A preliminary study of the drift of young-of-the-year (YOY) cyprinid fish in a dam-fragmented large river : the Belgian River Meuse.

Downstream dispersal of fish in rivers is mainly an unidirectional drift movement because of the entrainement by water current. Most of the Cyprinidae species exhibit a propensity to drift in their egg-larval and juvenile stages as young-of-the-year (YOY) in spring and early summer. Th drift of YOY chub (Leuciscus cephalus) and nase (Chondrostoma nasus) in the River Meuse was studied during the year 2001 by collecting fish impinged on the filtering screens of a cooling water intake (Tihange nuclear power plant). A first drift wave was observed in summer and was composed of healthy fast growing individuals. A second drift wave was observed in autumn and seemed to be related to high flows and turbid water at that time of the year. In both species, the mean body lengths were smaller in the autumn than at the end of the growing period in summer. Our hypothesis are : i) that the River Meuse is a common dispersal-drifting route for distinct sub-populations of YOY fish from the whole upstream basin ;ii) that populations drifting in autumn are different from those drifting in summer in term of causal factors involved.The ecological significance of the drift process of YOY fish in dam-fragmented rivers is briefly discussed based on the Meuse case study.