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The sinusoidal swimming of fish, previously interpreted as foraging behaviour, was studied with respect to season, density of large zooplankton, fish length, time of the day, weather condition and solar radiation in Řimov Reservoir, Czech Republic, using a bottom-mounted, split-beam transducer (7°, nominal angle; frequency 120 kHz). The proportion of sinusoidally swimming fish increased from April to August while this behaviour was absent in October. The occurrence of sinusoidal swimming showed an apparent pattern throughout the day; it increased sharply around sunrise, was highest within 5–6 h around solar noon, and sharply decreased around sunset. Significantly less frequent occurrence of sinusoidal swimming was recorded during cloudy days compared to sunny days. The vast majority of records came from fish of standard length ranging from 100 to 400 mm, which represents the typical size range of common bream Abramis brama and roach Rutilus rutilus of age >1+, the main zooplanktivores in the reservoir. The presence of these larger fish in the open water of the reservoir, as well as the presence of sinusoidal swimming, apparently correlates with the presence of large zooplankton (Daphnia, Leptodora and Cyclops vicinus) in the epilimnion. The increase of sinusoidal swimming between April, June and finally August resulted in an increase of zooplankton component in fish guts. It appears that high values of solar radiation, and stable calm weather during high pressure periods, result in optimal optical conditions for sinusoidal swimming, making this foraging behaviour more efficient and widely used in fishes exploiting the zooplankton production in the reservoir.

The orientation of fish in a horizontal plane has important consequences for estimating their true size from horizontal acoustic records. The aim of this work was to verify the assumption that during the daytime fish are randomly orientated in the lacustrine zone of the canyon-shaped Řimov reservoir and to compare distributions of fish orientation between the lacustrine and tributary (riverine) zones. Fish orientation was acoustically surveyed at fixed locations using the SIMRAD EK 60 split-beam echo sounder (elliptical beam, 120 kHz) with a horizontally aligned transducer. The horizontal aspect (angle between the fish body and the transducer axis) was used to describe their orientation. The conventional single-echo detector (SED) and the cross-filter detector (CFD) were applied. No trend was found along the reservoir when comparing records from four sites processed with the conventional SED. At all sites, most fish appeared to move predominantly in directions perpendicular to the central axis of the acoustic beam, i.e. the side-aspects (90°) of fish prevailed over other aspects. The CFD registered tracked fish several times more often than the SED. In the lacustrine zone the frequency distribution of fish aspect appears very similar when recorded by sonar beams oriented parallel to and across the longitudinal axis of the reservoir (criss-cross-beaming experiment). In the tributary zone, beaming perpendicular to the longitudinal axis of the reservoir revealed a significantly higher proportion of fish moving along the longitudinal axis. Therefore, the assumption of random fish orientation is not applicable in the tributary zones of such reservoirs.

Des campagnes acoustiques nocturnes et durant le jour ont ete effectuees pour comparer les estimations de stock de poissons de 8 lacs artificiels et un puits de mine inondee d'Europe centrale. Durant l'ete, de jour comme de nuit, des biomasses plus elevees de poissons ont ete observees dans la partie haute (jusqu'a 4 m de profondeur) de la colonne d'eau, par sondage acoustique horizontal que dans la partie plus profonde par sondage vertical. Des comparaisons du coefficient de retro-(reverberation) en fonction du volume d'eau (s v ), de la biomasse de poissons (kg ha -1 ), de leur abondance (ind. ha -1 ) et du poids moyen des poissons (g) ne montrent pas de schemas communs entre ces reservoirs mais varient significativement en fonction de la periode du jour ou de la nuit. Pour la plupart des reservoirs, les campagnes nocturnes presentent des estimations plus importantes que celles effectuees de jour. Cependant, le contraire est aussi observe pour certains reservoirs. Ces differences ne peuvent s'expliquer par la composition en especes des stocks de poissons. La repartition en tailles des poissons est differente entre la nuit et le jour. Une plus grande proportion de plus gros poissons (1+) est trouvee durant la nuit pour deux des trois lacs reservoirs et du lac minier. En general, il n'est pas possible de decider si des campagnes de jour ou de nuit fournissent des estimations plus fiables des stocks de poissons. Nous conseillons d'effectuer des transects de nuit et de jour avant de decider de la periode d'evaluation. Pour une estimation complete, il est souhaitable de combiner des campagnes acoustiques mobiles avec un echantillonnage au moyen d'un engin de peche.

The aim of this study was to quantify the efficiency of different lengths of beach seine nets for each diel period in freshwater reservoirs. Only fish older than young-of-the-year were considered. Nets of 10, 20 and 50 m length ("in-nets") were tested in an enclosed area framed by a 200 m long net (block net). The net efficiency estimate was calculated as the ratio of fish catches with the in-net and block net divided by the ratio of their respective areas. The net efficiency estimate was significantly different between day and night catches. At night, the efficiency estimate of nets depended on the size of the fish. The efficiency estimate of a 10 m long net decreased significantly in reverse correlation with fish size. A similar trend was found when using a 20 m long net. The efficiency estimate of a 50 m long net was independent of fish size. The variance in efficiency estimate between samples with a given net length was high, but decreased with longer nets. Of five species tested, only the efficiency estimate for catching bream (Abramis brama) increased significantly with the length of net. The biomass and abundance of larger fish was generally higher at night, although especially short nets exhibited a spuriously high efficiency estimate during the day, probably due to the concentrating (chasing) effect of the hauling ropes. We therefore recommend the use of a 50 m long net, since its nighttime efficiency estimate was about 0.9 in terms of both sampling abundance and biomass. A model relating the efficiency estimate and net length was developed with the data acquired.

