Spawning performance and plasma levels of GnRHa and sex steroids in cultured female Senegalese sole (Solea senegalensis) treated with different GnRHa-delivery systems

The aquaculture of Senegalese sole (Solea senegalensis) is limited by the poor spawning performance of the F1 generation cultured broodstock. The present study compared the efficiency of two sustained-release delivery systems loaded with gonadotropin-releasing hormone agonist (GnRHa) and a single GnRHa injection on the stimulation of oocyte maturation (OM), sex steroid hormone secretion and spawning. Analysis with an ELISA showed that the administered GnRHa was no longer detectable in the plasma 3, 7 and 14 d after administration via a single injection, microspheres and an implant, respectively. All GnRHa treatments induced a transient elevation of testosterone (T) and 17β estradiol (E2) plasma levels after 1–3 d. Only three spawns were obtained from controls on days 35, 41 and 42 of the study and ovarian biopsies showed abundant fully vitellogenic oocytes, but little OM. Fish given a GnRHa injection spawned sporadically with the first spawn at 20 days post-treatment (dpt). In contrast, GnRHa implants and microspheres stimulated OM and daily spawning starting at 4 and 5 dpt, respectively, and lasting for a period of 28 d. Total fecundity in the broodstock treated with GnRHa-delivery systems was 15–22 times greater than controls and 3–5 times greater than GnRHa-injected fish. Although egg quality appeared to be high, no fertilization was obtained. The results suggest that GnRHa-delivery systems are highly efficient on stimulating OM and spawning in Senegalese sole, presumably through the long-term elevation of pituitary luteinizing hormone release, which in turn stimulated the appropriate changes in ovarian steroidogenesis. However, the effectiveness of the treatment was negated by possible dysfunctions of the male breeders or of the reproductive behavior of the broodstock.