Annual reproductive cycle of the common snook: Endocrine correlates of maturation

Wild common snook Centropomus undecimalis were captured off the Gulf coast of Florida over a 2-year period and sampled for serum and gonad tissue to c haracterize their annual reproductive cycle. During the: summer months, lev els of the sex steroid hormones estradiol-17 beta (E-2) and testosterone (T ) in females were significantly elevated above basal winter values. Mean go nadosomatic index (GSI) was also significantly elevated in females during s ummer. In addition, fish whose ovaries contained oocytes in vitellogenic an d final maturation stages had elevated levels of alkali-labile protein phos phorus (ALPP, an indirect measure of the egg yolk precursor, vitellogenin. Peak levels of E-2, T, and ALPP were measured in females during the spawnin g season (late summer) when GSI was also maximal. Circulating levels of T a nd 11-ketotestosterone (11-KT) in males were also higher in the summer mont hs than at any other time of the year. Males showed an increase in GSI over basal winter values during the summer months, with patterns of change simi lar to those of circulating androgen profiles. The highest androgen levels and GSI were measured in males during the spawning season when a high degre e of sperm duct formation was evident histologically. Collectively, these d ata provide a basis for future evaluation and control of reproductive funct ion in cultured common snook and in environmental studies of this species.