InfluenImpact of individual morphological and physiological characteristics on the sexual maturation of the European eel (Anguilla anguilla)

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Since the 1980’s worldwide populations of eels declined steeply (Moriarty 1986, ICES 2002, Dekker 2003a). Several causes have been identified and studied, but mainly those related to the freshwater life phase. Marine causes for this decline have also been hypothesised such as deteriorating reproductive capacities of the adults (Feunteun 2002, Dekker 2003b). The European eel is believed to spawn in the Sargasso Sea (Schmidt 1922), approximately 6000 km from its continental habitat. It is only during the long-distance migration to the spawning grounds that the eel undergoes true sexual maturation (vitellogenesis and ovarian development), triggered by environmental factors encountered during this phase. However, the oceanic life phase of eels remains for the most part a mystery as no sexually maturing individuals have ever been caught or observed in the open sea. At present, the only way to obtain sexually maturing eels, is to artificially induce sexual maturation in migratory individuals (silver eels) caught in freshwater (for review Dufour et al. 2003). However, accounts of eels (particularly true for Anguilla anguilla) not responding to gonadotropin injections are frequent and yet unexplained (Pedersen 2003, Pedersen 2004, Palstra et al. 2005). Overall, there is evidence that although eels receive the same treatment, they display tremendous variability in the way they sexually mature and in their reproductive capacities (Dufour unpublished data, this study). The objective of this study was to describe the variability in the response of eels in terms of gonadal yield to standardized gonadotropic treatment (CPE injections), and to relate this variability to individual characteristics. To do this, sexual maturation was induced in groups of eels from different locations in France. Their capacity to mature was evaluated throughout the experimental periods using external indicators, and compared to their initial physiological and morphological characteristics.