Plasma Thyroid Hormone Levels in Migratory and Lake-Resident Coho Salmon Juveniles from the Karluk River System, Alaska

Abstract

This study tested the hypothesis that thyroid hormone levels in wild juvenile coho salmon Oncorhynchus kisutch are higher in fish beginning their seaward migration (smolts) than in lake-resident fish (parr). Average overall thyroxine (T4) levels were higher in wild smolts (3.5 ng/mL) than in wild parr (1.8 ng/mL); however, average overall triiodothyronine (T3) levels were similar in wild smolts (3.0 ng/mL) and parr (2.3 ng/mL). Whereas hormone levels in smolts varied between sampling dates (mean T4, 1.2–6.6 ng/mL; mean T3, 0.7–4.8 ng/mL; P < 0.05), hormone levels in parr remained fairly constant (mean T4, 1.3–2.6 ng/mL; mean T3, 1.8–2.8 ng/mL; P > 0.05). Partly because of these dissimilar patterns of variability, differences in thyroid hormone levels between smolts and parr tended to be subtle. Thus, wild coho smolts were more similar to wild parr than might have been predicted from studies of hatchery-reared fish. Furthermore, because hormone levels in migrants were sometimes similar to those in nonmigrants, predictions of the onset of the seaward migration by wild coho salmon probably should be based on more factors than thyroid hormone levels alone.