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 Mehaigne (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.
In three consecutive years (1998 to 2000), 20 adult grayling Thymallus thymallus (L.) (FL ± SD: 326 ± 43 mm) were radio-tracked during circum reproduction period (February to May) in the River Aisne, Belgium. Before the spawning period, grayling remained consistently in the pool-riffle sequence in which they had been captured. Pre-spawning migration time extended from 7 to 29 March. Distances travelled during the spawning migration ranged from 70 to 4980 m (mean ± S.E.: 1234 ± 328 m). Spawning migrations started under conditions of decreasing water level and increasing water temperature in a thermal range (daily mean T°) from 5 to 8 °C. Spawners remained from 1 to 31 days (mean ± S.D.: 10.4 ± 9.8 days) at the spawning grounds and performed a post-spawning homing from 28 March to 18 April in decreasing water flow and in a thermal range (daily mean T°) from 7 to 11 °C. This study demonstrates that migration patterns of grayling are similar between years, but with a timing adjusted as a response to annual variations of the hydroclimatic conditions.