Assessing the use of environmental flows and stocking for the persistence of a flow-dependent spawner in a drying climate

Context Where water is used to support ecosystem processes under a changing climate, it is vital that it is done efficiently. The Australian bass (Percalates novemaculeata) is a freshwater fish endemic to south-eastern Australia that has been adversely affected by anthropogenic disturbances. This has prompted investigations as to the use of environmental water in support of the species in addition to widespread stocking. Aims We investigate whether environmental flows and stocking in isolation support a population of Australian bass more effectively than they do in combination, under various climate-change scenarios. We also determine the cost efficiency of each strategy. Methods We used an age-based stochastic population model to determine outcomes of strategies. Key results Our model suggested that the application of two or three large environmental flows per year was highly effective in all but the most-extreme climate-change scenarios. Alternatively, the value of stocking increased with an increasing severity of climate change. The cost of delivery of environmental flows far exceeds that of stocking. However, stocking directly benefits only a single species (in this case Australian bass), whereas the provision of environmental water has multi-species benefits, in addition to being vital to maintaining the river ecosystem itself. Conclusions Under severe climate-change scenarios, stocking, and the use of environmental water in support of populations, may be successful only to a point. Implications Extreme climate change may therefore negate any attempts to halt the decline of populations of some species.