The article presents data on the effect of positive temperature trend of water in Rybinsk reservoir on changes in species composition, dynamics in population and fish growth. The data is base on stationary observation from Darwin State Nature Reserve during the 50-year period. Steady increase in water temperature and increased duration of ice-free period has led to significant changes in fish phenology and composition of a fish fauna. Shift of spawning to an earlier date, while remaining the same timing of reservoir filling, leads to adverse conditions of spawning phytophils. It is registered population depression of Holarctic fish species and population growth of Ponto Caspian spesies. Occurrence (appearance) of a new mass Ponto Caspian type of Clupeonella cultriventris L has led to almost complete disappearance of smelt and population growth and growth of predatory fish species of Ponto Caspian complex. Eutrophication of reservoir and overgrowing of coastal macrophytes leads to recovery (restoration) of rare spesies of limnophile.