SEASONAL-CHANGES IN THYROID AND REPRODUCTIVE STEROID-HORMONES IN FEMALE CHANNEL CATFISH (ICTALURUS-PUNCTATUS) IN POND CULTURE
Additional Document Info
Mature female channel catfish in pond culture in Texas were sampled monthly throughout 1 year to characterize changes occurring in ovarian histology and plasma concentrations of reproductive steroid and thyroid hormones. Oocyte growth, measured by oocyte diameter and gonadosomatic index, was initiated in the fall and continued until the time of spawning in May. Plasma estradiol and testosterone were lowest immediately after spawning and increased in the fall in association with the initiation of vitellogenesis. Both hormones showed significant increases in February; estradiol then remained elevated in prespawning animals, whereas testosterone returned to levels found in the fall. Increased plasma estradiol was accompanied by oocyte growth and preceded increases in plasma alkali-labile protein phosphorus, an indicator of circulating vitellogenin, supporting a role for estradiol in regulating vitellogenesis in channel catfish. A significant increase in the concentrations of both thyroxine and triiodothyronine occurred in February, coincident with the steroid hormone peak, indicating that endocrine interactions between thyroid activity and reproduction may be occurring at this time. A second peak in thyroid hormones occurred in the summer, unrelated to reproductive state and coincident with the period of maximum water temperature and weight gain, suggesting that in this species thyroid activation may also accompany periods of maximum anabolic activity. 1989.