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 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.