Spatial Distribution and Temporal Variance of Fish Communities in the Channelized and Regulated Vltava River (Central Europe)

Spatial distribution and temporal variations of fish communities in a channelized portion of the Vltava River (Central Europe) were investigated from May to October 1992. Fish abundance and species number were collected monthly during diel samples in the main channel and a large oxbow. To determine differences in fish communities we chose the low-flow period when maximal differences in physical environment between habitats are expected. Fish community patterns from the main channel and oxbow differed only in abundance of ecological groups; the main channel had higher abundance of rheophilic species, whereas eurytopic species were more abundant in the oxbow. However, our analysis revealed significant differences in temporal variations between habitats. Fish abundance usually varied unpredictably over day-night periods in the main channel, whereas in the oxbow we observed a pronounced peak in fish abundance from morning to mid-afternoon. Diel changes in fish abundance in the oxbow corresponded to occurrence of low dissolved oxygen during afternoon and night-time periods. The effect of oxygen depletion induced by strong organic pollution limited the oxbow suitability for fishes. Similar habitats are usually occupied by limnophilic and phytophilic species that were absent in our samples. Our results suggested that man-induced high variability of physical environment decreased variability in fish community.