The culture performance of monosex and mixed-sex new-season and overwintered fry in three strains of Nile tilapia (Oreochromis niloticus) in northern Vietnam

Two experiments (E1, E2) to evaluate the performance of new-season and overwintered fry in three strains (Viet, GIFT and Thai) of Nile tilapia (Oreochromis niloticus) were conducted at Research Institute for Aquaculture No.1 (RIA-1) in northern Vietnam. The performance of both mixed and monosex fish of the three strains was also compared. The experiments were carried out from May to November (E1) and August to November 1997 (E2) with overwintered and new-season fry, respectively. Each experiment included a comparison of production in fertilized earthen ponds (6×300 m2) and cages (18×1.2 m3) suspended within a single pond (1200 m2). In the pond trials, the three strains were stocked communally as either monosex or mixed-sex groups, whereas treatments (strain×sex) were separated into replicate cages. Individual size of fish stocked was 11.8±0.8 g (E1) and 9.4±0.4 g (E2). Floating pellets were fed at 5% (b.wt. day−1, 30% crude protein) initially, and declined to 2% by the third month of culture. Water quality and fish growth were monitored monthly. Overwintered fingerlings, both mixed and monosex, of all three strains performed well. Daily weight gain (DWG) ranged between 1.8 and 2.3 g day−1 in both ponds and cages. Among the three tilapia strains, the GIFT fish attained a significantly (P less than 0.01) larger individual final weight (387.7±21.0 g in ponds; 410.6±35.1 g in cages) compared to the Thai (351.6±26.1 g in ponds; 373.2±30.4 g in cages) and Viet strains (359.5±33.2 g in ponds; 350.3±33.4 g in cages). Overall, monosex fish of the three strains grew significantly faster than mixed-sex fish (P less than 0.05). DWG of new-season fingerlings (E2) in ponds ranged from 1.6 to 1.8 g day−1, showing slower growth compared to overwintered fingerlings. There was no significant difference in the growth between the three strains, and between mixed and monosex fish in ponds. Growth of all three strains was significantly slower in cages than ponds (overall DWG=0.8–0.9 g day−1; P less than 0.01). The results suggest that the performance of overwintered seed compare well with new-season fry. These results are discussed in the context of developing appropriate strategies for tilapia seed productions for northern Vietnam