Balancing temperature and timing: Strategies for enhanced Rainbow Trout hatchery returns in Australia

ABSTRACT Objective Rainbow Trout Oncorhynchus mykiss, which are not native to Australia, are confined to the cooler waters of the continent’s southeastern and southwestern regions, where their recreational fishery is often supported by fingerling stocking. High summer temperatures restrict the distribution of trout and pose challenges for growing out and stocking fingerlings, whereas cold winter water temperatures slow production and growth rates during early stages. By increasing water temperatures in the hatchery during early development and delaying release to coincide with more suitable environmental conditions, we sought to increase fish size at release and improve the survival rate of stocked fish. Methods This study evaluated the effectiveness of modified stocking practices in Lake Jindabyne and the Thredbo River, southeastern Australia, by tagging fish with coded wire tags and assessing the number and sizes of the tagged fish that returned to the spawning run or were caught by anglers. Results Rainbow Trout that were reared at 10°C were typically ∼30% larger than those that were hatched and reared in colder, ambient river water. The return rates to the spawn run and angler catches were higher for the Rainbow Trout that were reared in warmer waters and released in autumn than for those that were reared at ambient river temperatures and released in summer. Although the recapture sizes varied significantly between some autumn and summer stocking-groups, the mean size differences after 12 months at liberty were minimal (<27 mm). The fish typically returned to the spawn run or were caught by anglers at 350 to 400 mm fork length and at 2 or 3 years of age regardless of the treatment group. Conclusions Overall, stocking larger Rainbow Trout in autumn versus stocking fingerlings in summer (the conventional method) significantly improved angler catches and spawning returns. The optimal release conditions combined with access to larger prey items and increased capacity to evade predators appeared to make larger fingerlings more likely to survive and contribute to spawning runs and angler catches in Lake Jindabyne. The current study’s findings will inform future stocking strategies, with larger and potentially fewer fish being stocked in some circumstances.