Towards a standardized characterization of the potentially migrating silver European eel (Anguilla anguilla, L.)

We defined a standardized method for discriminating candidate silver eels that may undergo catadromous migration in the following season from the sedentary fraction of a population. A combination of two qualitative criteria (state of differentiation of the lateral line and colour contrast) and one quantitative criterion (Ocular Index OI) was used to determine the development toward silvering. In the non-migratory phase, we found a gradient of the three criteria between yellow (0 criterion), presilver (1 to 2 criteria) and silver (3 criteria) eels. In the migrant phase, silver eels had ended their metamorphosis process and were characterized at the same time by the presence of the 3 silvering criteria. A mark-recapture survey using PIT-tags provided evidence that only identified silver eels (3 silvering criteria present) in the catchment actually emigrated the following season. Moreover, the use of a single criterion of silvering among the three generated large variation in the estimated proportion of candidates for emigration which varied between -22% and +63 %. Such a result confirmed that a multicriteria approach is needed to characterize in a standard way the potentially migrating silver eel.