Seasonal patterns in serum levels of thyroid hormones and sex steroids in relation to photoperiod-induced changes in spawning time in rainbow trout, Salmo gairdneri.

Serum levels of thyroid hormones (T4 = L-thyroxine and T3 = 3,5,3'-triiodo-L-thyronine) were measured and correlated with previously published levels of 17 beta-estradiol, testosterone (T), and Ca2+ (index of vitellogenin) in four groups of female trout held for 1-3 years at 8.5-9.0 degrees on a ration of 0.5% of body weight day-1 under different constant photoperiod regimes. In group 1, trout under a regimen of 18L:6D became sexually mature (SM) in April/May and then SM again in September/October; in group 2, trout under a regimen of 6L:18D became SM in January/February; in group 3, trout under a regimen of 6L:18D became SM in March/April; in group 4 trout under a regimen of 18L:6D followed by 10L:14D became SM in September/October. In all groups, regardless of the photoperiod-induced changes in spawning time, serum T3 exceeded T4 and both serum T3 and T4 patterns showed a consistent relationship with the sequence of steroid hormone changes and spawning time. Thyroid hormone levels were high during previtellogenesis but fell as sex steroids and serum Ca2+ increased. T3 and T4 were lowest before spawning when sex steroids were at their peak and then increased sharply following spawning as sex steroid levels declined. Peak serum T coincided with ovulation and usually preceded the postreproductive surge in serum T3 and T4. The hypothesis is discussed that T3 enhances early ovarian development, but as energy-demanding vitellogenesis proceeds T3 formation is suppressed, thereby curtailing growth and favoring energy partition to the ova.