After Anguilla anguilla larvae reach the European coast, metamorphosing glass eels exhibit an estuarine migration phase and can potentially colonize the continental area. Associated behaviours to upstream movement in estuary and river basin differ strongly: passive tidal transport in estuary, active swimming beyond the upstream tidal limit. Moreover, the migratory behaviour may shift towards a density-dependent dispersal beyond this limit. A positive relationship has previously been established between glass eels' body condition and migratory behaviour in estuary and also higher in the river basin. An experiment was settled to test for the density-dependent versus migratory behaviour under controlled conditions. The relationships between some behaviours (swimming, grouping, feeding, and aggressiveness) and body condition was investigated at the individual level. Two controlled factors were crossed, leading to four combinations of high and low levels of food and temperature. The high level of food led to a lesser loss in body condition. Swimming activity was positively related to initial body condition and loss in body condition, but these two variables were not related to aggressiveness. We conclude that the density-dependent dispersal hypothesis was not reinforced by these present results.
RÉSUMÉRelations entre comportement individuel et morphométrie sous différentes conditions expérimentales de température et de nourriture chez les anguilles européennes (Anguilla anguilla) L'anguille européenne est un poisson migrateur dont les larves parviennent jusqu'à nos côtes où elles se métamorphosent en civelles qui colonisent le milieu continental après une phase de migration estuarienne, suivie ou non par la colonisation du bassin versant. Les comportements associés au déplacement amont dans ces deux milieux diffèrent fortement : une nage « portée » par le courant durant le flot en estuaire, une nage active en limite amont d'estuaire. Une corrélation positive a déjà été établie entre le facteur de condition et le potentiel migratoire de l'anguille (que ce soit en estuaire ou en phase de colonisation du bassin versant). Par ailleurs,