Reproduction of hatchery-produced meagre Argyrosomus regius in captivity I. Description of the annual reproductive cycle

The aim of the present study was to monitor the reproductive cycle of a hatchery-produced meagre Argyrosomus regius broodstock. Four-year-old fish of a mean weight (± SD) of 5.1 ± 1.1 kg for females (n = 7) and 4.6 ± 0.7 kg for males (n = 6), were maintained under simulated natural photo-thermal regime and sampled between November 2010 and July 2011. Vitellogenesis began in March, when the first increase in oocyte diameter was observed. Fully vitellogenic oocytes of mean (± SEM) diameter between 561 ± 23 and 621 ± 9 μm were observed between April and June. Two of the seven females spawned spontaneously in June, producing six major spawns of 390,000-940,000 eggs per day with fertilization success > 97%. Vitellogenesis was associated with relatively low levels of testosterone (T) ranging between mean values of 0.036 ± 0.01 ng ml− 1 and 0.207 ± 0.07 ng ml− 1, and of 17β-estradiol (E2) ranging between mean values of 0.194 ± 0.09 ng ml− 1 and 0.473 ± 0.12 ng ml− 1. Spermiation also began in March, but only 33% of the fish produced significant amounts of milt to allow sperm quality evaluations. In May 100% of the fish were spermiating, and in July spermiation ceased almost completely. No significant changes were observed during the spermiation period (March-June) in sperm concentration (mean ranged between 18.9 and 31.5 × 109 szoa ml− 1) or initial sperm motility (mean ranged between 44 and 80%). The mean duration of motility (mean ranged between 0.78 and 1.27 min) was also stable during the reproductive season, with the exception of a significant decrease in May. Mean plasma T and 11-ketotestosterone (11-KT) levels in male meagre increased gradually during the year, reaching their spermiation season peaks in March (0.818 ng 11-KT ml− 1) or in May (0.263 ng T ml− 1). The study demonstrated that gametogenesis proceeds normally in hatchery-produced meagre and under the influence of relatively low levels of sex steroid hormones. Although spontaneous spawning was observed for the first time in meagre maintained in captivity (2 of 7 females), the study demonstrated also that oocyte maturation is sporadic and inconsistent, and reliable spawning could be obtained using only exogenous hormones. Furthermore, it was shown that females failing to undergo oocyte maturation, maintain their vitellogenic oocytes in a viable condition without significant atresia for a period of at least 2 months (April-June). These oocytes could potentially be induced to undergo maturation, ovulation and spawning at any time during this “post-vitellogenesis” period.