Influence of river morphology and current velocity conditions on spawning site selection of Chondrostoma nasus (L.)
The spawning site of nase, Chondrostoma nasus, was investigated in the Fischa, a tributary of the Danube in Austria. Hydrological and river morphological variables (river width, water temperature, current velocity, water depth, substrate composition and oxygen content in the interstices) were measured at the spawning site, at resting places and randomly chosen river sections to analyse spawning habitat selection. The spawning temperature (7-years observation period) was 9.0 ± 1.7° C. At the spawning site a strong, significant selection for depths in the range of 0.2-0.3 m (nearly 60% of all measurements) and average water current velocities between 0.4 and 0.6 m • s-1 (> 50% of all measurements) was observed. The sediment composition at the spawning area was characterized by a high proportion of gravel and pebbles, and differed significantly from the overall available substrates (Mann-Whitney U-Test; p < 0.004, n = 42). The oxygen contents in the interstices at the two measured depth layers at the spawning site were significantly higher (Mann-Whitney U-Test; p < 0.005) than at adjacent downstream sections. The mean values at the spawning site were 9.0 ± 0.8 mg in 10 cm and 6.9 ± 2.6 in 20 cm sediment depth; at the adjacent downstream section the corresponding values were 5.0 ± 3.7 mg in 10 cm and 3.4 ± 3.3 mg in 20 cm sediment depth. These results may be used in the framework of river restoration projects to construct and predict usable spawning sites for this endangered rheophilic cyprinid.