MACKIE, P*; SHIH, J; WRIGHT, P; BALLANTYNE, J; University of Guelph, Ontario: Osmoregulatory effects of early introduction to cold seawater in Atlantic salmon (Salmo salar) smolts

Due to the costly practise of maintaining juvenile salmon through the winter, salmon producers in Europe have turned to producing out-of-season or 0+ smolts that are released into sea cages in the fall, a full six months prematurely. In Canada, cold seawater temperatures during the winter months make this practise problematic with extensive mortalities. One approach to solving this problem is to develop strains of Atlantic salmon with superior performance in cold seawater. We have undertaken extensive analysis of physiological performance on eleven families of 0+ Atlantic salmon (Salmo salar). Out-of-season smolts were transferred from freshwater and acclimated to cold (4°C to 1.9°C) seawater in January, 2002. Blood and tissue samples were collected from freshwater controls and seawater acclimated fish 0h, 24h, 96h, and 1 month after the seawater tanks reached full seawater concentration. In the majority of seawater acclimated families, plasma osmolality peaked at 24h. In contrast, some families reached a maximum value only after 96h. There were significant differences in plasma osmolality between families 24h post-seawater transfer indicating that families vary in their ability to osmoregulate in cold seawater. As well, all seawater fish had higher plasma osmolalities compared to freshwater control fish. Plasma Cl^– ion concentrations mirrored changes in plasma osmolality, further supporting a difference in osmoregulatory ability between families.