Timing of puberty in hatchery-produced greater amberjack (Seriola dumerili)

INTRODUCTION

The process of puberty involves the acquisition of competence of the brain-pituitary-gonad axis in supporting the proliferation, development, and maturation of germ cells. Under aquaculture conditions, the timing of puberty might differ from what is reported in the wild, and fish may either reach precocious or late maturity, while in extreme scenarios they never achieve sexual maturity. Farming of greater amberjack (Seriola dumerili) is still in its infancy and information on the reproductive biology of this species has been obtained mostly from wild-caught reared fish. We report here the first attempt to describe the first age of maturity in a first-generation (F1) greater amberjack stock held in sea cages.

METHODS

Hatchery-produced greater amberjack were reared in sea cages from juveniles (0+) to 5 years old (yo). Samplings were conducted every year in June, during the period of reproductive maturation and spawning of cultured reared broodstock in the Mediterranean basin. Fish were bled and plasma levels of eight sex steroids were measured using liquid chromatography tandem mass spectrometry (LC- MS/MS) and records were taken of body weight (BW), total length (TL), gonadosomatic index (GSI) and hepatosomatic index (HIS). At the age of 4 and 5 yo, fish were considered reproductively mature and eligible for spawning induction, and were treated with gonadotropin-releasing hormone agonist (GnRHa, 50 μg kg-1) and transferred to tanks for spawning. Egg production was evaluated for 3 weeks.

RESULTS & DISCUSSION

No growth-related sexual dimorphism was observed. The ovaries of 1 and 2 yo females consisted of primary oocytes, while at the age of 3 yo early vitellogenic (Vg) oocytes were also identified. At the age of 4 yo, late Vg oocytes were recorded, but extensive follicular atresia characterized the ovary content of 50% of females. Similarly, at the age of 5 yo, batches of late Vg oocytes were present in the gonads, but atresia was very limited. On the other hand, complete gametogenesis in males was evident already in more than 50% of the examined 2 yo fish. The percentage of males in an advanced spermatogenesis stage reached 100% in 3 yo fish and, therefore, it can be considered as their first age of maturity for males greater amberjack. In regards to the hormonal profile of the monitored F1 fish, low levels of testosterone (T) and 17 β-estradiol (E2) were found in all females that entered vitellogenesis. Moreover, high levels of androstenedione were detected in the plasma of atretic females. In males, high androgen levels were found during advanced spermatogenesis. The administration of GnRHa to 4 and 5 yo fish, induced one and two spawns, respectively, however no fertilized eggs were obtained. The results indicate the male F1 greater amberjack mature well and within the same period observed in wild males, albeit with smaller gonad size. On the contrary, females seem to mature later than in the wild, also with a smaller gonad size. Spawning in response to GnRHa treatment was not effective, contrary to what has been shown with wild-caught culture-reared broodstock.