Stunting of Wild Coho Salmon (Oncorhynchus kisutch) in Seawater: Patterns of Plasma Thyroid Hormones, Cortisol, and Growth Hormone

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Abstract
Under natural conditions, some coho salmon presmolts (Oncorhynchus kisutch) in Kamchatka have been observed to migrate prematurely to the sea and fail to grow in brackish water for prolonged periods (natural stunts). Plasma levels of growth hormone, cortisol, thyroxine, and triiodothyronine were measured in coho salmon parr captured in the river (freshwater), smolts migrating downstream captured at the river mouth (brackish water), smolts in the sea (seawater), and natural stunts in the inlet (brackish water). The physiological conditions of natural stunts seem analogous to those of hatchery-derived stunts observed in hatchery-reared juveniles in sea pens, with normal plasma sodium concentration, low levels of thyroid hormones and cortisol, and high growth hormone levels.