Using ecology and behaviour to optimise release strategies for mulloway Argyrosomus japonicus (Sciaenidae)

The release of hatchery-reared fishes is a popular management technique for augmenting depleted fish stocks and increasing fishery productivity. Fish stocking has been trialled in many countries with varied outcomes from total loss of stocked fish to enhanced catch rates. The success of fish stocking programs is dependent on a range of interacting factors which affect fish post-release, which relate to behavioural traits and ecological interactions This thesis assesses a number of these factors for mulloway Argyrosomus japonicus (Temmink & Schlegel 1843), and advances the science of restocking through the novel application of acoustic telemetry to address specific questions regarding release strategies. Chapter 1 provides an outline of contemporary stock enhancement methodologies and telemetry studies worldwide as well as a review of research on A. japonicus in Australia. Argyrosomus japonicus is a social fish when young, and the implications of school size and optimal density for release strategies are assessed in Chapter 2. Over- and under-crowding stocked fish leads to reduced growth rates driven by social effects, and the foraging ability of a school on wild live feed types was assessed using live forage species and growth trajectories. A novel acoustic tag method is presented in Chapter 3, and its effectiveness and impact on fish feeding and behaviour is examined in the laboratory. Traditional tag implantation methods had a significant impact on short term feeding behaviour and the new tag system overcame this adverse effect. This system was applied to test the ability of targeted stocking to reduce emigration in Chapter 4. Fish released directly into key habitat showed lower daily activity, and spent a greater proportion of time occupying key refuge habitat than fish released at the shoreline. To evaluate effects of abiotic factors on emigration of released fish, Chapter 5 assesses movements of 21 sub-adult A. japonicus, using passive acoustic telemetry over a period of 6 months in the Georges River, NSW, Australia. Released A. japonicus were associated most frequently around the site at which they were released, and showed strong site- and group-fidelity with little overlap in distributions between released cohorts. Emigration rates of stocked fish were also found to be lower than expected from previous published examples in the literature. Chapter 6 summarises the key findings from the four field and laboratory studies described in this thesis and concludes with recommendations and potential problems for future fish stocking programs in Australia