In fish, puberty (= first sexual maturity) is susceptible to multiple, interacting environmental cues. As a result, farming conditions may have varying influences on the age of pubertal onset causing precocious puberty in some species or delayed and even completely blocked puberty in others. Therefore, this study aimed at characterizing pubertal development in cultured grey mullet ( Mugil cephalus), an important fish candidate for domestication and aquaculture production.
Two stocks of grey mullet were compared: wild-caught (WC; Apulia, Italy) and hatchery produced (HP; National Center for Mariculture, Eilat , Israel) grey mullet, both subjected to captive conditions consisting of ambient seawater salinity (40 ppt; Gulf of Eilat, Red Sea) and photo-thermal regime. The age of the fish was estimated based on their scales. Growth performance and gonadal development were monitored in 2- and 3- year old fish (2y and 3y).
Our results revealed that in both groups all 2y fish had immature gonads. The majority of age 2y females had ovaries with late perinucleolar stage oocytes as the most advanced oocyte stage, however, some of these fish still had ovaries constituting mainly by oogonia intermingled with small groups of early perinucleolar stage oocytes. Yet, HP females had significantly larger oocytes than WC specimens. Testis from an immature age 2y HP specimen showed small seminiferous lobules. Only spermatogonia, along with somatic cells were visible. Gonad section from a 2y HP intersex showed the presence of all stages of spermatogenesis. Scattered perinucleolar stage oocytes were visible, indicating that both WC and HP, had immature testes. Following 3y age category revealed that WC females were larger and heavier than cognate males. Although not significant, the HP grey mullets appear to exhibit a similar trend. Gonadosomatic index (GSI) values in 3y HP females and males were significantly higher than those of the WC of the same age. Undifferentiated gonads were found in 20% of WC fish compared to 5% in the HP fish. While only 33% had reached vitellogenic oocytes level, 54% of the HP females reached this level of development. Males exhibited a significant difference in development: while 100% of the WC males showed first stages of spermatogenesis, 67% of the male population in the HP group produced mature sperm. HP females, sampled during October (spawning season, under our natural conditions), reached maturity and ovulation. Their GSI exceeded 15%. Interestingly, approximately 50% of the HP females’ GSI exhibited markedly developed gonads with GSI values ranging between 10-20% while all the others were lagging far behind, having GSI values between 0.2 to 0.3 %. GSI values in WC females were also divided into 2 sub groups and found to be significantly lower than those of the hatchery produced females (1-6% and <1% respectively). Males also exhibited two GSI groups pattern, higher GSI values were exhibited by HP fish (1.5%-2.75% and <1% compared to 0.2-0.5% and <0.15%) . Suggesting hierarchy plays a major role affecting gonadal development in mullets.
In summary, The 3 yr old HP mullet females and males exhibited enhanced gonadal maturation as compared to that of the WC captive-reared fish, probably as a result of domestication.