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Two experiments were conducted to evaluate the effects of lysine supplementation on the reduction of dietary crude protein (CP), and reductions of total ammonia nitrogen (TAN) and soluble phosphorus (soluble-P) discharged into water by rainbow trout. In Experiment 1, eight diets were prepared, such that Diet 1 contained 32.8% fish meal, 42% CP and 2.1% lysine. Diet 2 had the same amount of fish meal and lysine as in Diet 1, but only 37% CP. Diets 3–8 contained 16.4% fish meal, 37% CP, and 1.5%, 1.65%, 1.8%, 1.95%, 2.1%, and 2.25% lysine, respectively. Rainbow trout (initial body weight: 26.9±0.2 g) were randomly assigned to triplicate tanks per diet with 50 fish per tank. After an 8-week feeding trial, weight gains were 112.5, 111.8, 87.5, 94.4, 103.0, 105.7, 101.8, and 102.6 g; feed conversion ratios were 0.99, 1.01, 1.16, 1.15, 1.10, 1.08, 1.10, and 1.09 g feed/g gain for fish fed diets 1–8, respectively. Survival was greater than 99% for all treatments. In Experiment 2, 130 fish (102.4±8.3 g) per tank were moved to flowing water tanks (140 l) and fed the same diets as in Experiment 1 for 1 week, and then TAN and soluble-P in the wastewater were measured for 2 consecutive days. The values of TAN discharged by the fish 8 h after feeding were 9.69, 8.56, 8.44, 7.81, 7.19, 6.65, 6.24, and 6.29 mg/l, and discharged soluble-P were 1.01, 0.85, 0.45, 0.31, 0.30, 0.26, 0.25, and 0.14 mg/l for fish fed Diets 1–8, respectively. Results showed that lysine supplementation in plant protein-based diets allowed dietary CP to be reduced without reducing fish performance. TAN and soluble-P excretions by rainbow trout were reduced in plant protein-based diets (37% CP) with lysine supplementation compared to the control fish fed with fish meal diets with either 42% or 37% CP. |
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