The water balance in Atlantic salmon (Salmo salar L.) overwintering in sea water (34 ‰) was investigated. With a decrease of temperature from 5.6 to 1.0°C the drinking rate decreased from 13.9 to 5.7 ml/kg/day, and the absolute amount of water absorbed decreased from 8.9 to 5.0 ml/kg/day. A decrease in temperature led, however, to an increase in the proportion of water absorbed in the intestines from 60 to 96%. Blood serum osmolarity increased from 320 to 440 mosm/1 with decreasing temperature and there was a reduction in tissue water content from 75 to 69% The disturbance of water balance at low temperature may be one of the factors responsible for mortality of salmon overwintering in sea water.

Comparative growth observations (13 months) of European (Anguilla anguilla L.) and American (Anguilla rostrata Le Sueur) glass eels were undertaken. Plastic containers (60 x 30 x 35 cm, volume of 30 L) were combined with a sedimentation tank and submerged, gravel, biological filter to form a recirculation system. Feeding commenced on the second day after arrival of the fish and proceeded as follows : days 1 to 50, krill hydrolysate ; days 51 to 58, krill hydrolysate mixed with a salmon starter feed ; day 59 onwards, standard salmon feed. Feed was provided at 2-hourly intervals via automatic feeders for the first 161 days of rearing, and thereafter by hand 5 times a day at a feeding rate of 5 % of the fish biomass. The average individual weight of the European eels on arrival (x = 0.266 ± 0.005 g), was significantly higher than that of the American eels (x = 0.167 ± 0.006 g). After 45 days of rearing, the weight differences between the species became insignificant. Except for the different transportation mortalities on arrival, the European and the American eels demonstrated similar mortality rates during the experiment (12.5%), food conversion rate (2.3) and specific growth rate (0.90 and 0.99 respectively). No parasites or diseases were detected. No differences were observed in general behaviour and feeding between the two species. Key-words :Anguilla anguilla, Anguilla rostrata, culture, glass eels, growth comparison. Aucune différence dans le comportement général et la prise alimentaire n'a été détectée entre les deux espèces. OBSERVATIONS DE LA CROISSANCE DES CIVELLES EUROPÉENNES

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