Influence of salinity on the energetics of gill and kidney of Atlantic salmon (Salmo salar).

The effect of seawater acclimation and adaptation to various salinities on the energetics of gill and kidney of Atlantic salmon (Salmo salar) was examined. Smolts and non-smolts previously reared in fresh water were exposed to a rapid increase in salinity to 30 ppt. Plasma osmolarity, [Na(+)], [Cl(-)], [K(+)] and [Mg(++)] increased in both groups but were significantly lower in smolts than non-smolts. Gill Na(+), K(+)-ATPase specific activity, initially higher in smolts, increased in both groups after 18 days in seawater. Kidney Na(+), K(+)-ATPase specific activity was not affected by salinity in either group. Gill and kidney citrate synthase specific activity was not affected by seawater exposure in smolts but decreased in non-smolts. In a second experiment, Atlantic salmon smolts reared in fresh water were acclimated to 0, 10 or 30 ppt seawater for 3 months at a temperature of 13-14°C. Gill Na(+), K(+)-ATPase was positively correlated with salinity, displaying 2.5- and 5-fold higher specific activity at 10 and 30 ppt, respectively, than at 0 ppt. Kidney Na(+), K(+)-ATPase specific activity was not significantly affected by environmental salinity. Citrate synthase and cytochrome c oxidase specific activities in gill were slightly (6-13%) lower at 10 ppt than at 0 and 30 ppt, whereas kidney activities were lowest at 30 ppt. Oxygen consumption of isolated gill filaments was significantly higher when incubated in isosmotic saline and at 30 ppt than at 0 ppt, but was not affected by the prior acclimation salinity. The results indicate that although high salinity induces increased gill Na(+), K(+)-ATPase activity, it does not induce substantial increases in metabolic capacity of gill or kidney.