Studies of seasonal gonadal cycles and hormonal control of reproduction in winter flounder, Pseudopleuronectes americanus (Walbaum)

Studies were conducted using gonadotropic hormone- releasing hormone analog (GnRH-A) in adult winter flounder, Pseudopleuronectes americanus (Walbaum). This species was used as a model fish in an effort to understand seasonal gonadal cycles and the pituitary-gonadal axis regulation of reproduction in a seasonally breeding teleost. -- The natural seasonal reproductive cycle was investigated over a two-year period. In females, rapid ovarian recrudescence which begins in August, was largely completed by December, However, ovarian growth continued throughout the winter as shown by further increases in both gonadosomatic index and oocyte diameter. Analysis of oocyte-size frequency distribution indicated that in reproductively active females, previtellogenic and vitellogenic oocytes are present but just one clutch of vitellogenic oocytes matured per reproductive season. Plasma estradiol increased in parallel with gonadosomatic index and oocyte diameter, whereas plasma testosterone remained low during the early stages of ovarian recrudescence. Compared with females, male testicular development was more rapid, with GSI reaching a maximum in October followed by declining values prior to spawning. Plasma testosterone and 11-ketotestosterone levels rose slowly in association with the progress of testicular development. Plasma sex steroids reached their peak values just prior to the spawning period (May/June) in both sexes, while the minimum seasonal hormone values were observed in fish with regressed gonads. -- Seasonal responses of the pituitary, and indirectly the gonads, to GnRH-A were monitored by measuring plasma androgens and estrogen following in vivo hormone treatment. Both males and females were highly responsive to GnRH-A treatment throughout the gonadal recrudescence period and the prespawning phases; little or no response was observed during the regressed period. Plasma levels of estradiol, testosterone and 11-ketotestosterone were elevated following GnRH-A administration and the hormone levels were sustained for prolonged periods of time. The effects of GnRH-A treatment on the reproductive cycle were also observed by noting increases in gonadosomatic index and oocyte diameter in fish during the period of gonadal recrudescence. During the prespawning stage, ovulation and spermiation could be achieved following releasing hormone treatment. In contrast, the low gonadosomatic index remained unchanged by GnRH-A treatment during the regressed period. -- The capacity of GnRH-A to accelerate final egg maturation/ovulation and induce spawning was investigated in prespawning females prior to the natural spawning period. Hormone administration induced ovulation in some females in February, as early as three months prior to the normal spawning season. Egg/larval quality data indicated that GnRH-A can be used to advance spawning of female flounder without having serious detrimental effects on the rates of egg fertilization, hatching and larval survival