EVALUATION OF THE ARTIFICIAL OXYTOCIN EFFECT OF GNRH INJECTION AND ORAL ADMINISTRATION ON SMALL YELLOW CROAKER (LARIMICHTHYS POLYACTIS)

Marine economic fish such as Larimichthys crocea and Larimichthys polyactis cannot spawn spontaneously under artificial indoor breeding conditions, so they need artificial oxytocin to ovulate normally. The traditional way of artificial oxytocin is to give injections to the parent fish and squeeze eggs will cause great stress response, sometimes even die from stress, resulting in the reduction of the parent fish population. Oxytocin requires a lot of manpower and resources, therefore, a low-cost and effective way of oxytocin should be found. To evaluate the effect of gonadotrophin releasing hormone (GnRH) as an exogenous hormone on the spawning of small yellow croaker (Larimichthys polyactis), the saline injection group was used as the control, and the luteinizing hormone releasing hormone (LHRH-A2, 2.4 μg/kg) injection group, the gonadotrophin releasing hormone (GnRH, 75 μg/kg) injection group and gonadotropin-releasing hormone (GnRH, 6 mg/kg) oral administration group were designed to analyze the egg production, ovarian development changes and serum hormone levels. The results showed that the GnRH-injected group did not differ significantly from the LHRH-A2-injected group in terms of the initial spawning time and duration, while the total spawning quantity, fertilization rate and hatching rate were significantly higher (P<0.05). Histological analysis of the spawning process showed that the LHRH-A2 injection group as well as GnRH injection and oral administration induced ovarian development from stage Ⅲ to stage Ⅴ, and promoted oocyte maturation and excretion from stages Ⅲ and Ⅳ. Serum luteinizing hormone (LH), follicle stimulating hormone (FSH) and GnRH, increased rapidly with exogenous hormone treatment and could be maintained until late oviposition. Similar trends were observed in all treatment groups. In summary, GnRH injection could induce the spawning of the small yellow croaker and the effect of spawning was not significantly different from that of LHRH-A2. Oral administration of GnRH could induce spawning in small yellow croaker as well, but the spawning quantity and buoyant egg weight were significantly lower than those of the hormone injection groups. Further optimization of the oral administration of GnRH is needed in the future. The results provide a certain reference basis for further optimizing and supplementing the oxytocin mode of small yellow croaker, and oral hormone method is also a powerful tool to induce the spawning of small ornamental fish with too large body size and inconvenient operation.