Analysis of morphological and physiological parameters during the silvering process of the European eel (Anguilla anguilla) in the lake of Grand-Lieu(France).

The aim of the current study was to describe the morphological and physiolo gical changes, which occur during the transition from yellow to silver eel (Anguilla anguilla). We investigated the different stages present in an eel population prior to the downstream migration. We were thus able to observe the sequence of changes during the metamorphosis. Measurements were taken on 305 eels captured in the lake of Grand-Lieu (France). To evaluate the de gree of metamorphosis, gonads, digestive tract, and liver were weighed and the level of gonadotropic hormone (GtH) was measured. Principal component a nalysis (PCA) was carried out on a subsample of eels for which we had all f our physiological parameters in order to determine homogeneous groups accor ding to their sex and degree of maturation. Thus, three groups were identif ied: the first one corresponding to male silver and intermediate eels, the second one, silver and intermediate females, and the third group yellow eel s. Discriminant functions based on morphological variables were then calcul ated and used to reclassify individuals in their initial category. Eels wer e correctly reclassified at 94%. These functions were then applied on the w hole sample. Results of the classification were compared to initial data, a llowing the identification of four different stages as well as the sequence of events during the beginning of the metamorphosis.