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

Abstract

Introduction The reproductive cycle can be controlled by either placing the fish in an appropriate environment or by changing the internal regulating factors of the fish with injected hormones or other substances. Aim of the study The present work was designed to determine the suitable environmental and hormonal requirements for induced spawning of thin-lipped grey mullet, reared in freshwater fish farms. Materials and methods The application of a high-potency synthetic analogue of luteinizing hormone-releasing hormone agonists (LHRH-a) and human chorionic gonadotropin (HCG) for inducing the final stages of maturation, ovulation, and spawning in mullets was investigated in ripe spawners, reared in freshwater fish farms, with average egg diameters of 0.600 mm or more, in suitable water temperature and salinity. Results Two hormonal injection strategies were used. The first involved the use of HCG as 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 was HCG and the resolving injection was luteinizing hormone-releasing hormone agonists alone or in combination with HCG. The use of pregnyl (HCG) as a priming injection at a dose of 20 000 IU/kg body weight, followed by a second injection of 40 000 IU HCG/kg body weight 24 h later proved to be effective in inducing final oocyte maturation, ovulation, and spawning in Liza ramada at 44–50 h after hormonal injection. The optimum water temperature and salinity was 19oC and 35%, 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 the egg. The blastodisc was situated on the bottom side of the floating egg. Conclusion HCG is very useful for inducing and synchronizing spawning in L. ramada ripe spawners, reared in freshwater fish farms, in combination with suitable environmental factors.