Environmental and hormonal control of induced spawning inthin-lipped grey mullet, Liza ramada (Risso)

IntroductionThe reproductive cycle can be controlled by either placing the fish in an appropriateenvironment or by changing the internal regulating factors of the fish with injectedhormones or other substances.Aim of the studyThe present work was designed to determine the suitable environmental andhormonal requirements for induced spawning of thin-lipped grey mullet, reared infreshwater fish farms.Materials and methodsThe application of a high-potency synthetic analogue of luteinizing hormonereleasinghormone agonists (LHRH-a) and human chorionic gonadotropin (HCG)for inducing the final stages of maturation, ovulation, and spawning in mullets wasinvestigated in ripe spawners, reared in freshwater fish farms, with average eggdiameters of 0.600 mm or more, in suitable water temperature and salinity.ResultsTwo hormonal injection strategies were used. The first involved the use of HCGas a priming injection, followed, 24 h later, by a resolving injection of HCG.The second strategy followed the same time frame but the priming injection wasHCG and the resolving injection was luteinizing hormone-releasing hormoneagonists alone or in combination with HCG. The use of pregnyl (HCG) as apriming injection at a dose of 20 000 IU/kg body weight, followed by a secondinjection of 40 000 IU HCG/kg body weight 24 h later proved to be effective ininducing final oocyte maturation, ovulation, and spawning in Liza ramada at 44–50 hafter hormonal injection. The optimum water temperature and salinity was 19oC and35%, respectively. Translucent ovulated eggs had coalesced lipid and yolk masses.The fertilized eggs had one or more oil globules gathered in the upper space of theegg. The blastodisc was situated on the bottom side of the floating egg.ConclusionHCG is very useful for inducing and synchronizing spawning in L. ramada ripespawners, reared in freshwater fish farms, in combination with suitable environmentalfactors.