THE EFFECT OF WATER CURRENT ON FORAGING BEHAVIOR OF THE RHEOPHILIC CYPRINID CHONDROSTOMA-NASUS (L.) DURING ONTOGENY - EVIDENCE OF A TRADE-OFF BETWEEN ENERGETIC GAIN AND SWIMMING COSTS

The main aims of this study were to investigate (1) the relationship b etween the energy balance of young stages of a rheophilic cyprinid, Ch ondrostoma nasus, and water current and (2) the critical water velocit ies for this species during its early ontogeny, Experiments were condu cted in a recirculating flume. Water velocity and type, size and densi ty of prey were varied. The foraging behaviour of three size classes o f nase (15, 20 and 25 mm TL) was filmed and analysed with an Image Ana lysis System. Capture success and consumption were determined and the following critical water velocities were measured: (1) nul benefit wat er velocity (NBWV); (2) nul consumption water velocity (NCWV); and (3) maximum sustainable water velocity (MSWV). The capture success drasti cally 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 i ncreased 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 m m TL nase). Although the three defined critical water velocities did n ot differ from each other in early larvae, MSWV in 20-25 mm TL nase we re significantly higher than the NBWV and the NCWV. The MSWV of nase s howed 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. (C) 1998 John Wiley & Sons, Ltd.