Assessing brown trout population responses to alternative flow scenarios in a regulated Alpine river using an individual-based model

To achieve the environmental objectives of the Water Framework Directive, Italian river management policy is moving from a minimum flow release (Deflusso Minimo Vitale) to ecological flows (Deflusso Ecologico). This thesis applies inSTREAM, a mechanistic individual-based model that simulates the life cycle of stream salmonids and predicts how the abundance, biomass and persistence of the population depend on habitat characteristics and on the flow and temperature regimes. The model was applied to a regulated reach of the Cordevole River, first to verify its behaviour under the hydrological regime of the reach, then to assess the response of a brown trout population to three alternative releases: the minimum vital flow, the ecological flow and an increased ecological flow. Setting the model up required modelling the habitat in GIS, simulating depth and velocity at each discharge with HEC-RAS, assembling the flow, temperature and turbidity input series, and evaluating the parameters specific to brown trout. Two model experiments were run; the first reproduced the population under the actual hydrological regime of the reach for the period 2010-2019: the population persisted with an age structure, a density and a biomass comparable with those reported for alpine streams. The second experiment compared the three water releases using constructed time series from which hydrological variability was removed, to isolate the effect of the release: higher releases produce lower abundance in the juvenile classes and favour the adults in terms of growth. The results are preliminary, calibration on local data is still required. The model nevertheless proved a useful tool to support decisions in flow regulation, as it can be for other aspects of river and fishery management.