In August 2004 and 2005, an extensive study of the fish community was carried out in the largest water supply reservoir in the Czech Republic and Central Europe, the canyon-shaped Želivka Reservoir, using a fleet of Nordic multimesh gillnets. Fishes were sampled at eight locations along the longitudinal profile of the reservoir and at five benthic depth layers covering depths from the surface down to 18 m (benthic gillnet 1·5 m high), and at three pelagic depth layers down to the depth of 5 m above the bottom (pelagic gillnets 4·5 m high). Catches of both juvenile (age 0+ year) and adult (fishes >1 year) fishes were highest in the upper layers of the water column (i.e. in the epilimnion down to 5 m, and down to 10 m in the benthic habitats). Along the tributary–dam axis in the pelagic habitats, both juvenile and adult fishes preferred the upper part of the reservoir, where the maximum number of species and also the greatest abundance of zooplankton were found. In the benthic habitats, fishes selected location according to factors other than trophic status. More juvenile fishes were recorded in the benthic habitats than in the pelagic habitats. Depth had the largest explanatory power for predicting fish community composition, followed by the affiliation with benthic and pelagic habitats, and location on the longitudinal axis of the reservoir. The fish community was represented mainly by cyprinids and consisted of two distinct groups of species, with bleak Alburnus alburnus, rudd Scardinius erythrophthalmus and asp Aspius aspius dominating the offshore group while perch Perca fluviatilis and ruffe Gymnocephalus cernuus were affiliated with the inshore group of the adult fish community. Roach Rutilus rutilus, bream Abramis brama and pikeperch Sander lucioperca occurred in important proportions in both the inshore and the offshore zones. All species, with the exceptions of adult perch (1+ year and older), 0+ year perch and 0+ year roach, preferred the most eutrophic tributary part of the reservoir. The fish community was relatively stable between the 2 years sampled.

Spatial distributions of fish were studied in two types of reservoirs by means of hydroacoustics during the summer. Different patterns of fish distribution were found in non-cascade (Řimov, Želivka) and cascade (Kamýk, Slapy, Stěchovice, Vrane) reservoirs. Maximum biomass and density of fish were observed in the tributary area of the non-cascade reservoir. The biomass declined towards the dam area. Average weight of fish showed the opposite trend—maximum average weight was observed in the dam area and declined towards the tributary area. In the cascade reservoirs fish distribution was found to be more complicated but the maximum biomass and density of fish were observed in the dam areas, whereas the tributary areas were nearly fishless. Poor ecohydrological conditions in the tributaries of cascade reservoirs are likely to be responsible for the low abundance of fish in these areas. This was evident in the example of Vrane reservoir which has two major inflows. Low biomass and density of fish were observed in the cold and low oxygen saturated inflowing cascade Vltava River while higher biomass and density were observed in the warm and well oxygen saturated Sazava River inflow. Average weights of fish followed the same trend as in non-cascade reservoirs—the maximum was in the dam area and it declined towards the tributary area. The vertical distribution of fish is driven by the fully developed stratification of temperature and oxygen during the summer season in non-cascade reservoirs. A significantly lower biomass of fish was observed below the thermocline in vertical surveys in both non-cascade and cascade reservoirs. Summer stratification in cascade reservoirs is weakly developed or is not developed at all due to hypolimnetic releases of cold water and short retention times. High transparency suggests lower primary production in cascade reservoirs.

The composition of the surface layer of 13 low-cost jewelry samples with a high Cd content was analyzed using an energy-dispersive X-ray fluorescence spectrometer (ED XRF). The analyzed jewels were obtained in cooperation with the Czech Environmental Inspectorate. The jewels were leached in two types of artificial sweat (acidic and alkaline) for 7 days. Twenty microliters of the resulting solution was subsequently placed on a paper carrier and analyzed by an LIBS (Laser-Induced Breakdown Spectrometry) spectrometer after drying. The Cd content in the jewelry surface layer detected by using ED XRF ranged from 13.4% to 44.6% (weight per weight—w/w). The samples were subsequently leached in artificial alkaline, and the acidic sweat and leachates were analyzed using laser-induced breakdown spectrometry (LIBS). The amount of released Cd into alkaline sweat ranged from 24.0 to 370 µg Cd per week, respectively 3.23–61.7 µg/cm2/week. The amount of released Cd into acidic sweat ranged from 16.4 to 1517 µg Cd per week, respectively 3.53–253 µg/cm2/week. The limit of Cd for dermal exposure is not unequivocally determined in the countries of the EU (European Union) or in the U.S. Based on the US EPA (United States Environmental Protection Agency) approach used to establish the reference dose (RfD) for Cd contained in food and information about the bioavailability of Cd after dermal exposure, we assessed our own value of dermal RfD. The value was compared with the theoretical amount of Cd, which can be absorbed into the organism from jewelry in contact with the skin. The calculation was based on the amount of Cd that was released into acidic and alkaline sweat. The highest amount of Cd was released into acidic sweat, which represents 0.1% of dermal RfD and into alkaline sweat, 0.5% of dermal RfD. These results indicate that the analyzed jewelry contains Cd over the limit for composition of jewelry available within the territory of the EU. The determined amount of Cd in analyzed jewelry does not, however, pose a threat in terms of non-carcinogenic toxic effects.