Growth Pattern of Stream-Dwelling Brown Trout under Contrasting Thermal Conditions

Abstract

Growth of brown trout Salmo trutta was analyzed based on 3 years of simultaneous temperature and growth data from seven streams of contrasting chemical and biological character. A laboratory-based growth model was employed to examine geographical variation in growth performance among wild populations. A sensitivity analysis of model predictions was also performed based on simulated optimum temperatures for growth within the range of observed temperatures and on the optimum temperature used in the model. In spite of the diverse environmental conditions, the annual increment in body mass was not significantly different among most populations. However, the ratio of the actual growth rate to the maximum growth rate predicted by the model differed among rivers. A significant negative correlation was found between this ratio and mean annual water temperature. The growth model thus underestimated growth in the coldest rivers, suggesting that adaptations to local thermal conditions can occur at a small geographical scale. Seasonal changes in the fit to the growth model were also different among rivers. Our simulations showed that, in almost all rivers, the best fit to the model among age-1 trout was obtained at lower values of optimum temperature in both spring and summer. Our findings suggest that the parameters of the model may not be identical for all brown trout populations. The discrepancies between observed and predicted growth indicated that temperature alone did not account for all the spatial and temporal variation in growth rates.