Photoperiod and temperature regulation of smolting in steelhead trout (Salmo gairdneri).

Abstract

Photoperiod was the main environmental factor controlling the onset of par r–smolt transformation in steelhead trout (Salmo gairdneri) as evidenced by subsequent changes in migratory behavior and coefficient of condition. Photoperiod appeared to act as a synchronizer of an endogenous rhythm because smolt-like fish were observed in the dark at constant temperature from December through July for fish ≥ 19 cm. Phase and frequency or phase adjustment only of the annual photoperiod cycle advanced or delayed smolting. Increasing day length was the major photoperiod constituent stimulatory to smolting whereas day length, as such, and accumulated hours of exposure were of lesser importance. Temperature did not appear to influence the onset of smolting but fish reared under a variable temperature cycle (6.9° to 18.6 °C) generally migrated in larger numbers than did those reared under a constant temperature (12.3 °C). When the seasonal temperature cycle was out of phase and behind the photoperiod cycle, the migratory period was extended. Conversely, when the temperature cycle was in advance of the photoperiod cycle, the migration period was shortened.