Abstract:We investigated the mode (active versus passive) of larval downstream dispersal and its influencing factors in the nase carp (Chondrostoma nasus). Marked larvae (early and later stages), together with equivalent numbers of passive particles, were released in the main channel of the River Danube (Austria) at different flow (low, high) and current (over-critical, under-critical) conditions. Larvae and particles were recaptured with stationary nets at varying distances from release. We assumed that differences in the spatial dispersal patterns between larvae and particles were due to fish activity. We hypothesized that river discharge, developmental stage, current speed, and distance from release would influence these differences. We found that activity was independent of developmental stage or current speed at release, although activity was higher during low flow conditions. It may be that larvae deliberately enter the current during low flow, because the hydraulic conditions facilitate active dispersal. Furthermore, activity was greatest near the release site. This might be due to an intrinsically greater activity when fish are placed into novel surroundings or a result of rheoreaction. The discharge-dependent dispersal patterns observed represent an important ecological link between flow and recruitment and demonstrate the importance of inshore conditions for the early life stages of fish in large rivers, especially with regard to river modification and restoration schemes.RĂ©sumĂ© : Nous avons Ă©tudiĂ© le mode (actif ou passif) de dispersion des larves vers l'aval et les facteurs qui l'influencent chez le hotu (Chondrostoma nasus). Des larves marquĂ©es (stades prĂ©coces et plus avancĂ©s) et un nombre Ă©quivalent de particules passives ont Ă©tĂ© relĂąchĂ©es dans le chenal principal du fleuve Danube (Autriche) dans diffĂ©rentes conditions d'Ă©coulement (dĂ©bit faible, dĂ©bit Ă©levĂ©) et de courant (supercritique, sous-critique). Des larves et des particules ont Ă©tĂ© capturĂ©es dans des filets stationnaires Ă diffĂ©rentes distances du lieu de lĂącher. Nous sommes partis de l'hypothĂšse que les diffĂ©rences des motifs de dispersion spatiale entre les larves et les particules Ă©taient dues Ă l'activitĂ© des poissons. Nous avons postulĂ© que le dĂ©bit du fleuve, le stade de dĂ©veloppement, la vitesse du courant et la distance par rapport au lieu de lĂącher influenceraient ces diffĂ©rences. Nous avons constatĂ© que l'activitĂ© Ă©tait indĂ©pendante du stade de dĂ©veloppement ou de la vitesse du courant au lieu de lĂącher, l'activitĂ© Ă©tant toutefois plus forte durant des conditions de faible dĂ©bit. Il est possible que les larves entrent dĂ©libĂ©rĂ©ment dans le courant quand le dĂ©bit est faible, parce que ces conditions hydrauliques facilitent la dispersion active. En outre, l'activitĂ© Ă©tait la plus forte prĂšs du lieu de lĂącher. Cela pourrait ĂȘtre dĂ» Ă une activitĂ© intrinsĂšquement plus forte quand les poissons sont placĂ©s dans un environnement nouveau ou en raison d'une rhĂ©orĂ©action. Les motifs de dispersion dĂ©pendant du dĂ©bit observĂ©s constituent u...