The effect of water current on foraging behaviour of the rheophilic cyprinidChondrostoma nasus (L.) during ontogeny: evidence of a trade-off between energetic gain and swimming costs

Abstract

The main aims of this study were to investigate (1) the relationship between the energy balance of young stages of a rheophilic cyprinid, Chondrostoma nasus, and water current and (2) the critical water velocities for this species during its early ontogeny. Experiments were conducted in a recirculating flume. Water velocity and type, size and density of prey were varied. The foraging behaviour of three size classes of nase (15, 20 and 25 mm TL) was filmed and analysed with an Image Analysis System. Capture success and consumption were determined and the following critical water velocities were measured: (1) nul benefit water velocity (NBWV); (2) nul consumption water velocity (NCWV); and (3) maximum sustainable water velocity (MSWV). The capture success drastically declined above a certain value of water velocity which increased with body size. Consumption varied with density and size of prey and with fish size. It was high within a range of water velocities which increased with fish size and then declined to the NCWV. The profitable range of water currents increased during ontogeny, and swimming costs in early larvae were higher in proportion to the maximal energy intake, compared with larger fish (positive net energy gain between 0 and 7 cm s−1 in 15 mm TL nase and between 0 and 15 cm s−1 in 20 and 25 mm TL nase). Although the three defined critical water velocities did not differ from each other in early larvae, MSWV in 20–25 mm TL nase were significantly higher than the NBWV and the NCWV. The MSWV of nase showed a linear increase with fish size. A comparison with values from the literature for other fish species shows that MSWV may depend more on body shape than on species-specific life history patterns. © 1998 John Wiley & Sons, Ltd.