Growth modelling in accordance with daily water temperature in European grayling (Thymallus thymallus L.)

The model of von Bertalanffy has been and is still widely used to model fis h growth, mainly because of its good description of annual growth over the whole life span. However, it does not take into account a seasonal variabil ity in growth sate, an important phenomenon that appears quite well correla ted with water temperature fluctuations in temperate climates. In the prese nt study, we demonstrated that it was possible to model such variations by including daily water temperature in the von Bertalanffy growth formula owi ng to the correlation between the growth coefficient k and water temperatur e. The model we chose to describe such a correlation includes parameters wi th obvious biological significance and is mathematically well structured, w hich allowed an extensive use of our growth model. Hence, we use our new mo del to describe annual variability in the growth of European grayling (Thym allus thymallus L.) in a river section where water temperature could rise u p to the thermal tolerance limit for this species, inducing reduced growth rates and severe mortality events. Finally we were able to explain the grow th rate variability from one year to the next by interannual water temperat ure fluctuations.