Effects of salinity acclimation on pre-smolt growth, smolting and post-smolt performance in off-season Atlantic salmon smolts (Salmo salar L.)

The main objective of this study has been to investigate the effect of salinity acclimation on parr-smolt transformation and post-smolt performance in off-season Atlantic salmon smolts (Salmo salar L.). Four groups of potential off-season salmon smolts were raised under a phase advanced simulated natural photoperiod between 26 July and 14 November. After 9 weeks of photoperiod treatment, salinity in three of the tanks was changed from freshwater to a salinity of 6.0, 13.1 and 20.0 parts per thousand. All groups were transferred to full strength seawater on 14 November and monitored regularly during the next three months. Hypo-osmoregulatory ability improved in all groups during the first 9 weeks of photoperiod treatment. A significant increase in gill Na+,K+-ATPase activity was observed between weeks 9 and 16, with a higher activity in the 20.2 parts per thousand group at week 16. Growth and condition decreased in all saline groups and no positive effects on post smolt performance were observed. Final weight after three months in seawater was 13% lower in the 6.0 and 13.1 parts per thousand groups, and 28 parts per thousand lower in the 20.2 parts per thousand group, compared with controls. Our findings suggest that acclimating Atlantic salmon to saline water during off season smolting has no positive consequences, and is probably not necessary to stimulate the development of hypo-osmoregulatory ability and increase gill Na+,K+-ATPase activity during smolting or improve post-smolt growth. (C) 2002 Elsevier Science B.V. All rights reserved.