EFFECTS OF CONTINUOUS LIGHT AND SHORT-DAY PHOTOPERIOD ON SMOLTING, SEAWATER SURVIVAL AND GROWTH IN ATLANTIC SALMON (SALMO-SALAR)

Duplicate groups of 1-year-old Atlantic salmon were exposed to 5-7 wee ks of short days (LD 8.15:15.45) with artificial light, followed by a period of continuous light (LL) for 3 months (A), 2 months (B) or 1 mo nth (C) before transfer to sea cages. Duplicate groups were also expos ed to a 2-week period of LD 8.15:15.45, followed by a period of contin uous light for 2 months (D). Comparisons were made with fish that were reared under natural light conditions from October onwards (F) and co ntinuous light (E) only. Fish from all groups were transferred to sea cages on 9 June and their survival and growth were monitored until 31 October. There was a significant (P<0.001) interaction between light r egime and time for all smolt characters measured. Development of smolt colouration, decrease in condition factor, increases in gill Na-K-ATP ase activity and the ability to regulate plasma Na+ after 24 h in full -strength sea water, indicated that the fish in groups A, B and C comp leted smoltification 6-8 weeks after the end of the short-day periods. No decrease in the condition factor was recorded for fish in groups D and E. The mortality was 40% for group A and 34% for group B, and 15% or less for the other groups. Groups A and B had probably lost some o f the smolt characters by the time of seawater transfer, The specific growth rate in sea water was 1.5% for group F and for the survivors of groups A and B, 1.4% for group C and 1.2% for groups D and E. In conc lusion, a short-day regime of 5-7 weeks, followed by continuous artifi cial light, caused smolt-related changes similar to those found in out door-reared fish, whereas this was not so for fish exposed to either a short-day period of only 2 weeks or to continuous artificial light.