Changes in Plasma T 4 and T 3 Levels during Reconditioning and Rematuration in Male and Female Wild Atlantic Salmon ( Salmo salar ) Kelts Held in Freshwater under Two Photoperiod Regimes

• Published in: Canadian Journal of Fisheries and Aquatic Sciences
• Main Authors: Eales, J. G., Cyr, D. G., Finnson, K., Johnston, C. E.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 1991
• Subjects: Aquatic Science; Ecology, Evolution, Behavior and Systematics; Atlantic salmon; Salmo salar
• Online Access: http://dx.doi.org/10.1139/f91-286; http://www.nrcresearchpress.com/doi/pdf/10.1139/f91-286

The plasma level of L-thyroxine (T 4 ) 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 T 4 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 (T 3 ) level also rose in spring in both males and females. In males and nonreproducing females, plasma T 3 remained high during the growth phase in summer and decreased in fall and winter. Plasma T 3 was very low in nonfeeding fish and showed some influence by the 6-mo compressed photocycle. Plasma T 3 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 T 4 and T 3 appear to be controlled independently and that plasma T 3 is more susceptible than T 4 to vitellogenic state and other factors relating to metabolic demands.