Gonadotropin-releasing hormone analogue (GnRHa) in the form of saline injections or sustained- release microspheres was used to induce oocyte maturation, ovulation, and spawning in captive red porgy (Pagrus pagrus). Individually tagged vitellogenic females (n = 9 or 10) were treated at the beginning of the spawning season (March) with 20 µg/kg body weight (bw) GnRHa-loaded microspheres, a single injection of 20 µg/kg bw dissolved in saline, or physiological saline (control). Females were placed in tanks (one tank per treatment) connected to overflow egg collectors and monitored for 11 days. In addition to the eggs collected from the tank overflow, eggs were stripped from the fish on a daily basis. Only one spawn was obtained from the control fish, probably from a single female, given the small relative fecundity (700 eggs/kg bw). On the contrary, treatment with a GnRHa injection produced two spawns (9 and 11 days after treatment) and 50% of the fish ovulated. Treatment with GnRHa microspheres induced seven spawns (3 and 6-11 days after treatment) and 100% of the females ovulated. Females did not spawn all the eggs ovulated on a particular day, evident from the significant number of eggs obtained by manual stripping. Egg quality did not significantly differ among treatments, whereas number of spawned eggs and total relative fecundity were significantly higher in fish treated with GnRHa microspheres (ANOVA, p<0.05). The results demonstrate the potential of GnRHaloaded microspheres to induce spawning in red porgy, as a method of overcoming spawning failures in commercial hatcheries.
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