Changes in plasma estradiol and effects of triiodothyronine on plasma estradiol during smoltification of coho salmon, Oncorhynchus kisutch.

Plasma estradiol-17 beta levels were measured by radioimmunoassay in untreated coho salmon of both sexes or in fish fed triiodo-L-thyronine during the smoltification period. Mean plasma estradiol increased between February 25 and April 1 from 94 to 142 pg/ml, and then by May 13, it had decreased to 80 pg/ml. This hormonal cycle was followed by a second significant increase to 219 pg/ml on June 22. Plasma thyroxine level covaried with that of estradiol. Treatment with triiodothyronine had no effect on plasma estradiol or thyroxine levels. Plasma estradiol surges during the smoltification period must be considered along with other significant endocrine changes for a possible role in the developmental phenomena that characterize this period.