We propose a matrix population modelling approach in order to describe the dynamics of a grayling (Thymallus thymallus, L. 1758) population living in the Ain river (France). We built a Leslie like model, which integrates the climate changes in terms of temperature and discharge. First, we show how temperature and discharge can be related to life history traits like survival and reproduction. Second, we show how to use the population model to precisely examine the life cycle of grayling : estimated numbers of individuals at the beginning of the reproduction season, estimated number of individuals in summer, comparison between observed and simulated numbers of individuals. Finally, we discuss our results in the framework of the assessment of impact of possible climate changes on grayling populations.