Freshwater biodiversity is in crisis. Increasing human population growth continues to stress riverine ecosystems through the compounding impacts of river regulation, acknowledged as a leading cause of biodiversity decline. The global expansion of dams in the 21st century provides water security for consumptive purposes often at the expense of aquatic biota. Many studies have detailed the negative impacts of dams on fish, including the loss of migratory pathways, reduced access to preferred habitat and cold-water pollution and focus on downstream impacts, with fewer studies on the impacts of dams on fish upstream. In this study, we modelled the impact of reservoir expansion on a threatened native fish, two-spined blackfish, Gadopsis bispinosis, residing above the Cotter Dam and below Bendora Dam. Two processes were examined in the model, 1. loss of breeding habitat and, 2. increased predation. These threatening processes were assessed using the expected minimum population size as an indicator of risk.

Model outcomes suggest that the two-spined blackfish population is vulnerable to the loss of breeding habitat, particularly if the species is unable to spawn in the lentic dam waters. However, impacts are ameliorated when the ‘whole of reach’ (between the two dams) is considered. Two-spine blackfish were also found to be vulnerable to low levels of predation by trout residing in the reservoir, particularly if the trout population have a high growth rate. We hypothesised that reservoir expansion (and subsequent inundation) would result in an increase in primary productivity, driving an increase in goldfish (trout food) numbers, resulting in rapid trout population growth that would have detrimental impacts on the two-spined blackfish population. We use a population model to predict the outcomes of dam expansion on a threatened native fish species.