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

Two experiments (E1, E2) to evaluate the performance of new-season and over wintered fry in three strains (Viet, GIFT and Thai) of Nile tilapia (Oreoch romis niloticus) were conducted at Research Institute for Aquaculture No.1 (RTA-1) in northern Vietnam. The performance of both mixed and monosex fish of the three strains was also compared. The experiments were carried out f rom 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 X 300 m(2)) and cages (18 X 1.2 m(3)) suspended within a single pond (1200 m2). In the pond trials, the thr ee strains were stocked communally as either monosex or mixed-sex groups, w hereas treatments (strain X sex) were separated into replicate cages. Indiv idual 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) initiall y, 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 perf ormed 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 attai ned a significantly (P < 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 +/- 2 6.1 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 < 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 betwee n mixed and monosex fish in ponds. Growth of all three strains was signific antly slower in cages than ponds (overall DWG = 0.8-0.9 g day(-1); P < 0.01 ). The results suggest that the performance of overwintered seed compare we ll with new-season fry. These results are discussed in thr context of devel oping appropriate strategies for tilapia seed productions for northern Viet nam. (C) 2000 Elsevier Science B.V. An rights reserved.