Advancement of ovulation in yellowtail founder (Pleuronectes ferrugineus) using gonadotropic hormone-releasing hormone analogue (GnRHa)

Yellowtail flounder (Pleuronectes ferrugineus) are being considered for aquacultural development and in order to maximize productivity within a hatchery, a thorough knowledge of the reproductive cycle and how it may be manipulated, must be attained. -- Ovarian biopsies, using polyethylene cannulae, were used to follow the annual oocyte growth pattern in yellowtail flounder. Vitellogenesis, as measured by an increase in both oocyte diameter and opacity, began in October-November, and progressively continued until the spawning season, between June and August The use of cannulae for ovarian biopsies in yellowtail flounder was validated by assessing homogeneity of oocyte sizes throughout the ovary, indicating that oocytes from the anterior ⅓ of the ovary were representative of the entire left and right ovaries. -- The capacity to advance the yellowtail flounder spawning season using gonadotropic hormone-releasing hormone analogue ([D-Ala⁶,Pro⁹-NHEt]GnRHa) was studied in maturing females by administering the hormone in three separate spawning trials, four months, two months and two weeks prior to the natural spawning season. From ovarian biopsies taken at time of treatment, it was found that the spawning response was dependant upon oocyte size reaching approximately 400μm before final egg maturation, and ovulation, could be induced. In addition, the response to GnRHa treatment was quicker, more uniform, and better synchronized closer to the natural spawning season, with 25%, 100% and 100% of the females spawning in each group, respectively, and the mean number of days to first ovulation being 25, 20, and 14 days, respectively. -- Egg quality was measured by morphological criteria of unfertilized eggs (viability rates or the percentage of potentially fertilizable eggs), fertilization rates, and hatch rates and were high and similar in each group. Viability rates for each group and the control group were 87.1%, 83.0%, 89.9% and 82.9%, respectively. Fertilization rates were 71.0%, 63.7%, 70.7% and 54.6%, whereas hatch rates were 79.2%, 85.4%, 86.8%, and 80.5%, respectively. Advancement of spawning by GnRHa had no deleterious effects on egg quality when compared to spontaneously ovulating control fish