Induction of sexual maturation of the female honeycomb grouper, Epinephelus merra, in the non-breeding season by modulating environmental factors with GnRH analogue implantation

Abstract

Grouper culture assumes importance like any other economically important food fish aquaculture practice in Japan. Regular availability of sexually mature brood stock at appropriate time and stable seed production are considered vital for prolific fish farming. Despite the fact that previous studies have induced sexual maturation and spawning in some grouper species that had ovaries with vitellogenic or full-grown mature oocytes, little attention has been paid to artificial induction of sexual maturation and spawning of immature fish in off-spawning season. Developing such a method will promote grouper aquaculture immensely. In an aim to facilitate such a task, we used one of the popular experimental model fishes, the honeycomb grouper, Epinephelus merra, to induce sexual maturation during non-breeding season by modulating environmental factors with gonadotropin-releasing hormone analogue (GnRHa) implantation using osmotic pump. We performed three different experiments flanking various periods of non-breeding season under artificial/natural photothermal conditions with or without GnRHa implantation. Induction of sexual maturation is scored by the presence of vitellogenic to mature oocytes after various treatments in addition to the data of gonado-somatic index and plasma estradiol levels. Among various treatments, high water temperature with long photoperiod conditions induced sexual maturation in immature groupers during October to December. Implantation of GnRHa to these fishes induced sexual maturation at all periods of non-breeding season tested in this study. In fact some of the fishes belonging to these groups have even spawned during the course of treatment indicating the effectiveness of the treatments. On the other hand, neither GnRHa implantation alone nor exposure to natural photothermal conditions evoked sexual maturation. Likewise, combination of short and long photoperiod with low to high water temperature exposure over the duration period was found to be least effective with or without GnRHa implantation. Similarly, exposure to long photoperiod or high water temperature alone did not induce sexual maturation. Hence, combination of artificial photothermal conditions with GnRHa implantation seems to be a decisive method to induce sexual maturation in immature fish. Based on these results, we demonstrate a superior strategy for successful breeding of sexually immature marine teleost fish during non-breeding season by modulating environmental variables with GnRHa implantation.