Changes in Plasma T4 and T3 Levels during Reconditioning and Rematuration in Male and Female Wild Atlantic Salmon (Salmo salar) Kelts Held in Freshwater under Two Photoperiod Regimes

The plasma level of L-thyroxine (T4) of wild reconditioned Atlantic salmon (Salmo salar) kelts held in freshwater at seasonal temperatures on a natural simulated photoperiod showed a robust seasonal profile with a marked peak in early spring followed by low levels throughout the summer. Plasma T4 was low in kelts that did not recondition and was uninfluenced by gender, eventual reproductive state, or imposition of a 6-mo compressed photocycle. The plasma 3,5,3′-triiodo-L-thyronine (T3) level also rose in spring in both males and females. In males and nonreproducing females, plasma T3 remained high during the growth phase in summer and decreased in fall and winter. Plasma T3 was very low in nonfeeding fish and showed some influence by the 6-mo compressed photocycle. Plasma T3 was particularly low in females at the time of vitellogenesis when 17β-estradiol was high. It is concluded that in reconditioned Atlantic salmon kelts, levels of plasma T4 and T3 appear to be controlled independently and that plasma T3 is more susceptible than T4 to vitellogenic state and other factors relating to metabolic demands.