Reproductive ecology of three rheophilic fish species in the Austrian Danube River system: Insights into the spawning of Chondrostoma nasus, Barbus barbus, and Vimba vimba, and the larval development of C. nasus in the River Traisen

This study investigates the reproductive ecology of three rheophilic fish species (Chondrostoma nasus, Barbus barbus and Vimba vimba) in the River Traisen, Austria. The main objectives were to describe the spawning sites of these species and to describe the development of nase eggs and larvae in situ. The study also examines the immigration patterns from the Danube into the Traisen in relation to discharge and water temperature. The fieldwork was carried out between March and June 2020 in the lower reaches of the Traisen (11 km renaturalized river section and 4 km residual water section). In addition to visual counts, a drone (UAV) and underwater cameras were used to quantify and document spawning behavior. At the spawning sites the parameters water depth, flow velocity, temperature and substrate type were recorded. The spawning site characteristics of nase and barbel were consistent with previous studies. The vimba bream spawned at a water temperature of 13–17 °C, a water depth of 39 ± 9 cm and a flow velocity of 93 ± 19 cm/s and was described for the first time for a Central European river population. The development of the nase from eggs to fry was documented in the field; the incubation of the eggs lasted 30 days with a mean water temperature of 11.4 °C. Only a few barbel and vimba bream used the restored section for spawning, the majority of both and all nase spawned in the residual flow stretch upstream, where they were prevented from migrating further due to the low discharge.