Estimating the stocking potential of fish in impoundments by modelling supply and steady‐state demand

Summary 1. Fish stocking is an increasingly common management tool for freshwater and marine environments and is often used to create and maintain fisheries in closed waters. The densities at which fish are stocked can have a large impact on a stocking programme’s success and sustainability. Stocking densities in impoundment sport‐fisheries, for example, are often based on social or practical factors, and ecologically based stocking models are needed to assist the selection of stocking densities that are appropriate for the environment. 2. In this study, stocking density is calculated with a numerical model that balances the supply of prey production with the energetic demand of stocked fish. The model aims to deliver outcomes over a range of potential management objectives, by providing specific consumption scenarios that represent the trade‐off between population abundance and individual body size in stocked fisheries. 3. The model uses a steady‐state population approach to calculate stocking density, which optimises population consumption by maintaining a consistent biomass distribution and encourages sustainable stocking by considering the energetic needs of all cohorts. Carrying capacity is represented by the steady‐state stocking density under a minimum consumption scenario (when fish meet only their minimum energetic needs). The comparison between a desired consumption rate and the existing level of production is used to assess the status or ‘health’ of the existing population and is used to determine whether stocking can occur and whether stocking densities can be sustainably increased. The probability of incorrectly assuming populations are achieving a given consumption level is also estimated, which is an ideal approach for interpreting multiple probability distributions. 4. A Monte‐Carlo analysis of uncertainty was used to provide a probability distribution of stocking density of Australian bass ( Macquaria novemaculeata ) in three Australian impoundments under various seasonal and consumption scenarios. The likely consumption rates of the existing populations were determined using historical stocking densities, which showed that the three populations were of reasonable health, although one impoundment may be overstocked. The steady‐state stocking densities depended on the desired consumption rate, and there was an eightfold difference in the stocking density aimed at providing large ‘trophy’ fish and the density required to reach carrying capacity. 5. Model outputs of existing abundance and biomass density agreed with empirical estimates of abundance and relative density in these impoundments, which provides support to the model’s accuracy. This supply–demand approach to estimating the range of appropriate stocking densities shows promise as a decision‐support tool for stocked impoundments and other closed fisheries.