Inter-annual and diel patterns of the drift of cyprinid fishes in a small tributary of the River Meuse

The biological processes underlying larval cyprinid drift are still unclear, in particular the distinction between the active and passive components. The present study examined fish drift, water temperature and light intensity in the River Méhaigne (Belgium) over a two-year period using a two-fold method that combines punctual 24 h samples with 3 weekly 1 h dusk samples over the drift season. Inter-annual comparisons revealed variations in the relative abundance of roach Rutilus rutilus and chub Leuciscus cephalus, explained by a reduced spawning success of chub due to a lower mean water temperature in 2004. Cyprinid larval drift peaked under increasing temperatures, whereas river discharge and turbidity had no apparent effect. A nocturnal repartition of the diel drift distribution was observed, with the highest drift density during late dusk (< 1 Lux) suggesting a loss of rheotaxis in dark conditions. Moreover, the mean body size of roach and chub varied significantly between different times of the day, roach TL increasing from dusk to dawn while chub TL was smallest at night. Biological significance of the drift is discussed with respect to specific larval ecology and morphology.