Thyroid hormone concentrations in the gonads of wild chum salmon during maturation

Changes in gonadal and plasma concentrations of thyroid hormones were examined at various stages of maturation in chum salmon (Oncorhynchus keta) caught in the Bering Sea and the Bay of Alaska. Plasma concentrations of thyroxine (T₄) were less than 5 ng ml^-1, and those of 3,5,3′-triiodo-L-thyroxine (T₃) were less than 2 ng ml^-1 I in both males and females, regardless of the degree of sexual maturity or the gonadosomatic index (GSI). There was no clear relationships between circulating thyroid hormone levels and tissue levels. The ovarian T₄ concentrations were undetectable (less than 0.2 ng g^-1) or less than 2 ng g^-1 when GSI was lower than 1%, but increased thereafter and reached a plateau of 8-10 ng g^-1 when GSI became 2%. The ovarian T₃ concentrations were about 5 ng g^-1 when GSI was 1%, increased to a maximum level (20 ng g^-1) when GSI was about 2%, and decreased to a constant level of 10 ng g^-1 thereafter. The T₄ and T₃ content in single oocyte increased proportionally to the oocyte volume, indicating a constant incorporation of the hormones into the oocyte. The T₄ concentrations in the testis were 1 ng g^-1 or less regardless of the GS1. On the other hand, the T₃ concentrations were highest (15 ng g^-1) when the GSI was less than 1%, decreased thereafter when spermatocytes appeared in the testis, and became about 5 ng g^-1 I in testes containing spermatozoa, raising the possibility of a role for T₃ during early gamete and/or gonad maturation of testes.