Modelling the effects of drought on the population of brown trout in Black Brows Beck

Records from a 30-year study of the population of migratory brown trout, Salmo trutta L., in Black Brows Beck (English Lake District) are used to construct a model for predicting population numbers during normal rainfall conditions and under the influence of drought. Twice-yearly samples of different age-groups of trout were recorded, together with a more detailed set of records for a further 8 years. These data are used to develop and calibrate a time-dependent population model. A concurrent record of rainfall data for Windermere is aggregated to form seasonal averages and used as an indicator of drought for the region. Drought conditions result in lower than expected stock which can be identified as outliers from the Ricker stock-recruitment curves for each sample. A set of five difference equations, which describe the overlapping trout age-groups, is identified and used to forecast the trout population up to a few years ahead. The model is particularly successful at forecasting population numbers 1-year ahead if the following years drought is taken into account. Incorporation of drought effects into the model significantly improves model forecasts. A linearised stability analysis suggests that long-term drought conditions will have the effect of increasing population stability, although a particularly severe drought, or series of severe droughts, may lead to population decline.