GnRHa-induced spawning of the Eastern little tuna (Euthynnus affinis) in a 70-m3 land-based tank

• Through GnRHa administration, we successfully induced spawning in Eastern little tuna in a land-based tank.
• Multiple and continuous spawning events were induced by the GnRHa administration.
• The fertilized eggs obtained following induced spawning developed and grew normally.
• This technique represents the first step toward more reliable production of artificial Eastern little tuna seeds.

A suitable recipient species is essential for efficient production of donor-derived gametes. With its relatively small body size, short generation time, and physiological and genetic similarities to Pacific bluefin tuna (PBT; Thunnus orientalis), Eastern little tuna (ELT, Euthynnus affinis) is a suitable candidate recipient species for spermatogonial transplantation. To provide a stable source of recipient larvae for spermatogonial transplantation, in the present study we developed a hormone administration technique for reliable seed production in captive ELT. In June 2009, 32 wild-caught 1-year-old ELTSwere transferred to a 70-m3 land-based tank at our facility (situated at 34°97′ N, 139°76′ E) and reared in the tank for 13 months. Fourteen of the fish (43.8%) survived until July 2010. During the 2010 spawning season (August–October), no spontaneous spawning was observed. To induce spawning, gonadotropin-releasing hormone agonist (GnRHa) was administrated to 9 of 14 ELT broodstocks (5 females and 4 males) in September 2010. The first spawning was observed at 3 days post-implantation, and continual spawning events were observed over the following 8 days. The number of eggs collected per day ranged from 50,000 to 170,000. The mean (± SD) fertilization and hatching percentages were 87 ± 12% and 30 ± 19%, respectively. Parentage assignment by DNA genotyping revealed that 3 of the 5 implanted females produced viable offspring, and one of these females participated in up to nine consecutive spawning events. For the second spawning induction trial, twelve 2-year-old ELTS (6 males and 6 females) were maintained in a net pen before the trial and were implanted with GnRHa pellets in August 2011 when they were transferred to the same tank used for the first trial. Production of fertilized eggs was observed continually for 11 days after GnRHa administration. The mean (± SD) number of hatched larvae per day was 138,059 ± 115,352, and the mean (± SD) fertilization and hatching percentages were 61 ± 42% and 42 ± 33%, respectively. Thus, GnRHa administration is effective for inducing spawning in ELT maintained in captivity and leads to a more reliable production of ELT seed. The reproduction enhancement techniques developed in the present study may ultimately facilitate development of a surrogate broodstock technology for PBT.