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 examined the effect of size/age and feed composition on body composition and P content in rainbow trout. A 2×2 factorial design was used with two 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 with protein:fat ratios of 40:25 and 46:25 respectively. Fish were fed 3×/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 commercial feeding table. Results indicated that both size/age and feed composition had highly significant effects (p < 0.001) on body composition. 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 moisture content than did a high fat diet (protein:fat, 40:25). Body composition also correlated with the K-factor (p < 0.001) showing that slender 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 difference of 0.06 g/kg.

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