RESUMEN
The influence of energy content of glass eels (Anguilla anguilla) on their migratory behavior was investigated in November 2004 and February 2005 under laboratory conditions. Fish were sorted according to their response to a decrease in light intensity: Glass eels becoming active at dusk and using water flow to move with the current were considered as "active", whereas those remaining in the substratum throughout the catching period (24 h) were categorized as "inactive". "Active" glass eels exhibited higher energy content than "inactive" ones, whatever the catching month, and were more pigmented. Differences in percentage dry weight between "active" and "inactive" fish were already significant in the first pigmentary stages, suggesting that energy content plays a causal role in the estuarine migration of glass eels. The possible impact of energy state on the development of different migratory tactics in glass eels is discussed.