Timing of puberty in F1-generation hatchery-produced greater amberjack (Seriola dumerili)

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We evaluated the onset of puberty of first-generation (F1) hatchery-produced greater amberjack (Seriola dumerili) reared in sea cages for 5 years. Fish were sampled every year in June, at the expected peak of the spawning period in the Mediterranean Sea. No sexual dimorphism in body weight was observed in the study. The ovaries of 1 and 2-year-old (yo) females consisted of primary oocytes only, while at the age of 3-yo early vitellogenic (Vg) oocytes were also identified, but with extensive follicular atresia. At the age of 4-yo, late Vg oocytes were observed, but again extensive follicular atresia characterized the ovaries of 50 % of females. At the age of 5-yo, follicular atresia of Vg oocytes was very limited. In males, gametogenesis was evident already in 1- and 2-yo fish, and 100 % of sampled 3-yo males produced collectable viable sperm. Plasma testosterone (T), 17β-estradiol (E2), and 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) remained similar in 3 – 5-yo females, with T and E2 levels being highest in females in advanced vitellogenesis or with significant follicular atresia, compared to immature females. In males, plasma T declined over the years, while 11-ketotestosterone (11-KT) and 17,20β-P were highest in 4 and 5-yo males, with spermatozoa motility characteristics being improved from the 4th year onwards. The administration of GnRHa implants to 5-yo fish induced only two spawns, albeit no fertilized eggs were obtained. The results indicate that hatchery-produced greater amberjack males mature well and within the same age observed in the wild, however with smaller gonad size. On the contrary, females mature later than in the wild, also with a smaller gonad size. Spawning in response to GnRHa treatment was not effective, suggesting that Mediterranean hatchery-produced broodstocks may be dysfunctional, and further research is needed to document any improvement as the fish get older, or to determine if the results may be related to the specific stock of fish.