Changes in plasma thyroxine, estradiol-17 beta, and 17 alpha,20 beta-dihydroxy-4-pregnen-3-one during smoltification of coho salmon.

Circulating levels of plasma estradiol, 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta PROG), and thyroxine were measured during smoltification of coho salmon in two successive years (1984 and 1985). Two elevations of thyroxine occurred in both years, the first occurred in April followed by a second increase in May. Estradiol levels were dynamic with variable changes particularly in postsmolts when estradiol levels were either depressed in 1984 or high in 1985. However, elevations of estradiol preceded the second elevation of thyroxine in May in both years when the salmon were undergoing the later stages of smoltification. There were no consistent changes of plasma 17 alpha,20B PROG during this period. Although, in 1984, there were significant elevations and depressions of 17 alpha,20B PROG, and in 1985, there was a major unexplained depression of 17 alpha,20B PROG on April 24. There were no sexual differences noted for plasma steroid levels in the juvenile fish. These data suggest that there are seasonal changes of estradiol coinciding with other developmental changes during smoltification.