Application of a dynamic population model for brown trout (Salmo trutta) in the l'Oir valley (Lower Normandy): First simulations.

Francés. Increasing numbers of simulation tools are currently becoming available to evaluate the impact of various types of water uses on the aquatic ecosystem; one of these is a model of the dynamics of brown trout (Salmo trutta) populations, now being developed. To complement the considerable data already available on the l'Oir valley (Lower Normandy, France), inventories as well as specific experiments were conducted in 1996 to verify and complete the different model components, especially in terms of the biological functions involved. In particular, a habitat study enables determining the carrying capacity of the environment and therefore a density-dependent component in survival of young troutlets; reproductive potential is estimated with the aid of blood samples and the sacrifice of a few mature females. Any exchanges with the downstream reaches of the valley are quantified by means of a trapping system. The results of these experiments enable an initial application of the model to the valley as a whole and a comparison of the simulations performed with data obtained from fish inventories. Use of a weekly time step for calculations appears more appropriate than a monthly step, in representing the effect of temperature on the population. However, the model in its present state does not give a satisfactory representation of the impact of the major climatic event observed in summer 91; it does, on the other hand, provide a good representation of the restoration of the population of genitors beginning in 1992.