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.

Artificially fertilized eggs and yolk‐sac larvae of a freshwater tropical/subtropical fish Clarias gariepinus receiving no external food were incubated at 22, 25 and 28° C until full yolk resorption. Developmental time, size and matter composition (CHNS‐O Analyzer and ashing) were assessed at egg fertilization, hatching and yolk resorption; respiration was measured every 4–5 h. The course of acceleration of C. gariepinus embryonic developmental rate with temperature (Q10dev) was compared over the temperature range to those of Cyprinus carpio and Oncorhynchus mykiss; they differed greatly, but were similar when compared on the basis of effective temperatures specific to each fish. Specific growth rates for energy (88, 150 and 183% per day at 22,25 and 28° C, respectively) as well as the conversion efficiencies of egg energy (64, 71 and 68%, respectively) and protein (71, 78 and 76%, respectively) in C. gariepinus larval tissues were higher than those known for the endogenous feeding period of coldwater and temperate fish species. In C. gariepinus at the end of yolk resorption, the carbon percentage and caloric values of dry weight, size (in terms of dry matter, minerals, protein and energy per larva) and transformation efficiencies were lowest at 22° C, highest at 25° C and had slightly decreased at 28° C. A tentative mechanism which leads to the positive or negative response of body size to temperature over the viable temperature range is defined.

The aim of this study was to assess the biochemical profile of tench blood plasma during preand postspawning period under the conditions of hormonally-induced artificial reproduction. A total of 59 females and 27 males were examined during the postspawning period of 1999 and 52 females and 25 males were examined during the prespawning period of 2000, as well as 48 females after reproduction. Biochemical indices determined in blood plasma were as follows: cortisol, glucose, total protein (TP), triacylglycerols (Tcg), cholesterol (Chol), transaminases (ALT and AST), creatine kinase (CK), alkaline phosphatase (ALP) and electrolytes (Na). In females in the pre-spawning period, higher values of TP (P < 0.05) and Tcg (P < 0.01) were found compared to males. Immediately after reproduction, males had higher TP (P < 0.01) and Chol (P < 0.01) than females. No significant sex-related differences were found in other indices under study. Higher values of glucose (P < 0.01), Tcg (P < 0.01), Chol (P < 0.05), AST (P < 0.01) and ALP (P < 0.01) were found for females after reproduction in June compared to values found in April, i.e. two months prior to reproduction. Differing water temperature (10.3 °C in April; 22 °C in June) associated with metabolic rate also played an important role. Induction of ovulation by GnRH synthetic analogue and carp pituitary was not successful in all females. However, between the spawned and unspawned female fish, differences were found in glucose concentration (P < 0.01) but non-significant differences were recorded for other biochemical indices. The blood plasma biochemical profile enabled to assess the state of internal milieu of broodstock during the reproduction period.

Effect of incubation temperature (range: 9-36 °C; interval: 3 °C) on artificially propagated weatherfish (Misgurnus fossilis) early ontogeny (during interval from egg fertilization to the finish of hatching) was investigated. Both, the amplitude of the incubation period (evaluated in four crucial moments), the total hatching period duration was inversely proportional to the incubation temperature and ranged from 17.5 days at 9 °C to 1.8 days at 24 °C (expressed at H 50 ) or from 137 hours at 9 °C to 9 hours at 24 °C, respectively. There were no influence of rising temperature on the total length of newly hatched larvae (T L = 4.23-4.67 mm), in contrast to negative correlation with developmental stage (9-18 °C: stage 37; 21-24 °C: stage 36), i.e. the length might determine the age at hatching, rather than the age at hatching determines the hatching length. The thermal tolerance range in term of survival lies between 9 and 24 °C (the thermal optimum 15-24 °C, i.e. weatherfish is a warm-mesothermic species). Temperatures above 24 °C (in our study 27-36 °C) are considered the lethal temperatures already during embryonic period. It is highly recommended to distinguish an impact of suboptimal temperatures 9-12 °C on development during explored interval only, in contrast to possible other effect of these lower temperatures in context of the whole early ontogeny. RÉSUMÉEffets de la température sur les premiers stades de vie de la loche d'étang, Misgurnus fossilis (L. 1758) L'effet de la température d'incubation (gamme : 9-36 °C, intervalles : 3 °C) sur les premiers stades ontogéniques de loches d'étang (Misgurnus fossilis) (de la fécon-dation à la fin de l'éclosion) a été étudié. La durée de la période d'incubation (évaluée à quatre moments clés) et la durée de la période d'éclosion des oeufs d'un lot sont inversement proportionnelles à la température d'incubation et s'étalent de 17,5 jours à 9 °C à 1,8 jour à 24 °C (pour l'indice H 50 ) et de 137 heures à 9 °C à 9 heures à 24 °C, respectivement. Il n'y a pas d'influence d'une élévation de température sur la longueur totale des larves à l'éclosion (T L = 4,23-4,67 mm), alors qu'il y a une corrélation négative avec le stade de développement (9-18 °C : stade 37 ; 21-24 °C : stade 36), i.e. la longueur semble déterminer l'âge à l'éclo-sion, plutôt que l'âge à l'éclosion déterminerait la longueur à l'éclosion. La gamme

Sperm quality of Barbus barbus L. was compared among the three following dietary regimes: Group A, fed 100% commercial diet (Karpico containing 33% crude protein and 6% fat), Group B, fed 78% commercial diet and 22% frozen chironomid (Chironomus plumosus) larvae, and Group C, fed 56% commercial diet and 44% frozen chironomid larvae. Concentrations of polyunsaturated fatty acids (PUFAs) in Group A, B, and C were 39.1, 42.0, and 44.6, respectively, as a percentage of total fatty acids. Sperm morphology, volume, concentration and motility, total number of spermatozoa, and osmolality of the seminal plasma were compared during the spawning season. Dietary regime did not influence sperm volume, concentration, or total number of spermatozoa, osmolality of seminal plasma, or the percentage of motile sperm, but significantly affected sperm morphology (except for anterior and posterior parts of the midpiece) and sperm velocity (P < 0.05). Groups B and C showed similar sperm characteristics during the spawning season compared to Group A. Almost all parameters changed either among or within groups during the spawning season, suggesting differences in terms of the optimal time for sperm collection. The best time for sperm collection was March for Group A, but April for Groups B and C, when the osmolality of the seminal plasma measured 289 mOsmol kg(-1) and sperm motility was maximal. Spermatogenesis, hydration, and cell decomposition were confirmed as the three major parameters controlling sperm characteristics during the spawning season. The possible correlation between sperm morphology and motility requires further study.