Histological study of the effects of treatment with gonadotropin-releasing hormone agonist (GnRHa) on the reproductive maturation of captive-reared Atlantic bluefin tuna (Thunnus thynnus L.)

The effects of gonadotropin-releasing hormone agonist (GnRHa) administration on the gonads of 66 Atlantic bluefin tuna (Thunnus thynnus L.) reared in floating cages (La Azohía, Murcia, Spain) were evaluated using histological analyses. During two consecutive reproductive seasons (June-July 2004 and 2005), about half of the fish in each cage were administered a GnRHa implant using a spear gun, without being anesthetized or physically restrained, and were sacrificed for sampling 2-8 d later. Rearing for 1-3 years in captivity did not prevent vitellogenesis and oocyte growth, since yolk granule formation and oocyte diameter of fully vitellogenic oocytes were similar to wild fish. However, a lower gonad size and number of vitellogenic oocytes were observed compared to wild spawners. None of the 14 untreated Controls had oocytes at final oocyte maturation (FOM) at the time of sampling and postovulatory follicles (POFs) were observed only in three of them. In addition, a higher volume fraction of atretic oocytes was observed in untreated Controls when compared to the wild or GnRHa-treated fish, indicating a shut-down of reproductive activity at this time. On the contrary, all GnRHa-treated females had oocytes at FOM and/or POFs, with the exception of two fully atretic individuals. In the males, no differences in testicular histology were observed between GnRHa-treated and Control fish, and individuals were at early or late spermatogenesis. The study indicates that wild-caught Atlantic bluefin tuna reared in captivity undergo vitellogenesis and spermatogenesis, but females have a low capacity for spontaneous FOM and ovulation, whereas GnRHa treatment is effective in inducing multiple cycles of FOM and ovulation.