Primary responses of rainbow trout to dietary phosphorus concentrations

A series of experiments was conducted to research sensitive indicators for phosphorus status of fish using post-juvenile rainbow trout. Fish were fed up to 24 days with semipurified diets that varied in phosphorus content. Concentrations of glucose-6-phosphate, ATP, creatine phosphate, glucose, total lipids and total cholesterol in blood or skeletal muscle were relatively unchanged by the 24 days of dietary phosphorus restriction. Inorganic phosphorus and ATP levels in the blood, however, correlated significantly and positively. Inorganic phosphorus levels in plasma and urine were significantly lower in fish fed phosphorus-deficient diets than phosphorus-supplemented diets. Urinary phosphorus excretion peaked 6-10 h after feeding fish with diets containing potassium phosphate. Fish receiving either commercial feeds or experimental diets containing phosphorus as fish bone excreted trace amounts of phosphorus in the urine. Faecal content of phosphorus significantly increased when the diet containing potassium phosphate was supplemented with calcium carbonate. Urinary phosphorus concentration was found to be a rapid and sensitive indicator for dietary intake of phosphorus and probably phosphorus status of the fish, and had an advantage over conventional response variables in estimating dietary phosphorus requirement especially with large commercial-size fish.