A Stochastic Trout Population Model for Supporting Fish Management

Brown trout (Salmo trutta fario) is an ecologically and economically important fish species in many Swiss rivers. Since the 1980s, a significant decrease of brown trout catches was reported across Swiss rivers. To better understand the causes of this decline, a trout population model was developed a few years ago. It predicted trout densities at single river reaches. We improved this model to better guide management decisions for river restoration. We extended the model by distinguishing stocked and resident fish. This allows us to account for the empirical evidence of different behavior of stocked and resident fish in order to better consider the effect of the most important fish management practice, stocking. We use different mortality rates for stocked and resident fish populations by introducing a mortality rate ratio between stocked and resident fish as a model parameter. The impact of this parameterization is studied by sensitivity analyses of the deterministic fry survival submodel as well as the full, stochastic brown trout life cycle model.