Brno Reservoir (259 ha) is one of the most intensively utilised waterbodies in the Czech Republic, being used simultaneously for recreation, sport, fisheries, boating and electric power production. Despite this, no consistent fish survey has yet been performed at the reservoir. Between 2009 and 2012, a number of measures, including fish biomanipulation, were applied to improve water quality. Large cyprinids (mainly common bream Abramis brama) were removed and predatory fish (e.g. pike Esox lucius, zander Sander lucioperca) stocked. In September 2012 and 2013, we carried out an extensive study of the fish community in order to describe the present fish assemblage and evaluate the success of biomanipulation. Fish were sampled at 11 locations using electrofishing (inlet zone) and beach seining (lake zone). Twenty-three species and one hybrid were recorded (inlet zone – 20, lake zone – 14), with roach Rutilus rutilus and bleak Alburnus alburnus (plus perch Perca fluviatilis) dominant in inlet samples, but carp Cyprinus carpio dominant by biomass; and white bream Blicca bjoerkna and roach dominant in the lake zone, both by abundance and biomass. Predatory species represented 14-17 % of biomass in both the inlet and lake zones in both years. The final results of biomanipulation were questionable. While a low proportion of adult bream suggests successful removal, populations of small cyprinids, such as roach and white bream, increased in compensation. There was little evidence for an increase in predatory fish following stocking, probably due to angling pressure. Our results indicate that biomanipulation to improve ecological water quality in reservoirs is unlikely to be successful when they are managed specifically for carp and predator angling.
The natural diet of two different common carp (Cyprinus carpio) phenotypes (mirror and scaly) was studied in earth ponds in the Czech Republic. The proportion of different items was assessed through analysis of gut contents in order to establish differences and similarities between the phenotypes. In addition, temporal changes through the growing season, the influence of fish weight on proportion of main items ingested and the relationship between gut content and natural food availability was also assessed. The results showed no difference in feeding behaviour between mirror and scaly carp, the two phenotypes displaying a diet overlap of around 0.8.
A shift in food resources from benthic to planktonic organisms was observed between early and late summer, which was reflected in changes in electivity index for some taxa. Fish weight did not influence the proportion of organisms ingested. No correlation was found between natural food in the pond and that in the gut, any variation in diet probably reflecting the microhabitats in which individual carp choose to feed.
Food composition of otter, Lutra lutra, was studied by the analysis of 349 spraints found during one year period (2003–2004) at the River Kamenice (Czech Republic), where Atlantic salmon, Salmo salar, fry have been stocked regularly since 1998 in a reintroduction programme for the species. Brown trout, Salmo trutta m. fario, dominated otter diet and formed 29% of all prey items and 62% of biomass of all fish eaten. The second most abundant prey (27%) was common sculpin, Cottus gobio, followed by Atlantic salmon, and grayling, Thymallus thymallus. The proportion of salmon in the diet of otters amounted to 14.5% in numerical abundance of all prey items taken and 2% in biomass of fish component of the diet. The majority (71.5%) of fish eaten by otters had a total length between 61 to 200 mm.
Otter (Lutra lutra) diet was studied by analysing of 180 spraints and 29 partly consumed fish remains found in winter around fishponds in South Bohemia (Czech Republic). The proportion of fish found in spraints was 95.6% of all prey items, roach (Rutilus rutilus), topmouth gudgeon (Pseudorasbora parva) and common carp (Cyprinus carpio) being the most numerically abundant species with 37.3, 21.3 and 19.0% respectively. Carp dominated the diet bymass (47.8%), whilst roach and “other commercial fish species” (predatory and herbivorous fish, tench, Tinca tinca) formed 21.8 and 15.3% of total biomass, respectively. Small fish (< 200 mm) predominated in the diet and only 4.0% of all fish found in spraints exceeded 300 mm TL. Large fish remains were very scarce in spraints and were best recorded from uneaten prey remains. The partially eaten remains of carp, pike (Esox lucius) and common bream (Abramis brama) were found on banks or ice, but most (86%) were carp. The original length of carp corpses ranged between 283 and 530 mm TL, and the proportion of body mass consumed varied between 5.0 – 90.1%. The length of pike remains ranged from 386 to 754 mm TL, of which 84.0% of body mass on average was consumed by otters. The otter diet apparently reflects food availability in fishponds and supply channels. Small water basins with a high stock density can be vulnerable to serious damage especially during the winter period and at such places, where no better accessible source of food is available to otters. However, in many such places simple mitigation measures may be able to reduce otter predation.