Background: The construction of large dams has substantially altered the instream conditions that support freshwater fish populations worldwide. In the Murray-Darling Basin, large dams and subsequent flow regulation has contributed to widespread declines in native fish through habitat fragmentation, disruption of natural flow regimes, and altered thermal conditions caused by cold-water pollution. Whilst the detrimental effects of cold-water pollution on the survival, growth, and recruitment of select native fish species has been well established, how large dams and cold-water pollution influence the broader fish assemblage remains poorly understood.
Methods: To address this, we used fine-scale environmental DNA (eDNA) surveys to characterise freshwater fish assemblages downstream of large dams in four rivers of the northern Murray-Darling Basin. We quantified changes in fish community composition in relation to distance from large dams and the extent of water-temperature augmentation.
Results: Multivariate analyses reveal distinct fish assemblages that were strongly influenced by both proximity to dams and water temperature. Univariate analyses further identify key species whose distributions were most strongly associated with these environmental gradients.
Conclusion: Together, these findings demonstrate that large dams influence fish communities beyond direct barriers to movement, playing a key role in shaping assemblage structure. This work highlights the value of eDNA for assessing riverine fish communities and provides insights that will inform fisheries management and threatened-species conservation in increasingly variable and human-modified freshwater ecosystems.