The distribution patterns of alpine bullhead Cottus poecilopus in three tributary streams of the Rožnovská Bečva River (Danube basin) were studied with respect to temperature, oxygen concentration and saturation, shading, current, conductivity, total organic carbon (TOC), nitrates and phosphates, biochemical oxygen demand (BOD ), pH, redox potential, bottom grain structure, density of macroinvertebrates and the abundance of brown trout Salmo trutta. Sites with lower abundance per hectare of C. poecilopus differed significantly in dissolved oxygen saturation, density of macroinvertebrates during the autumn period (positive correlation with C. poecilopus) and in abundance per hectare of S. trutta (negative correlation). These results indicate that these factors significantly influence the distribution of this endangered species in the studied catchment and that stocking of S. trutta will impair its recovery.

Summary In freshwater streams, flooding is a typical source of natural disturbance that plays a key role in the dynamics of animal populations and communities. However, habitat degradation and fish stocking might increase the severity of its impact. We tested the effects of a flash flood on the abundance of three size classes of headwater dwelling Alpine bullhead, Cottus poecilopus, in the streams of the Carpathian Mountains in the Czech Republic, that are stocked with hatchery‐reared brown trout, Salmo trutta. We showed that the overall abundance of Alpine bullhead was highest at the sites with the least degraded habitat (i.e., natural habitat) and we caught almost no Alpine bullhead at the sites with the most degraded habitat. The flash flood had a strong negative effect on the abundance of the largest individuals of Alpine bullhead. Abundance of small and medium size Alpine bullhead was negatively affected by the abundance of adult stocked brown trout before as well as after the flash flood. However, negative effect of adult brown trout abundance on abundance of large Alpine bullhead was not significant before the flash flood, and it became significant after the flash flood. This could indicate an accumulation of negative impacts of trout stocking and flash flood on this size class. Overall, our results suggest that stocking of hatchery trout and habitat degradation can reinforce the impact of flash floods on the population of Alpine bullhead in the streams of the Carpathian Mountains.

In the last centuries, gravel-bed rivers in developed countries have undergone rapid changes in channel morphology. The most serious problems include channel transformation related to progressive channel narrowing, incision or bed sediment coarsening. The main reasons for transformations were connected to the human interventions, which affected water and sediment fluxes in the basins. This paper summarizes contemporary research activities focused on these negative effects of channel transformations in the Czech flysch Carpathian rivers (the Morávka, the Olše and the Ostravice draining the highest mountainous areas of the Beskydy Mts). As the result of channel transformations, progressive changes in fluvial ecosystem were observed. The initial phytosociological survey demonstrates a higher biodiversity in the floodplain along the preserved multi-thread river channel than along the deeply incised channel in the Morávka River basin. Our observations of aquatic communities demonstrated that the channel transformation connected with incision and coarsening of bed sediments negatively affected fish or lamprey populations in the studied rivers. Regulation, damming and incision of channels caused changes of hydrological regime linked with gradual drying of floodplain. Additionally, a large set of hydraulic structures, bridges or weirs were affected by undercutting and progressive destruction in the Ostravice, Olše and Morávka River basins, which is assigned to increased transport capacity of regulated channels together with decreased sediment supply from mountainous parts.