TIMING OF PARR-SMOLT TRANSFORMATION IN ATLANTIC SALMON (SALMO-SALAR) - EFFECTS OF CHANGES IN TEMPERATURE AND PHOTOPERIOD

Water temperature modified the development of smelt characteristics an d their duration after an abrupt increase in daylength. A temperature change alone was not a sufficient environmental cue to induce a simila r development. To complete parr-smelt transformation after an abrupt i ncrease in daylength, a thermal sum of approximately 400 degree-days ( degrees d) seemed to be necessary. Three groups of Atlantic salmon (1 +) were reared under continuous light (LL) from hatching and then expo sed 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 fro m the continuous light regime, to make it possible to continue the exp eriment with only six tanks and still use duplicates in each combinati on of light and temperature regime. Thereafter, three temperature regi mes were applied: 7.7 degrees C (mean) (4.5-10.8) (range), 10.7 degree s C (7.6-13.7) or 13.7 degrees C (10.5-17.3). Mean temperature from Ja nuary 31 to April 2 was 9.1 degrees C. The medium temperature regime i ncluded no abrupt temperature shift. The short-day regime caused a low er thermal-growth coefficient (TGC) than the continuous regime until t he increase in daylength. After this increase and in the rest of the e xperiment, the TGC was still lowest (P < 0.05) in the groups previousl y exposed to short days. The short-day regime caused a reduction in th e 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. Slivery colour and the ability to regulate plasma Cl- l evel in a 24-h seawater challenge test developed under both light regi mes. The decrease in condition factor was not significantly affected b y temperature. Silvering and Na-K-ATPase activity developed mon rapidl y at the high temperature regime (P < 0.001), but no difference was fo und between the medium and low temperatures. An elevated fill Na-K-ATP ase 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 cont inuous 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 tempe rature (P < 0.05). (C) 1998 Elsevier Science B.V.