Background. Roach, Rutilus rutilus (Linnaeus, 1758), is an omnivorous fish species that is able to utilise a range of food resources. Both juvenile and older roach can negatively affect zooplankton abundance in freshwater bodies of water; hence populations are often reduced (biomanipulated) in order to increase zooplankton populations. The aim of this study was to assess the roach diet after large-scale removal of cyprinids (bream, roach) from a reservoir. The study was done to clarify the roach diet after three years of intensive reduction of cyprinid fish and also to find out how the roach feeding behaviour impacts the quantity of filtering zooplankton. As such, this study may help to explain more general relations within the aquatic food web and specify the roach diet during the vegetative season (from spring to autumn). Materials and methods. This study was undertaken at the Hamry water supply reservoir in the Czech Republic. Samples of macrozoobenthos, periphyton, and zooplankton were collected as representative food resources. Fish were caught using a 100-m littoral beach seine during the April to October growing season in 2011. Supplementary fish were caught using a pelagic Nordic gillnet in August and September 2012 and a 15-m beach seine in June and August 2012. Gut contents were preserved in 4% formaldehyde for later laboratory analysis (frequency of occurrence, index of preponderance, index of gut fullness). Results. ‘Detritus’ was the major component found in roach guts, with no difference observed in age category or locality (littoral vs. open water areas). Significant differences were observed, however, between younger (0+ and 1+, 36–92 mm) and older (>3 years, >92 mm) fish. The 0+ and 1+ age groups also fed on zooplankton (P < 0.008), accompanied by Chironomidae (1+), while diet of older roach (3–4+; 6–8+) included macrophytes and periphyton, together with Cladocera (fish from open water; P < 0.008). Conclusion. The results demonstrate that detritus was the main ‘dietary’ component of roach during the growing season, with macrophytes and periphyton as complementary dietary items. Zooplankton was an important dietary component of mainly younger roach age classes. Roach appear to be an important component in ichthyo-eutrophication of the Hamry Reservoir, mainly through transfer of phosphorous from plants to water.

Carp of the initial individual weight of 1020 g (+55 g) were fed in cages with four kinds of extruded, isoprotein feeds differing in the lipid content (7.5% in feed A, 11.6% in feed B, 15.3% in feed C, and 19.5% in feed D). The main source of lipids was a poultry fat. After the completion of the 52-day-long experiment the following values of the SGR and FCR indices were achieved (respectively): A-1.59 and 1.58, B-1.47 and 1.42, C-1.59 and 1.30, and D-1.59 and 1.29. The stage results, calculated every 7 days, as well as, the above-mentioned final values f FCR and SGR indicated, that throughout the entire period of study, the most favourable results of rearing were achieved while feeding the carp with the feed containing 15.3% of lipids, regardless of the water temperature, which ranged from 22.0-30.0°C at the time of the experiment.

Siberian sturgeon weighing 50 ±5 g, cultured in cages stocked at 45 specimens/m3 density and placed in cooling water, were fed Dan-ex 2545 trout feed for 112 days. Experimental treatments differed in the amount of ascorbic acid + glucose mixture (1 : 9) added to the feed. The mixture was applied, in individual treatments, in doses of 0; 1; 5; and 10 g/kg feed, corresponding to ascorbic acid doses of 0; 0.1; 0.5; and 1 g/kg feed. The highest mixture doses resulted in a significant increase in the fish body fat level; however, no significant effect on fish growth was observed.