The effect of reduced temperature and salinity on the blood physiology of juvenile Atlantic cod

The osmolality and ionic composition of the blood of juvenile Atlantic cod Gadus morhua and their response to conditions of reduced temperature and salinity in summer- and winter-acclimated individuals was investigated. Haematocrit percentage was relatively stable throughout the experimental procedures. Summer-acclimated juvenile Atlantic cod had higher plasma osmolality than winter-acclimated fish in ambient conditions. Plasma Na+ levels were, however, higher in winter conditions, while Cl- did not vary between seasons. Temperature reduction (12, 9 and 6degreesC in summer and to 6 and 4degreesC in winter) induced a significant response in plasma osmolality and Na+ levels in summer, but only in Na+ levels in winter-acclimated fish. A pronounced effect was seen in the summer 6degreesC treatment. Salinity treatments (24, 16 and 8) had a significant effect on almost all the variables in both summer and Winter and resulted generally in dilution of ionic and osmotic concentrations of the plasma. This effect was pronounced in the lowest temperature treatments, with the greatest reduction observed in the summer 6degreesC treatment. This could suggest that winter-acclimated fish are physiologically adapted to cope with lower seawater temperatures as opposed to summer-acclimated fish. (C) 2004 The Fisheries Society of the British Isles.