Assessing outcomes of environmental flows for an estuary-dependent fish species using a novel stochastic population model approach

Altered flow regimes remain one of the most serious threats to ecological sustainability of estuarine fish populations throughout the world. Estuary Perch (Percalates colonorum) is an estuary-dependent fish native to south-eastern Australia. The species is highly valued by recreational fishers and has greatly declined in abundance in recent decades. Strategies to recover populations include the use of environmental flows. We developed an age-based stochastic matrix population model based on the ecology of the species to explore the likely benefits of present and constructed environmental flow rules on recruitment and population response of the species in the lower Snowy River for 20 years into the future. The model outcomes indicate that the present environmental flow rules produce minimal population response and are of little benefit to recruitment of Estuary Perch. The flow prescription that produced the best modelled outcome, and that was most risk adverse to decline of females, was the annual release of 3 × 15,000 ML/day flow events, spaced 30 days apart in late winter/spring. Our approach highlights the importance of considering the whole life cycle of a species and identifying the key life history traits that can be influenced to achieve the desired conservation outcome. Although the model developed here was used to test management activities at a single site (and for a single species), it provides a novel approach that may be tailored to test and guide management activities aimed at benefiting similar flow-dependent species in estuaries throughout the world.