The effect of citric acid supplementation on growth performance, phosphorus absorption and retention in rainbow trout (Oncorhynchus mykiss) fed a low-fishmeal diet

Two feeding experiments were conducted to investigate the effect of a low-phosphorus (P) diet in combination with citric acid (CA) on rainbow trout growth performance and P utilization. In the first 12-week trial, duplicate groups of 30 fish (12.0 +/- 1.7 g) were fed one of five diets. The basal diet was formulated with low-P protein sources and 15% fishmeal (FM) without inorganic P supplementation. The test diets contained 0.5% monocalcium phosphate (MCP), 3% CA or both. A FM-based diet served as the positive control. The diet with MCP showed good growth and feed performance comparable to the control. Growth was not improved by the addition of 3% CA. In the second trial, duplicate groups of 20 fish (80.5 +/- 18.5 g) were offered one of four diets during a 12-week period. The basal diet from the first trial with MCP supplementation served as the positive control, and the same basal diet without MCP supplementation again served as the negative control. The test diets were supplemented only with CA at 1 and 2%. The diet with 1% CA had growth performance similar to the positive control. Thus, the addition of 1% CA to a low-FM diet containing no inorganic P liberated enough available P to produce juvenile fish performance similar to that achieved with a diet containing supplemental inorganic P