Cold-water pollution impacts on two 'warm-water' riverine fish: interactions of dam size and life-history requirements

Context: Temperature regulates most ecological processes in freshwater ecosystems. Anthropogenic changes to natural thermal regimes, especially cold water released from stratified impoundments (cold-water pollution, CWP), is a widespread and major threat to fish populations globally. Aims: Because mitigation options for CWP are often expensive, we aimed to provide robust ecological evidence to inform expenditure. Methods: We modelled population responses to CWP remediation for two warm-water fish species (Murray cod and golden perch) downstream of two impoundments (Copeton and Pindari dams) in the semi-arid northern Murray-Darling Basin, Australia. Key results: Predicted populations of both species were severely affected by CWP at both sites; however, impacts were species- and site-specific. Effects were greater on golden perch than Murray cod because of their higher temperature requirements for spawning. Predicted spawning opportunities decreased by 77-100% for golden perch and by 38-92% for Murray cod. The larger Copeton Dam had greater impacts than did the smaller Pindari Dam. Conclusions: Remediation of CWP can help restore populations and meeting the needs of golden perch may benefit a range of other species. Implications: Globally, CWP is a serious threat to warm-water fishes that may compromise and even nullify other restoration efforts. However, restoration may be impeded by current low abundances, angler harvest and restrictions to fish passage.