PLASMA SEX STEROID PROFILES AND THE SEASONAL REPRODUCTIVE-CYCLE IN MALE AND FEMALE WINTER FLOUNDER, PLEURONECTES-AMERICANUS

The pattern of seasonal gonadal development and variations in plasma sex steroids were investigated in adult male and female winter flounder, Pleuronectes americanus (Walbaum), from Conception Bay Newfoundland beginning August 1987 to December 1988. The winter flounder reproductive cycle can be divided into five consecutive phases of relative reproductive activity including: (1) rapid gonadal recrudescence in the fall (August-December); (2) continued slow gondadal growth in females, or maintenance of the well developed gonads in males, during the winter (December-February); (3) a prespawning phase of gonadal maintenance in the spring (March-April); (4) spawning early in the summer (May-June) after the female gonads reach peak weight; and (5) the summer postspawned period (June-August) when the gonads remain regressed. Female gonadal recrudescence in August is characterized by small increases in plasma estrogen levels and recruitment of small oocytes (greater than or equal to 150 mu m) into yolk accumulation. For the winter months, estradiol-17 beta levels in the plasma remain stable, approximately 15 ng ml(-1), until rising again together with testosterone to peak hormone levels just prior to spawning in conjunction with the highest seasonal values for the gonadosomatic index (GSI) and oocyte diameter. After spawning, the ovaries are regressed and sex steroids in the plasma fall to very low levels. Rapid seasonal recrudescence of the gonads in males is evident from rising GSI values, which reach a maximum in October, and from substantial early seasonal increases in plasma testosterone and 11-ketotestosterone. Afterwards, although GSI values subsequently decline presumably reflecting the process of spermiogenesis and/or onsect of spermiation in some males, the testes remain relatively well developed with the presence of sperm throughout the spawning season. As the proportion of spermiating males increases, the plasma levels of the androgenic steroid hormones rise to peak circulating levels from April to June. At the end of the summer reproductive season, the testes of post-spawned males become regressed and plasma testosterone and 11-ketotestosterone levels fall, reaching the lowest seasonal values.