Grey mullet, Mugil cephalus fingerlings is strongly requested for extensive culture in Tunisian reservoirs. Currently they are collected in the wild in the estuaries. This production sector is essential for improving food security of inland regions. The average annual production of inland mullet production is about 315 Mt with high variations (from 195 and 466 Mt) depending to hydro-climatic conditions. This production represents 30% national freshwater fish production and play a significant socio-economic role in supporting local low-income fisherman. Obviously, a regular fingerlings supply from hatchery instead of wild catch is a requisite for the sustainability of this activity.
In the frame of the USaid-MERC project “Stock enhancement and production of the Grey Mullet fry – a sustainable choice”, the INSTM is establishing a captive broodstock of Mugil cephalus. The fish have been captured during two summer campaigns in 2018 and 2019 from an extensive culture pond in the region of Mahres (governorate of Sfax) and from a fixed fishery at Tinja (governorate of Bizerte). Survival rate after hatchery acclimation in INSTM Monastir was 40% in 2018 and improved to reatch 70% in 2019 (table 1). This by improving transport conditions of fish (water exchange during transport, individual capture by landing net, handling in water, preventive treatment …).
A total of 70 mullet broodstock consisted by 28 females (body weight [BW] 1300 ± 218g) and 42 males (BW 655 ± 201g) were acclimated in captivity. Fish have been rearranged in three tanks of 20 m3-tank in flow through system located in greenhouse and subjected to natural conditions of light and water temperature. Tanks are supplied with seawater at 38-42 psu salinity from the lagoon of Monastir. Fish were fed daily at the rate of 1-2 % of their biomass using a specific prepared diet enriched with dried spirulina.
During 2020 spawning season (August, September), ovarian biopsy of females revealed only the presence the previtellogenic stage with 150-200 µm oocytes and no spermiating mature males.
During 2021 spawning season, the fish with adequate size have been submitted to hormonal treatment since June (both GnRHa and dopamine antagonist for females and methyltestosterone for males) for enhancing gonadal development. Fully mature females (presence of oocyte with a diameter greater than 500 µm) and spermiating mature male (milt production) have been observed during September 2021 and final hormonal spawning injections for induction spawning are scheduled for the coming days, when adequate thermal conditions are reached.