Nitrite Poisoning of Fish in Aquaculture Facilities with Water-recirculating Systems

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. Methaemoglobinemia, Cl -to N-NO 2 -weight ratio, Silurus glanis L., Tinca tinca L., Oreochromis niloticus, biological filters, nitrificationThe increasing need for growing fish under controlled conditions coupled with limited capacity of water resources has led to the building of aquaculture facilities where up to 95% of water is being recirculated.The main product of protein metabolism of fish, i.e. ammonia (Lloyd 1992;Wood 1993;Smutná et al. 2002) is removed from the tanks by nitrification (conversion to nitrites and subsequently to nitrates) by means of biofilters.Imbalances of the nitrification process, which might be due to a number of factors, will, however, often lead to an increase in the nitrite concentrations.Nitrite toxicity to fish varies considerably and depends on a large number of external and internal factors.Among the most important ones are water quality (Crawford and Allen 1977;Russo et al. 1981;Eddy et al. 1983;Lewis and Morris 1986; Bartlett and N e um an n 1998), fish species (Palachek and Tomasso 1984;Lewis and Morris 1986), fish size (Bartlett and Neumann 1998), and individual fish susceptibility