Since the introduction of sunbleak (Leucaspius delineatus) to southern England in 1986, its life history characteristics (such as reproductive behaviour, early sexual maturity and an unusually small adult size) have contributed to its rapid dispersal. This study examines the length-weight relationships and age of this non-indigenous cyprinid to highlight the potential threat to native 0+ cyprinids. Sunbleak populations demonstrated an unusual growth pattern for a cyprinid, with an average of 42 % of its maximum growth occurring in the first year, followed by extremely low annual growth until death. Very few significant differences were found between the mean length of several sunbleak age groups and the length of native 0+ bream Abramis brama, roach Rutilus rutilus, bleak Alburnus alburnus and rudd Scardinius erythrophthalmus. We have also found that young-of-the-year of these cyprinids share the same food and habitat with all sunbleak year class, which in some places has had a detrimental impact on the recruitment of native species.
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.