A nutritional approach to reduce phosphorus pollution in hatchery effluent

The phosphorus load in aquaculture discharge water can adversely impact aquatic ecosystems by stimulating eutrophication. Although phosphorus is required in fish feeds for proper growth and nutrition, a significant portion of dietary phosphorus is excreted into the water. The main objective of this study was to determine the effects of suboptimal phosphorus intake on metabolic response and phosphorus utilization. This information will contribute to efforts to reduce dietary phosphorus excretion from rainbow trout while maintaining optimum nutrition. The effects of vitamin D{dollar}sb3{dollar} and triiodothyronine (T3) on improving bioavailability and retention of dietary phosphorus were also examined.; Four feeding trials were conducted using rainbow trout (Oncorhynchus mykiss) having initial average weights of 1.8 g-100 g/fish. Fish were fed diets containing varying concentrations of phosphorus (0.1-1.8 g phosphorus/kg diet) for 8-14 weeks. Ash, phosphorus, calcium, and magnesium concentrations were measured in whole body, skin (with scales), plasma, vertebrae, liver, muscle, and scales. Alkaline phosphatase activity was determined in plasma and in a crude intestinal homogenate. Individual vertebrae were subjected to compression testing.; Mineral concentrations in skin, whole body, and vertebrae were significantly (P {dollar}<{dollar} 0.005) positively correlated with dietary phosphorus concentration. Of all tissues and metabolites measured, mineral concentrations in skin decreased most rapidly in response to suboptimal dietary phosphorus concentrations. Trout fed suboptimal phosphorus diets had significantly lower plasma phosphorus concentrations and alkaline phosphatase activity, and significantly higher whole body lipid than those fed adequate phosphorus diets. Vertebral size and strength, and cumulative weight gain were significantly positively correlated with dietary phosphorus concentration. Tissue indices of suboptimal phosphorus intake were significantly affected well before weight gain.; Dietary T3 enhanced whole body phosphorus and calcium retention, but resulted in abnormalities and mortality. Excess dietary vitamin D3 did not affect bioavailability of dietary phosphorus, but caused growth depression in phosphorus deficient fish. These studies show a relationship between fish size, skin phosphorus depletion, and growth, which could be used to determine the extent of phosphorus depletion tolerated by trout before growth is affected.