OSMOTIC PERFORMANCE AND SURVIVAL OF ATLANTIC COD (GADUS-MORHUA) AT LOW SALINITIES

This study examines the capacity of Atlantic cod (Gadus morhua) to wit hstand exposure to low salinities ranging from 1 to 7 g/l. Cod were ex posed to hypo-osmotic waters (1, 3, 5 and 7 g/l water salinity) at 10- degrees-C over a period of 6 days. Mortality rate, changes in osmotic parameters and changes in plasma glucose were monitored as indices of stress. No cod survived more than 20 h after transfer to 1 g/l water s alinity, while survival rate increased to 5% in the 3 g/l treatment, t o 95% in the 5 g/l treatment and to 100% in the 7 g/l treatment. The 1 -7 g/l range in water salinity brought about a gradation in the osmoti c response. Plasma osmotic and ionic concentrations dropped sharply in the first 48 h of 1, 3 and 5 g/l treatments. Cod surviving exposure t o 3 g/l had low ionic and osmotic concentrations, similar to those of moribund fish, whereas fish in the 5 g/l treatment returned, after 5 d ays, to higher concentrations close to the average for animals kept in 7 g/l salinity. Cod in the 7 g/l treatment maintained concentrations slightly lower than the average for control fish throughout the experi ment. Plasma glucose concentration decreased considerably, in both the control and experimental treatments. The rate of decrease was lower a t 7 and 28 g/l salinities than at 1, 3 and 5 g/l salinities. Blood glu cose does not appear to be a reliable osmotic stress indicator in Atla ntic cod.