Eel facultative catadromy is not clearly understood. Initial work suggested a possible control by energy
content and energy expense in young stages. The present work aimed to better understand the mechanisms
involved, by studying the influence of the salinity rearing environment in the loss of mass and
therefore on fish energy and migration. Glass eels (Anguilla anguilla L.) caught on the Atlantic coast were
identified according to their rheotactic response to the dusk signal, and thus to their propensity to
migrate which can be low (sedentary, M
fish) or high (flow-wise catches in early night, Mþ
fish)