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.

Fishponds were and are purposeful water structures. Fish production is their main function and rational management is an inevitable condition for their existence. The present high level of fishpond eutrophication results in nutrients overloading. The effect of the high level of nutrients is emphasized by top-down control of zooplankton by high fish stock densities. Currently the zooplankton is represented by small species such as nauplii, small cyclopoid copepods, small species of Cladocera and rotifers that are not so effective filtrators. The high eutrophic level brings about high primary production (mostly with predominance of inedible Cyanophytes) which cannot be used by this type of zooplankton. The main consequences of high eutrophication are large fluctuations in basic environmental parameters and a decrease in production effectivity. It is expected that the utilization efficiency of the enormous primary production through zooplankton into fish production is low. Therefore, maintaining ecological stability and healthy, sound functioning of the ecosystem, meaning without considerable fluctuations, represents an important task in sustainable fishpond management.

Supplemental cereal feeding (maize, wheat and triticale compared with a control group with natural food only) and its effect on fatty acid (FA) expression in the flesh during long-lasting storage of common carp (Cyprinus carpio) was investigated. The fish were cultured in earthen ponds in the Trebon region (Czech Republic). The content of fatty acid was investigated in the flesh of carp during 8 months oflong-lasting storage without additional feeding. Sixty common carp in their third year of life were used for the analyses. The weight of the fish (marketable fish) ranged from 1,358 g to 2,221 g. Polyunsaturated fatty acid (PUFA, n-3) content and composition in fish flesh were determined by gas chromatography (VARIAN 3300). Supplemental cereals caused lower levels of PUFAs and n-3 PUFAs in fish fat. The content of these fatty acids did not decrease, even during 8 months of fish storage. The average percentage of PUFAs in total fat from edible parts was: for maize 13.7% ± 1.58%, for wheat 11.6% ± 1.17% and for triticale 10.7% ± 1.00%. The percentage of n-3 PUFA for maize was 2.5% ± 0.36%, for wheat 3.38% ± 0.44% and for triticale 3.1% ± 0.39%.

Four production fishponds in the Czech Republic were investigated in 2003 and 2004 during a feeding experiment on common carp (Cyprinus carpio). In 2003, topmouth gudgeon (Pseudorasbora parva) was detected in enormous amounts in all of the investigated fishponds. P. parva got into the fishponds spontaneously by water inflow from connecting channels. The objective of this paper is to describe the condition of natural food in the presence of P. parva and its subsequent effect on carp production. The estimation of the population density of P. parva achieved at least 44 kg ha −1 in 2003. In 2004, precautions against P. parva invasion were taken and its presence wasn't recorded during the season. The impact of P. parva on natural food structure was described in terms of zooplankton and zoobenthos amounts and main fish production parameters. A special focus has been taken on the density of Daphnia genus and chironomid larvae, the most preferred zooplanktonic and zoobenthic groups in feed of P. parva, respectively. In 2003, P. parva suppressed the zooplankton populations significantly, especially large cladocerans of the Daphnia genus. Influence of P. parva on zoobenthos structure and density was not detectable. Unfavorable natural food condition in 2003 caused extremely low carp production which fell to the mean value of 283 kg ha −1 and food conversion ratio reached 3.5. On the contrary, in 2004 the mean carp production and food conversion ratio attained 634 kg ha −1 and 1.6, respectively. The results described harmful competitive effect of huge populations of P. parva and its surprising economic consequences. Costs per 1 kg of growth were increased by approximately 100% in 2003 compared with results from season 2004.