Using multiple lines of evidence to assess recovery potential of a warm water fish population in a cold water impacted river

Abstract

IntroductionHumans have substantially altered landscapes across the globe, generating novel ecosystems with varying states of modification. The principles of reconciliation ecology emphasise that such novel ecosystems must also be considered for conservation outcomes. This requires an understanding of how anthropogenic habitat alterations in a novel ecosystem may disrupt life cycle processes of key biota, thereby enabling the development of management strategies that may bypass or ameliorate potential bottlenecks in life history stages. We explore this by assessing Murray cod (Maccullochella peelii), a native fish species of considerable cultural, recreational and conservation significance in a severely altered river reach in south-eastern Australia. This species was considered to have disappeared in the reach following alterations to the river’s thermal profile (cold water pollution - CWP), but instead has persisted.MethodsWe examined the life cycle of Murray cod and use multiple lines of evidence to assess the impacts of CWP pollution on key processes such as adult migration, recruitment and juvenile survival. We also evaluated the potential for recovery and persistence of the species in this novel system using mitigation measures such as flow management and stocking and opportunistically tested their effectiveness.ResultsWe found the key impact pathway of CWP on the Murray cod population is via its effects on the key processes influencing natural recruitment. Conversely, the effects of CWP on emigration and survival of larger juveniles and adults did not appear to be a major bottleneck for the population. Using a stochastic population model, we showed that the population could be sustained by stocking and opportunistic water management while still meeting irrigation demands. Monitoring to test our recommendations showed natural recruitment and improved survival of stocked fish during years with limited flow releases, thereby corroborating our predictions.DiscussionOur use of multiple lines of evidence to assess a species’ responses to impacts of habitat alteration in a novel ecosystem enabled the formulation of recommendations for management that could then be tested. The success of such actions add to the growing body of literature that shows species of conservation significance can be maintained in novel ecosystems which should be specifically considered within the context of conservation planning across the landscape.

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

“…It is especially important that CWP mitigation options are included in the design and construction of new dams or the expansion of existing dams. There may also be some operational alterations, such as water delivery times, that could assist in reducing the downstream impacts, particularly in years when delivery of irrigation water is limited (Tonkin et al 2023). Additional to any planned remediation or construction of new dams, the design must predict daily temperatures and these temperatures need to be modelled to ascertain their appropriateness for the species affected.…”

“…It has over 100 large dams, 49 of which have been assessed to cause CWP, affecting over 3000 river-km (Ryan et al 2001;Preece 2004;Lugg and Copeland 2014). CWP has been studied in the southern MDB on species such as Murray cod (Maccullochella peelii), trout cod (Maccullochella macquariensis) and Macquarie perch (Macquaria australasica) (Koehn et al 1995;Todd et al 2005;Sherman et al 2007;Raymond et al 2022;Tonkin et al 2023). It is less well documented for the semi-arid northern MDB where CWP impacts are likely to be more severe owing to warmer water temperatures, more variable temperatures and flows, and for fish species such as golden perch (Macquaria ambigua) with higher temperature requirements (Koehn et al 2020a).…”