Natural mortality is a major regulating factor of fish stocking programs, with many fish succumbing to predation during the acclimation period soon after release. Stocking larger fish can result in higher proportions of fish making it to the fishery but comes at significant production costs and typically lower release numbers. An understanding of predation levels on fish can aid managers in developing stocking programs, including size at release, but determining predation rates is difficult. Mulloway ( Argyrosomus japonicus ) are the focus of a stock enhancement program in New South Wales, Australia. Survival of stocked juvenile mulloway was evaluated using acoustic telemetry using predation tags, which record the consumption of tagged fish by predators. Thirty fish were tracked for 5 months, of which 13 were predated upon, all within the first 23 days post-release, with most of these fish not surviving the first week. There was no relationship between the distance moved by fish and their likelihood of being predated. Fish surviving the initial 23 days appeared to acclimate to their new environment and were tracked moving throughout the estuarine system. Survival data allowed an instantaneous mortality rate to be determined, from which length-based mortality relationships were developed. Based on these relationships, 7.3 times as many fish would need to be stocked at 10 cm compared to 50 cm for equal numbers to survive and reach the fishery at 70 cm. Future research could replicate this study using different sized fish to refine length-based mortality estimates and further investigate optimal size at release.
Becker et al. (Fri,) studied this question.