Diel pattern of plasma T3 and T4 levels in Baltic salmon parr (Salmo salar L.) during two seasons

Abstract

In March (sunrise at 0530, sunset at 1800, water temperature 0.04 °C, and the fish not feeding) plasma T3 showed no diel variation and plasma T4 varied only slightly. In May (sunrise at 0200, sunset at 2120, water temperature 9.0 °C, and the fish feeding) plasma T4 showed a pronounced diel variation with its peak value appearing just before sunset and its nadir occurring at 0800. The plasma T3 level varied slightly with its peak value at 0400. The constant or slightly varying diel levels of plasma T3 suggests an efficient regulatory mechanism that maintains the plasma T3 concentration relatively stable throughout the 24 h. It is concluded that the diel patterns of T4 and T3 are dependent on factors related to the season. The influence of photoperiod and feeding on the diel pattern of T4 is discussed.