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Pikeperch (Sander lucioperca), European catfish (Silurus glanis), common carp (Cyprinus carpio), and gobies (Neogobius gymnotrachelus, Neogobius melanostomus) were collected from the Danube River (Belgrade section), and samples of liver, muscle, or whole-body composites (in the case of gobies) were analyzed for As, Cu, Fe, Hg, Mn, and Zn with inductively coupled plasma optical spectrometry to find out if there was a correlation between accumulation of these elements in predatory and prey species, as well as in pairs of species with overlapping diets. Concentrations of all analyzed elements were either higher (Cu, Fe, Mn, Zn) in liver than in muscle, or equal (As, Hg), except for Hg in carp, which was higher in muscle. Mercury concentration in liver and muscle of predators (catfish, pikeperch) was significantly (<10(-4)) higher than in prey fishes (carp and gobies). The results indicate that Hg concentration was biomagnified through the food chain. Concentrations of As, Fe, and Hg in carp liver and gobies whole-body composite were similar, but carp had significantly (<10(-4)) higher values of Zn and Cu in liver. The regression analysis and trendline equations indicate that the concentrations of all tested elements, except for As in liver, and Mn and Fe in muscle, were similar in predatory fish (pikeperch and catfish), on one hand, and in prey fish (carp and gobies), on the other hand. Distinctly high Zn concentration in carp is very common in this species due to its physiology. Concentrations of Hg and Zn were higher than the maximum acceptable concentration due to the high pollution level in this section of the Danube River, accordingly posing a risk for the human consumption of these fish species.

The analysis of 18 elements in muscle, liver, gills, and gonads of sichel (Pelecus cultratus), ruffe (Gymnocephalus cernua), and European perch (Perca fluviatilis), caught at a polluted segment of the Danube River near Belgrade, was carried out with the aim to expand the knowledge about the ecotoxicology of these species for monitoring purposes and the possible impact on human health. Generally, the elemental concentration significantly differed between species and tissues (p<0.0001), and a statistical interaction between these two factors was observed (p<0.0001). In muscle and liver, concentrations of Hg and Se were statistically higher in ruffe than in sichel and European perch. In gills, statistically highest concentrations of Mn, Sr, and Zn were found in sichel, and of Fe in European perch. In gonads, statistically highest concentrations of As were detected in sichel, of Zn in ruffe, and of Mn and Mo in European perch. The highest number of coefficients of partial correlations between fish weight and element levels was found in sichel (11 in total). Of all analyzed elements, Al and B had the highest number of partial correlations in tissues. The levels of Hg exceeded the maximum acceptable concentration (0.5 mg kg −1 ) in all muscle samples, which can pose a risk for human consumption. Different diet preferences of the investigated fish species resulted in a different accumulation of elements in tissues, and ruffe (as a species that consume mainly benthic macroinvertebrates) accumulated the highest level of Hg, which makes it suitable for monitoring of this element in water bodies.

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1. Prey preference and seasonal variation in the diet of the Eurasian otter, Lutra lutra (Linnaeus, 1758), were examined by the analysis of 789 spraint samples collected along a 10-km section of a small lowland salmonid river in Serbia, with a trout farm situated in its lower part, from June 2009 to March 2012. Of particular interest was any possible impact of the otter on brown trout, which is important to investigate for preventing and managing any potential conflicts with fishers and fish farmers. 2. Fish were by far the most important otter prey, followed by crayfish. Cottus gobio Linnaeus, 1758 (the European bullhead) was the most common prey species, probably because of the combination of its abundance and easy catchability. Other prey contributed very little to the diet of the otters. Of 11 fish species present in the river, five were found in the otter diet: the European bullhead, Salmo trutta Linnaeus, 1758 (brown trout), Squalius cephalus (Linnaeus, 1758) (chub), Barbus balcanicus Kotlík, Tsigenopoulos, Ráb & Berrebi, 2002 (the Danube barbel), and Phoxinus phoxinus (Linnaeus, 1758) (the Eurasian minnow). The bullhead was the only positively selected species, however. The share of fish prey in the otter diet was stable throughout the year, except for a decrease noticed during the summer.3. The results suggest that otters on the River Gradac target smaller, sedentary, and less nutritious prey. Owing to a small percentage of brown trout in otter spraints, we surmise that otters do not represent a major biological or economic threat, either to the native trout population in the river or to the trout from the nearby fish farm.4. Although there are no reports of otters causing damage to the trout population in the local fish farm, it is advisable to establish mechanisms for the compensation of fish farmers in case such damage does occur.5. The data presented in this paper can be used to contribute to the conservation of the otter in stream habitats, especially because there are no published studies on the dietary requirements of otter in Serbia.