Assessment of the effects of captivity on first sexual maturity of wild-caught and hatchery produced mullet (Mugil cephalus)

INTRODUCTION In fish, puberty is susceptible to multiple, interacting environmental cues. As a result, farming conditions may have varying influences on the age of first sexual maturity, causing precocious puberty in some species or delayed and even completely blocked puberty in others. Therefore, this study aimed at characterizing pubertal development in grey mullet (Mugil cephalus), an important fish candidate for domestication and aquaculture production. MATERIALS AND METHODS 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 consist 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). RESULTS AND CONCLUSIONS Our results revealed that all 2y fish had immature gonads. The majority of 2y females exhibited late perinucleolar oocytes as the most advanced oocyte stage. 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 than cognate males. Although not significant, the HP 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: 100% of the WC males showed first stages of spermatogenesis, while 67% of the male population in the HP group produced mature sperm. HP females, sampled during natural spawning season, 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. The project received funding from the European Union 7FP (GA 603121, DIVERSIFY) .