EFFECT OF SIZE AGE AND FEED COMPOSITION ON BODY-COMPOSITION AND PHOSPHORUS-CONTENT OF RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

In fish the environmental load of nitrogen (N) and phosphorus (P) may be calculated indirectly by a mass balance, subtracting the content of N and P accumulated relative to the amounts fed. The present study ex amined the effect of size/age and feed composition on body composition and P content in rainbow trout A 2x2 factorial design was used with t wo size/age groups (avg. wt 282g [1 yr]; 980g [2 yr]) of sib fish, fed an experimental high energy diet or a commercially available diet wit h protein:fat ratios of 40:25 and 46:25 respectively. Fish were fed 3x /day to avoid feed loss using isocaloric and restricted feeding (1.0%/ body weight [1 yr] and 0.7%/body weight [2 yr] at start) using a comme rcial feeding table. Results indicated that both size/age and feed com position had highly significant effects (p < 0.001) on body compositio n. Older fish had higher fat and protein content than 1 yr olds, while a protein rich diet (protein:fat, 46:25) gave lower fat and higher mo isture content than did a high fat diet (protein:fat 40:25). Body comp osition also correlated with the K-factor (p < 0.001) showing that sle nder fish contained less fat. P content increased with K-factor. Feed composition had a significant effect (p < 0.05) on the P content with fish fed the high fat diet containing the highest amount, with a diffe rence of 0.06 g/kg.