Modelling the impacts of current flow regulation and future flow restoration on fish populations in naturally intermittent lowland streams

Understanding the relationships between flow regime and the distribution of biota is critical for managing flows in regulated rivers. In northern Victoria, Australia, efforts are presently underway to restore a natural, intermittent flow regime to several streams which, for over 100 years, have received perennial diversions as part of a stock, irrigation, and domestic water supply. Bayesian, model-averaged, binomial regression was used to predict probabilities of occurrence for 13 fish species, including 5 non-native species, based on hydrologic variables characterising both the current and modelled future flow regimes at 10 sites representing a range of hydrologic regimes (categorised here as heavily regulated, moderately regulated, and unregulated). Regression models accurately predicted current probabilities of occurrence for most species across all sites. The models predicted a reduced likelihood of large, native, flow-dependent species occurring at regulated sites following flow restoration, while the distribution of two small native species currently restricted to unregulated sites were predicted to increase throughout the system. Predictions regarding widespread species were much less certain, and these species may be much less affected by flow restoration. This study illustrates the effects of artificially induced perennial flow on lowland fish distribution. Furthermore, the combination of pre-restoration data together with predictive modelling provide valuable insights into the likely outcomes of flow regime shifts. This information is now helping guide management decisions and the development of mitigation strategies to protect highly-valued species in the system. Such strategies include the provision of alternative flow scenarios that provide summer base-flows together with the maintenance and creation of critical deep pool habitats. The study is unique in that it examines the impacts of perennial flows on otherwise intermittent streams, and clearly demonstrates the value of combining empirical research and modelling in guiding environmental flow and ecosystem restoration decisions.