Three cases of severely compromised fish health and death in newly commissioned aquaculture facilities with water-recirculating systems are described. The cause of the damage and death was increased concentrations of water-borne nitrites and the subsequent methaemoglobinemia. The aim of the study was to better understand the aetiology of these cases of poisoning to help prevent them, and to examine effects of some water quality parameters on nitrite toxicity. The increased NO 2 -concentrations in water were caused by impaired functionality of biological filters in the second stage of nitrification, i.e. the conversion of NO 2 -to NO 3 -. Chloride concentrations in water were considered the main factor influencing NO 2 -toxicity in all of the cases described. In the case of death of catfish and tench, the Cl -to N-NO 2 -weight ratios were in the range of 13 -28 and 11 -19, respectively. In the case of tilapia health impairment without symptoms of toxicity, the ratios were between 50 and 150. In the water tank inflow, the Cl -to N-NO 2 -weight ratios were between 2000 and 10000. Blood methaemoglobin levels of catfish and tench (severe symptoms of poisoning) and of tilapia (no signs of impairment, only brownish discolouration of gills) were over 80% and 21%, respectively). In order to minimize risks in culture of fish in water-recirculating systems, it is necessary to choose a proper stock of fish and a proper feeding ratio, not to treat the fish with antibiotics in the form of baths, to check meticulously the quality of water. In case of increasing concentration of nitrites, to administer sodium chloride to get the chloride concentration increased at least to 100 mg·l -1 Cl -. Better operation of a biological filter can be speeded up by inoculation with activated sludge.

Tissue samples from 1,117 fish of 25 species were collected from 1991 through 1996 at 13 locations along the River Elbe. The principal indicator species were perch (Perca fluviatilis) (n=118), chub (Leuciscus cephalus L.) (n=113) and roach (Rutilus rutilus) (n=138). Mercury (Hg) concentrations in muscle and liver were determined by atomic absorption spectrometry. The liver/muscle index in three indicator species from heavily contaminated and lightly contaminated localities were significantly different. In fish from heavily contaminated localities, Hg was deposited preferentially in the liver (the depository for inorganic and organic forms of Hg), while in lightly contaminated areas, it was deposited preferentially in muscle.

Haematological parameters of 2-year-old carp (Cyprinus carpio L.) were assessed to study the protective effect of chloride on the health of ¢sh exposed to elevated nitrite concentrations. Four groups of carp were exposed to di¡erent concentrations of nitrite and chloride for 96 h (group E1: 67 mg L À 1 NO 2 À , 11mg L À 1 Cl À ; group E2: 67mgL À1 NO 2 À , 100 mg L À 1 Cl À ; group E3:0 mg L À1 O 2 À ,100 mg L À 1 Cl À and group C: 0 mg L À1 NO 2 À ,11mg L À 1 Cl À ). The main haematological response of carp to an acute exposure to nitrite (group E1) was a signi¢cant decrease (Po0.05) in haemoglobin concentrations (53.40 AE 6.61g L À1 ), haematocrit (0.21 AE 0.02 LL À1 ), erythrocyte count (1.13 AE 0.12 TL À1 ), leucocyte count (7.1 AE 4.19 GL À 1 ) and lymphocyte count (5.28 AE 2.51 GL À1 ), and a signi¢cant increase in methaemoglobin concentration (90.50 AE 4.38%, Po0.01) and mean corpuscular haemoglobin concentration (0.27 AE 0.2 LL À1 , Po0.05). At higher chloride concentrations (group E2), a lower nitrite toxicity was observed. In group E2 carp, methaemoglobin made up 38.32 AE 13.30%. Erythrocytes in carp exposed to nitrite showed qualitative changes. Compared with the control group C, group E1 carp showed a signi¢cantly higher number (Po0.05) of elongated erythrocytes, with the nucleus located at one cell pole (0.519 AE 0.388 TL À 1 ). All erythrocytes of group E1 carp had remarkably clear cytoplasms compared with the cytoplasm in the control group C. The biochemical values found were comparable with those found in controls. The main histological lesions were found in the gills of carp exposed to nitrite and consisted of hyperplasia and an elevated number of chloride cells.

Common carp (Cyprinus carpio L.) were exposed to nitrite (1.45 mmol·l -1 NO 2 -) for 48 hours at 14 °C and 20 °C, in order to investigate the mechanism of nitrite poisoning at these water temperatures. The effect of nitrite exposure on fish was assessed on selected haematological and biochemical indicators of the blood. Moreover, nitrite accumulation in the blood, liver and muscle was measured. Nitrite exposure produced high levels of methaemoglobin (88.2 ± 3.3% and 92.9 ± 6.1%) at both water temperatures compared with controls (0.3 ± 0.6% and 2.6 ± 3.0%). High fish mortality occurred in experimental groups (30% and 51%) compared with controls (0%). Nitrite exposure also resulted in an accumulation of nitrite in the fish body. The highest nitrite levels developed in the blood plasma, followed by the liver and muscle, respectively. Carp concentrated nitrite in the blood plasma and tissues to markedly higher levels at higher temperature (20 °C). The plasma nitrite concentrations (10.5 ± 1.9 mmol·l -1 ) were in this case more than 7 times higher than the environmental one. At lower temperature (14 °C), plasma nitrite concentration reached 5.0 ± 1.5 mmol·l -1 . In either event, plasma K + levels increased and Cl -levels and osmolality remained unchanged. Plasma Na + levels slightly decreased at the higher temperature. Nitriteexposed fish showed lower haematocrit values (PCV) at both experimental temperatures compared with controls. At 20 °C, the blood haematocrit decrease (0.20 ± 0.02 l·l -1 ) was accompanied by a low erythrocyte count (1.05 ± 0.12·10 12 l -1 ) and by a low haemoglobin level (51 ± 11 g·l -1 ). At the lower temperature (14 °C), the haematocrit decrease (0.25 ± 0.02 l·l -1 ) was caused by a low mean corpuscular volume (167 ± 27 fl). No significant changes were observed in the mean corpuscular haemoglobin (MCH), mean corpuscular haemoglobin concentration (MCHC), or selected erythrocyte dimensions (major axis, minor axis and aspect ratio).

The aim of the study was to evaluate the use of leeches of the genus Erpobdella as a means of assessing polychlorinated biphenyl contamination of watercourses. The River Skalice, heavily contaminated with PCBs, was selected as a model. The source of contamination was a road gravel processing factory in Rožmitál pod Třemšínem from which an estimated 1 metric ton of PCBs leaked in 1986. Levels of PCB were measured in leeches collected between 1992 to 2003 from 11 sites covering about 50 km of the river (the first sampling site upstream to the source of contamination and 10 sites downstream). The PCB indicator congeners IUPA no. 28, 52, 101, 118, 138, 153, and 180 were measured. Levels were highest at the four sampling sites nearest the source of pollution. The highest values of PCB congeners were found in 1992. PCB content decreased from 1992 to 2003 and with distance from the source. The study indicated that leeches of the genus Erpobdella are a suitable bioindicator of contamination in the surface layer of river sediments.