Assessment of key reproductive markers after hormonal induction of spawning, using gonadotrophin-releasing hormone in female yellow belly flounder (Rhombosolea leporine):.

Abstract

Yellow belly flounder (YBF) (Rhombosolea leporina) are of interest to the New Zealand aquaculture industry as a novel culture species. This is due to their high commercial value and low trophic feeding level. However, when held in captive settings, YBF are observed to undergo reproductive failure. GnRHa has been used as a spawning inducing agent within many cultured fish species. Flounder gonadotrophin levels were traced after induction and oocyte development was histologically assessed. At pituitary level it was seen that the GnRHa induction resulted in increased follicle stimulating hormone (FSH) levels. Luteinising hormone (LH) was seen to be unaffected. However, neither of these changes were significant P>0.05. These weak results were most likely caused by seasonality. Oocyte development was seen to follow a similar trend to other flounder and flatfish species, when oocyte size, development stages and features were tracked using histological analysis. Relationships between Gonadosomatic index (GSI) and gonadotrophins FSH and LH displayed weak correlations (P>0.05). This again could be linked to seasonal variability in temperature and photoperiod. The presence of large amounts of atretic oocytes observed in the gonads indicated that the ovary had already spawned for the season before initial capture, or that oocytes had been aborted. A likely cause of this would be captivity -induced stress, or fish having previously spawned before capture. All fish that ovulated were part of the GnRHa treated group. Fish were strip-spawned, fertilised and embryonic development was tracked. Incubation temperatures were 17 and 19°C. It was established that of the two temperatures, 17°C was optimum. A larger percentage of these embryos survived from fertilisation to hatch, which took approximately 68 hours. Bacterial infection was seen to be a problem in eggs incubated at the higher 19°C temperature. Egg quality was assessed by looking at the 8 cell blastomere stage, and grading charts were made for reference. these assessed blastomere size, shape, symmetry and cohesion.

It is suggested that future research efforts focus on seasonal variations of gonadotrophin levels and gonadal development, in order to gain clearer understanding of these seasonal effects. Effects of cortisol on the reproductive axis would also be a beneficial research area. This would provide insight into how stress affects sex steroid production and the maturation of the gonad and oocytes in YBF.