Timing of parr-smolt transformation in Atlantic salmon ( Salmo salar): effects of changes in temperature and photoperiod

Abstract

Water temperature modified the development of smolt characteristics and their duration after an abrupt increase in daylength. A temperature change alone was not a sufficient environmental cue to induce a similar development. To complete parr-smolt transformation after an abrupt increase in daylength, a thermal sum of approximately 400 degree-days (°d) seemed to be necessary. Three groups of Atlantic salmon (1 +) were reared under continuous light (LL) from hatching and then exposed to a 7-week period of short days (LD 8.15:15.45) which ended April 2. In parallel, three groups were exposed to continuous light alone. All six groups were then split into two subgroups of equal numbers. A subgroup from the short-day regime were then mixed with a subgroup from the continuous light regime, to make it possible to continue the experiment with only six tanks and still use duplicates in each combination of light and temperature regime. Thereafter, three temperature regimes were applied: 7.7°C (mean) (4.5–10.8) (range), 10.7°C (7.6–13.7) or 13.7°C (10.5–17.3). Mean temperature from January 31 to April 2 was 9.1°C. The medium temperature regime included no abrupt temperature shift. The short-day regime caused a lower thermal-growth coefficient (TGC) than the continuous regime until the increase in daylength. After this increase and in the rest of the experiment, the TGC was still lowest ( P<0.05) in the groups previously exposed to short days. The short-day regime caused a reduction in the condition factor ( P<0.001) and an increase in gill Na–K-ATPase ( P<0.001) after the increase in daylength in contrast to the continuous light regime. Silvery colour and the ability to regulate plasma Cl − level in a 24-h seawater challenge test developed under both light regimes. The decrease in condition factor was not significantly affected by temperature. Silvering and Na–K-ATPase activity developed more rapidly at the high temperature regime ( P<0.001), but no difference was found between the medium and low temperatures. An elevated gill Na–K-ATPase activity and low plasma Cl − levels in the 24-h seawater challenge test were found for a shorter period at higher temperatures. Plasma Cl − levels below 150 mM were found for approximately 1, 2 or 6 weeks in the high, medium and low temperature regime, respectively. In the continuous light regime, the temperature regime had no significant effect on the development of silvering, condition factor or gill Na–K-ATPase activity, but the ability to regulate plasma Cl − was affected by temperature ( P<0.05).