International eel symposium, 2003 - Quebec August 11, 2003
Oral Presentation

The Silvering Process of the European Eel: Characterization of Intermediate Stages for Identification of Downstream Migrants

Durif, C.* Cemagref, 50 avenue de Verdun, 33612 Cestas, France
Elie, P. Cemagref, 50 avenue de Verdun, 33612 Cestas, France

Presenter email address: Caroline.Durif AT cemagref.fr

Abstract Text: Before they start to migrate downstream eels undergo a metamorphosis called silvering, which primarily corresponds to the beginning of the reproductive phase. The fact that the European eel (Anguilla anguilla) is now considered as threatened lead us to consider ways of developing indices to evaluate quantitatively the fraction of downstream migrants.A physio-anatomical and morphological characterization of the transitional stages between the yellow and silver phases was achieved on a sample of more than a thousand eels, captured at eight sites in France. Data analysis methods were carried out to define five successive stages for female eels and three for males. These are based on the development of gonads, regression of the digestive tract and increase of pituitary gonadotropin hormone (GTH II). Throughout this study, knowledge obtained on the eco-physiology of silver eels concerns the definition of each silvering stage up to the typical migrating stage. In terms of application, these results allowed us to build indices based on biometrical measurements for the identification of the silvering stages. These results are directly applicable to the quantitative estimation of the fraction of migrants through the surveys, which are routinely done on the French territory as well as through fisheries data.