DEVELOPMENT OF SALINITY TOLERANCE IN UNDERYEARLING SMOLTS OF ATLANTICSALMON (SALMO-SALAR) REARED UNDER DIFFERENT PHOTOPERIODS

Groups of Atlantic salmon (Salmo salar), first fed in mid-February 198 9, were reared under continuous light (LL) at 12-15 degrees C until la te June. One group remained on LL, while the FEB, MAR, and MAY groups were subjected to segments of a simulated naturally increasing photope riod (61 degrees N), with initial daylength corresponding to February 1, March 15, and May I, respectively. The MAR and FEB groups developed important smolt characters (increase in gill Na+,K+-ATPase activity, hypoosmoregulatory ability, and salinity tolerance) during the experim ent whereas LL and MAY groups showed only incomplete changes in smelt status. The combination of continuous light during first feeding and e arly parr stages with a subsequent reduction to short daylength follow ed by a segment of simulated natural photoperiod allow important parts of the parr-smelt transformation to be completed. The period of expos ure to short photoperiod is critical for the synchronization of smelt characters in underyearling Atlantic salmon.