Sex steroid hormone and vitellogenin levels in striped bass (Morone saxatilis) maturing under 6-, 9-, and 12-month photothermal cycles.

Adult striped bass (Morone saxatilis) were matured under compressed 6- and 9-month photothermal cycles or under a 12-month control cycle. The length of their reproductive cycle was proportional to the length of the photothermal cycle to which they were exposed. Maturational changes in circulating levels of the yolk precursor vitellogenin (VTG) and of sex steroid hormones were similar in all groups of fish. Levels of VTG increased progressively in the blood plasma of maturing females and were highly correlated with oocyte diameter, but profiles of circulating estradiol-17 beta (E2) and testosterone (T) were biphasic. Plasma levels of E2 and T were low during most of oocyte growth, increasing sharply to peak levels just before spawning. Spent or regressed females had nondetectable or low plasma levels of the hormones and VTG. In males, plasma 11-ketotestosterone (11-KT) and T were sustained at low levels during most of the reproductive cycle, increasing only during spermiation. The main endocrine effect of compressing the photothermal and gametogenic cycles was to shorten the early period of sustained low hormone levels, during which most oocyte growth and the early stages of spermatogenesis occur. Consequently, final oocyte size and ovary or testes diameter were diminished in fish matured on short cycles. Photothermal cues appeared to initiate and terminate the reproductive cycle, but between these times, maturation rate and circulating levels of gonadal steroids were unresponsive to photothermal periodicity.