The silvering process of Anguilla anguilla: a new classification from the yellow resident to the silver migrating stage

The identification of five stages for female and two stages for male eels Anguilla anguilla using multivariate analysis was carried out on a large sample of individuals collected at six different locations in France. Stages corresponded to a growth phase (stages I and II), a pre-migrant phase (III) and two migrating phases (IV and V). It is likely that an important period of growth triggered silvering through the production of growth hormone (GH) in stage III eels. In migrating eels gonad development, gonadotropin hormone (GTH-II) production and increase of eye surface were similar at all sites. Differences among locations were found in gut regression and pectoral fin length. As variability for these increased with the size of the watershed and values were highest for the most downstream locations, fin length and gut regression may indicate the time since an eel started its migration. # 2005 The Fisheries Society of the British Isles