylus sp were present in the gillnet. Simple modifications to the operations of beach seines and gillnets and/or post-capture handling procedures, such as close regulation of size selectivity for the target species, careful removal of fish from meshes, and avoiding setting during high abundances of jellyfish will maximise the survival of discarded bycatch.