Predicting growth and mortality of brown trout (Salmo trutta) in the Goulburn River after mitigation of cold-water discharge from Lake Eildon, Australia

Published empirical models for growth and natural mortality were examined to investigate some of the potential effects on brown trout populations of flows managed to increase river temperatures downstream of a large reservoir. Models were fitted to length-at-age and size-at-maturity observations for brown trout (Salmo trutta) and to 3 years of river temperature data, to predict growth and mortality. Best fits of the models suggested that both energy intake and temperature regime currently limit growth. Flow management scenarios for the Goulburn River downstream of Eildon Reservoir, Australia, that increase the average daily river temperature by 2 and 4 degreesC, substantially reduce the potential for brown trout growth. An average daily 2 degreesC increase reduced the potential maximum size of brown trout by 4-38% and increased the time taken to reach 300 g by up to 31%. An average 4 degreesC rise reduced the potential maximum size of brown trout by 30-43% and increased the time taken to reach 300 g by 66-144%. Existing models of the effects of environmental temperature on rates of natural mortality also suggest that raised water temperature will result in increased brown trout mortality rates although uncertainty associated with empirical temperature-mortality models is high. Currently the Goulburn River downstream of Eildon supports a socially and economically valuable recreational brown and rainbow trout fishery. The present study shows that the quality and quantity of brown trout that would be available under strategies proposed to mitigate cold-water flows are unlikely to continue to sustain a viable, high quality trout fishery.