Diel dynamics of habitat use by riverine young-of-the-year Barbus barbus and Chondrostoma nasus (Cyprinidae)

Diel dynamics of habitat use by larvae and juveniles of barbel (Barbus barbus) and nase (Chondrostoma nasus) were investigated in a lotic river (River Ourthe, Belgium), from July 1995 to January 1996. In July, larvae of both species consistently used calm, shallow inshore bays throughout the day. In August and early September, juveniles of both species moved into bays at dawn, reached maximum densities at mid-day, when the bay was the warmest compared to the stream, then left the bay and moved into neighbouring shallow riffles in the late afternoon. Small fish immigrated earlier into the bay and emigrated later than larger fish. By early autumn, most nase and barbel had left the bay but occasionally returned there at night. While overwinter-ing, nase occupied shelters in bays all day and showed no diel movements whereas barbel overwintered elsewhere. Because juveniles of the two species, the omnivorous barbel and the herbivorous nase, showed almost identical diel dynamics of habitat use throughout the growth period, it is suggested that habitat structure and predation pressure, rather than food availability, are the driving forces behind these patterns.