Population dynamics modelling in an hierarchical arborescent river network: An attempt with Salmo trutta

The balance between births and deaths in an age-structured population is st rongly influenced by the spatial distribution of sub-populations. Our aim w as to describe the demographic process of a fish population in an hierarchi cal dendritic river network, by taking into account the possible movements of individuals. We tried also to quantify the effect of river network chang es (damming or channelling) on the global fish population dynamics. The Sal mo trutta life pattern was taken as an example for. We proposed a model which includes the demographic and the migration proces ses, considering migration fast compared to demography. The population was divided into three age-classes and subdivided into fifteen spatial patches, thus having 45 state variables. Both processes were described by means of constant transfer coefficients, so we were dealing with a linear system of difference equations. The discrete case of the variable aggregation method allowed the study of the system through the dominant elements of a much sim pler linear system with only three global variables: the total number of in dividuals in each age-class. From biological hypothesis on demographic and migratory parameters, we showed that the global population dynamics of fish es is well characterized in the reference river network, and that darns cou ld have stronger effects on the global dynamics than channelling.