Osmoregulation and growth in a wild and a selected strain of Atlantic salmon smolts on two photoperiod regimes

Atlantic salmon (Salmo salar) smolts originating from wild (Imsa) and selected (AquaGen) strains were maintained on simulated natural photoperiod (SNP) or constant light (LL) in replicate tanks between 3 December 1998 and 26 May 1999. Gill Na+,K+-ATPase activity, plasma growth hormone (GH) levels, condition factor (CF) as well as individual food intake, growth and food conversion were monitored regularly in all groups during the experimental period. Freshwater temperature was kept constant at approximately 12°C. In both strains, GH levels and gill Na+,K+-ATPase activity rose steadily in the SNP groups, whereas the abrupt increase in day-length caused a significant increase in GH levels within 4 weeks, concurrent with an increase gill Na+,K+-ATPase activity. The close relationship between GH levels and gill Na+,K+-ATPase activity indicates that GH plays and important role in the development of hypoosmoregulatory ability during smolting. Overall, a higher growth rate was seen in smolts from the AquaGen strain compared with the Imsa strain. No differences in food consumption were seen between strains, whereas an improved food conversion was observed in smolts from the AquaGen strain. Photoperiod was found to enhance growth through stimulation of food intake. It is concluded that the observed differences in growth and food conversion between wild and selected smolts are genetic, and probably reflect the fact that the AquaGen fish have been bred for rapid growth for several generations. © 2003 Published by Elsevier Science B.V.