Spawning induction of blue mackerel Scomber australasicus and eastern little tuna Euthynnus affinis by oral administration of a crude gonadotropin-releasing hormone analogue

Scombridae species, such as tunas and mackerels, often do not spawn in land-based fish tanks without hormone treatment. To induce spawning in various fishes, a gonadotropin-releasing hormone analogue (GnRHa) is often administered by pellet implantation. Noninvasive administration is desired to induce spawning in scombrids that are sensitive to handling stress. Spawning induction by oral administration has been reported in several fishes, yet this method has not been put into practice in the aquaculture industry since a considerable amount of GnRHa is needed. Utilization of peptide synthesizers is widespread, and antigen-grade GnRHa (AgGnRHa) produced by a custom-peptide supplier is approximately 100-fold cheaper than conventional reagent-grade GnRHa (RgGnRHa), although the purity of AgGnRHa is lower. Here, we confirmed that the spawning induction potency of AgGnRHa was similar to that of RgGnRHa by pellet implantation in blue mackerel Scomber australasicus. Oral administration of AgGnRHa [6.0 mg/kg body weight (BW) per day] showed an equivalent ability to induce spawning of the mackerel as pellet implantation (0.1 mg/kg BW). We could also induce spawning of eastern little tuna Euthynnus affinis by oral administration of the AgGnRHa. Further, the obtained eggs showed higher survival. Thus, the oral delivery of AgGnRHa could be a powerful tool to induce spawning in Scombridae.