Effects of photoperiod on juvenile growth and smolting of Atlantic salmon and subsequent survival and growth in sea cages

Abstract

Hatchery-reared Atlantic salmon parr were grown in the laboratory from November to May under two photoperiod (simulated natural and continuous light) and temperature (12-7-12° and 17-6°C) regimes. Growth rates of both photoperiod groups under the 17-6°C regime were greater from November to February but were lower from February to May than those under the 12-7-12°C regime. Accumulated degree days were slightly greater for the 17-6°C groups. Parr from both thermal regimes under continuous light grew much faster from November to February and were larger in May than those under natural photoperiod, but had low tolerance to salinity when challenged at 37.5‰ Most of the natural photoperiod test group survived the high salinity more than 96 h. The fish were acclimated to normal sea water (29–30‰) in May and moved to a sea cage for observations on survival and growth during the summer and autumn under natural thermal and photoperiod conditions. Salmon from the continuous light regime survived only slightly less well than those from the natural photoperiod regime; the former appeared to be healthy and grew reasonably well but by October were only about two-thirds the size of those from the natural photoperiod regime. It was concluded that the fish reared under continuous light, though large and healthy and smolt-like in appearance, were not smolts. These results are discussed in relation to the environmental conditions required to produce smolts and the importance of appropriate photoperiod treatment during juvenile growth in fresh water.