In 2000-2001, consequences of continuous predation of winter flocks of cormorants on fish communities of the secondary trout zone of the Dyje River in the Dyje River National Park (situated on the border between Czech Republic and Austria) were studied. This study was in continuation of previous ichthyological investigations realized in the area after mass invasion of large flock (250 specimens) of cormorants in years 1997-1998. At the same 4 localities, repeated ichthyological research was carried out using electrofishing. Zoobenthos samples were also taken in annual cycle to determine saprobity indexes. Fish community was evaluated applying basic ichthyological methods. In this study indexes of diversity (H΄) and equitability (E), characterizing stability of fish community ranged between 0.082-1.739 (H΄) and 0.045-0.806 (E) respectively, with the lowest values in sections 2 and 3. Abundance of brown trout fluctuated from 68 to 349 fish·ha^-1 and of grayling from 0 to 23 fish·ha^-1 (with absence in localities 2 and 4). Total biomass of the fish community varied from 15.3 to 128.5 kg·ha^-1. Bullhead (Cottus gobio) occurrence increased significantly (abundance of 30 557 fish·ha^-1 and biomass of 123.3 kg·ha^-1 in the section 3 in year 2001) as a result of considerable decrease in brown trout population density. At present (2000-2001), total fish biomass reaches only 48.6 kg·ha^-1 (54% decline), biomass of brown trout 18.9 kg·ha^-1 (60% decline) and grayling 1.1 kg·ha^-1 (90% decline). This unfavorable state of fish community corresponds with long-run reduction in angler catches, which in case of grayling is near zero (in 2001 only 0.1 kg·ha^-1). Zoobenthos analyses confirmed (index of saprobity 1.18-1.48) high water quality (oligosaprobity) typical of trout and grayling zone.

247-254