Modelling the effects of drought on the population of brown trout in BlackBrows Beck

Records from a 30-year study of the population of migratory brown trout, Sa lmo trutta L., in Black Brews Beck (English Lake District) are used to cons truct a model for predicting population numbers during normal rainfall cond itions and under the influence of drought. Twice-yearly samples of differen t age-groups of trout were recorded, together with a more detailed set of r ecords 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 indicato r of drought for the region. Drought conditions result in lower than expect ed stock which can be identified as outliers from the Ricker stock-recruitm ent curves for each sample. A set of five difference equations, which descr ibe the overlapping trout age-groups, is identified and used to forecast th e trout population up to a few years ahead. The model is particularly succe ssful at forecasting population numbers 1-year ahead if the following years drought is taken into account. Incorporation of drought effects into the m odel significantly improves model forecasts. A linearised stability analysi s suggests that long-term drought conditions will have the effect of increa sing population stability, although a particularly severe drought, or serie s of severe droughts, may lead to population decline. (C) 2000 Elsevier Sci ence B.V. All rights reserved.