Dispersal in metamorphosing juvenile eel Anguilla anguilla

The morphological, physiological and behavioural changes occurring during metamorphosis reveal adaptations to drastic shifts in habitat and life style. We have investigated how dispersal behaviour changed during completion of the larval metamorphosis in migrating European eels Anguilla anguilla, as they reached the limit between the tidal and non-tidal parts of a large river. We show that late-metamorphic glass eels arriving from the sea rapidly migrated in the freshwater zone of the upper estuary by means of selective tidal stream transport. Then, due to the loss of tidal advection and despite the absence of an osmotic barrier, glass eels accumulated at the break point of tidal streams. Newly transformed small yellow eels were homogeneously distributed around the point where they initially accumulated as glass eels. This suggests that completion of larval metamorphosis induced the end of upstream migration (settlement) and a switch to density-dependent dispersal linked to food search. This ontogenetic pattern probably evolved to maximise growth through optimal utilisation of productive marine and estuarine habitats.