This study was conducted in 20 reservoirs, ranging in size from 4 to 30 ha, in the mountainous, northern region of Vietnam, in ThaiNguyen and YenBai provinces, over two growth cycles in 2002/03 and 2003/04. The reservoirs are leased by farmers for ¢shery activities from the provincial administration, and the trials were managed by the lessee farmers. Three species combinations in ratios (by ¢ngerling weight) of grass carp: silver carp: bighead carp: common carp: mrigal 1:2:1:1:3 (A), 1:3:1:1:2 (B) and 1:2:1:1:2 (C) were used as seed stock. The overall mean yield of stocked ¢sh in 2002/03 and 2003/04 growth cycles in reservoirs in ThaiNguyen and YenBai provinces was 165 ( AE 21) and 190 (39), and 287 ( AE 22) and 325 ( AE 24) kg ha À1 respectively. The yield in reservoirs in both provinces, in both growth cycles and irrespective of the species combinations, increased in relation to stocking density. In reservoirs in ThaiNguyen Province, the species combination B gave the lowest yield (both growth cycles and overall), and differed signi¢cantly (Po0.05) from combinations A and C. The stocking e⁄ciency (ratio of the yield of stocked ¢sh in kg ha À1 to the weight of the stocked ¢sh in kg ha À1 ) in reservoirs inThaiNguyen Province ranged from 2.9 to 5.1over the two growth cycles and that in YenBai from 2.8 to 3.9. There was no discernible trend between growth cycles and/or between species combinations. The major cost incurred was for ¢ngerling procurement. In all instances, a net pro¢t was accrued. The mean ( AE standard error (SE)) net pro¢t ha À1 (in 10 3 VN dong; approximately 15500 VND 51 US$) was 885 ( AE 270) and 864 ( AE 214), and 1322 ( AE 176) and 1600 ( AE 150) for the growth cycles 2002/03 and 2003/04 for reservoirs in ThaiNguyen and YenBai provinces respectively. Between the two growth cycles, the net pro¢t increased in eight and three reservoirs from YenBai and ThaiNguyen, respectively, the maximum increase being recorded in Khuan Gio (165%) and Dong Man (39%) reservoirs.The overall mean values, for each growth cycle, with the same superscript are not signi¢cantly di¡erent from each other (P40.05).Gc, grass carp; Sc, silver carp; Bhc, bighead carp; Cc, common carp; Mrg, mrigal.Aquaculture Research, 2005, 36, 1037^1048 Culture-based ¢sheries in reservoirs in northernVietnam H S Nguyen et al.
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