EFFECT OF STOCKING DENSITY ON THE GROWTH OF RAINBOW TROUT (Oncorhynchus mykiss) AND WATER QUALITY IN RECIRCULATING AQUACULTURE SYSTEMS

Abstract

Farming rainbow trout (Oncorhynchus mykiss) in Vietnam started in 2005. Farming in water flowing through rearing systems is applied as the main farming technique with low density. Floods and water shortages cause great damage to rainbow trout farming in the Northwest of Vietnam. This study aims to evaluate the impact of stocking density on water quality and the growth performance of rainbow trout cultured in a low-head recirculating aquaculture system (LH-RAS). Two stocking densities, including 45 fish/m³ and 55 fish/m³ were raised in a 150 m³ LH-RAS separately. The LH-RAS has two raceway farming sections combined with a rough filtration and biological filtration treatment section. The system was supplemented with new water from 0.5 - 1l/second to compensate for the wastewater discharged from the system. The initial body weight of fish was 450 ± 5 g/fish and fed daily with industrial pellets containing 40 - 45% crude protein. The total body weight of the fish was measured every 25 days. Water temperature, DO, and pH were measured daily. N-NO₂^-, CO₂, H₂S, Alkalinity, TSS and N-NO₃^- were measured weekly. The results indicated that water parameters were significant differences between the two groups, specifically DO (6.09 ± 0.68 vs. 5.73 ± 0.55 mg/l), N-NO₂^- (0.014 ± 0.005 vs. 0.035 ± 0.0027 mg/l), N-NO₃^- (3.46 ± 1.22 vs. 8.63 ± 4.42 mg/l), CO₂ (6.82 ± 0.89 vs. 7.83 ± 1.25 mg/l) and TSS (4.11 ± 1.18 vs. 5.63 ± 1.10 mg/l) (P < 0.05). At day 125, the weights of the fish in 45 fish/m³ and 55 fish/m³ groups started to diverge with the size of 861 ± 28 g and 800 ± 39 g per fish, respectively (P < 0.05). After 180 days, the fish at a density of 45 fish/m² reached 1,340 ± 82 g per fish, while those at a density of 55 fish/m³ reached 1,170 ± 47 g per fish (P < 0.05). The DWG indices for these two groups were 4.72 and 3.75 g/day, respectively. The better water quality contributed to better growth of the 45 fish/m³ group compared to 55 fish/m³.