ABSTRACT:Nitrite is an intermediate in the oxidation of ammonium to nitrate. An elevated ambient nitrite concentration is a potential problem for freshwater fish since nitrite is actively taken up across the gills in competition with chloride. Nitrite is a well-known toxicant for fish as well as a disrupter of multiple physiological functions including ion regulatory, respiratory, cardiovascular, endocrine and excretory processes. One critical consequence of nitrite accumulation is the oxidation of haemoglobin to methaemoglobin, compromising blood oxygen transport. 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 (e.g. pH, temperature, cation, anion and oxygen concentration), length of exposure, fish species, fish size and age, and individual fish susceptibility. Chloride concentration in water is considered one of the most important factors influencing nitrite toxicity to fish. The importance of individual factors is assessed and re-evaluated continuously.

Around 20 progestins (also called gestagens, progestogens, or progestagens) are used today in assisting a range of medical conditions from endometrial cancer to uterine bleeding and as an important component of oral contraception. These progestins can bind to a wide range of receptors including progestin, estrogen, androgen, glucocorticoid, and mineralocorticoid receptor, as well as sex hormone and corticosteroid binding globulins. It appears that only five of these (four synthetic and one natural) progestins have so far been studied in sewage effluent and surface waters. Analysis has reported values as either nondetects or low nanograms per liter in rivers. Seven of the progestins have been examined for their effects on aquatic vertebrates (fish and frogs). The greatest concern is associated with levonorgestrel, norethisterone, and gestodene and their ability to reduce egg production in fish at levels of 0.8-1.0 ng/L. The lack of environmental measurements, and some of the contradictions in existing values, however, hampers our ability to make a risk assessment. Only a few nanograms per liter of ethynodiol diacetate and desogestrel in water would be needed for fish to receive a human therapeutic dose for these progestins according to modeled bioconcentration factors. But for the other synthetic progestins levels would need to reach tens or hundreds of nanograms per liter to achieve a therapeutic dose. Nevertheless, the wide range of compounds, diverse receptor targets, and the effect on fish reproduction at sub-nanogram-per-liter levels should prompt further research. The ability to impair female reproduction at very low concentrations makes the progestins arguably the most important pharmaceutical group of concern after ethinylestradiol.

Environmental compounds can interfere with endocrine systems of wildlife and humans. The main sink of such substances, called endocrine disrupters (ED), are surface waters. Thus, aquatic vertebrates, such as fish and amphibians, are most endangered. ED can adversely affect reproductive biology and the thyroid system. ED act by (anti)estrogenic and (anti)androgenic modes of action, resulting in abnormal sexual differentiation and impaired reproduction. These effects are mainly driven by direct interferences of ED with sex steroid receptors rather than indirectly by impacting synthesis and bioavailability of sex steroids, which in turn might affect the hypothalamic-pituitary-gonadal axis. Recent findings reveal that, in addition to the human-produced waste of ED, natural sources, such as parasites and decomposition of leaves, also might act as ED, markedly affecting sexual differentiation and reproduction in fish and amphibians. Although the thyroid system has essential functions in both fish and amphibians, amphibian metamorphosis has been introduced as the most sensitive model to detect thyroidal ED; no suitable fish model exists. Whereas ED may act primarily on only one specific endocrine target, all endocrine systems will eventually be deregulated as they are intimately connected to each other. The recent ecotoxicological issue of pharmaceutically active compounds (PhACs) present in the aquatic environment indicates a high potential for further endocrine modes of action on aquatic vertebrates by ED derived from PhACs, such as glucocorticoids, progestins, and beta-agonists.

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.

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The aim of the study was to assess the effects of the anthelminthic, praziquantel, on the haematological and biochemical indices of the blood of common carp (Cyprinus carpio L.). Fish were divided into six groups: two groups received 30 mg kg(-1) body weight (bw) of praziquantel, and two groups were given 50 mg kg(-1) bw of praziquantel mixed into the heat-treated amyloid vehicle. Fish in the remaining two groups were given only the amyloid vehicle and were used as controls. Fish were examined either 24 or 96 h after administration. Praziquantel treatment was characterised by a significantly lower erythrocyte count, haemoglobin level, packed cell volume and total protein at both dose levels after 24 h compared with the controls, but these parameters were similar to the control values at 96 h. The activity of alanine aminotransferase (ALT) was significantly higher after 96 h in the treated groups, which could be attributed to slight hepatocyte damage caused by praziquantel. However, the differences between the treated and the control groups were not great, and we presumed that they were reversible.