The European eels (Anguilla anguilla), which are catadromous fishes, grow in continental waters such as rivers, lakes, and lagoons before reaching the presumed Sargasso Sea spawning ground. This species, considered as a pest for a long time, is now classified as critically endangered. While attempts to preserve this species are now developed, they are limited by lacks of data on factors influencing eel population dynamics in continental environments. The aims of this PhD thesis were to study the factors influencing the abundance and features of adults (silver eels) when they leave continental waters to go to reproduce in the sea. Methods used were long-term mark-recapture monitoring (between 6 and 17 years) in natural and experimental conditions in ordered to study patterns of growth and survival in three French sub-populations. Results of the first experiment showed that the level of carrying capacity reached by the systems influences sexual orientation, which is not genetically determined in this species. For instance, individuals developed as males when the level of competition is strong, trading off to maximize survival at the cost of growth, probably to limit the costly investment in reproduction experienced by the female sex. Another experiment conducted in a freshwater march located in Camargue (South of France) consisted in successively increasing eel density up to system saturation. At saturation of the system, cohorts introduced suffered higher mortality than those previously installed, suggesting a strong competition between the different introduced cohorts. Finally, this PhD thesis provides strong support that the hydrosystem characteristics (saturation level and water quality) are determinant for eel populations as they directly influence the abundance and characteristics of futur spawners. Implications of these results for eel preservation are discussed.
Key words: European eel, population dynamic, life-history traits, growth, survival, stocking, preservation, mark-recapture, PIT-tag