Thyroid Hormone Content of Eggs and Early Developmental Stages of Five Oncorhynchus Species

Abstract

Measurements were made of thyroxine (T4) and triiodothyronine (T3) levels in the embryos and larvae of five species of the genus Oncorhynchus. Pink salmon (O. gorbuscha) embryos and larvae contained relatively low levels of both T4 and T3 throughout the early developmental stages. Sockeye salmon (O. nerka) had the highest levels of T4, with coho (O. kisutch), chum (O. keta), and chinook salmon (O. tshawytscha) having similar total T4 content. The chinook salmon embryos contained the highest T3 content of the five species studied, pink, sockeye, and chum salmon embryos had relatively low levels, and coho salmon embryos had intermediate levels of T3. In the pink salmon, there was little evidence of significant change in either T4 or T3 during development. In the other four species, the T4 content changed little between fertilization and hatch, but fell thereafter to stabilize at levels close to the lower detectable range of the assay used, except in the sockeye salmon larvae in which the values were considerably higher. The T3 content of chinook and coho salmon changed with development in a manner comparable with that observed for T4, but there was little evidence of a comparable posthatch decline in T3 levels in chum, sockeye, or pink salmon.