Silvering of European eel (Anguilla anguilla L.):: seasonal changes of morphological and metabolic parameters

The transformation of yellow eel into silver eel is called 'silvering', and takes place prior to migration. We found the sedentary yellow phase in spring, the migratory silver phase in autumn, while August was a cross-over month. We used principal component analysis (PCA) to characterise the morphological and physiological changes that accompany silvering in the European eel (Anguilla anguilla L.). Silvering is positively related to external parameters such as eye size, internal maturation parameters like GSI, vitellogenine (VIT), and blood-substrates such as phospholipids, Free Fatty Acids (FFA), and cholesterol. The Hepatosomatic Index was not significantly different between yellow and silver groups. In contrast, a significant difference was observed for parameters of body constitution (fat, protein, dry matter) between yellow and silver stages. Furthermore, the process of silvering is accompanied with increased levels of cortisol in autumn, which plays a role in mobilisation of metabolic energy from body stores towards migratory activity and gonadal growth. Based on Principal Component Analysis (PCA) with physiological, morphological and endocrinological parameters, it is concluded that during the process of silvering, several developmental stages can be recognised, with a timeframe of the premigratory sedentary yellow phase from April until July, August is a cross-over month, and the migratory silver phase is found from September until November.