Optimization of dietary macronutrients for Atlantic salmon post-smolts using increasing ration levels

Summary

Three fishmeal-based diets differing in macronutrient levels (crude protein/crude lipid (%) ratios of 40/26, 49/26 and 60/18) were studied in a requirement by ration level (RRL) trial with Atlantic salmon post-smolts. The diets were fed in excess (N = 6, 3, 3, respectively) and feed intake was estimated. Triplicate groups of fish were also fed each diet at ration levels of 20, 40 and 75% of the full ration. The fish were fed for 53 days. In fish fed full rations, feed intake and growth rates were not significantly affected by dietary treatment, but feed efficiency was significantly higher in fish fed diets 49/26 or 60/18 compared with those fed diet 40/26. No significant effect of diet was found on utilization efficiency of digestible energy (DE) for energy gain above maintenance (pooled slope 0.82). DE maintenance was estimated at 38 kJ kg−0.8 d−1. Linear regressions between digestible protein (DP) intake and protein gain were found for diets 49/26 or 60/18, but a second-order polynomial relationship was found for diet 40/26. Using the two highest ration levels for diet 40/26 (near satiety), the efficiency of utilization of DP for protein gain (kDP) was 0.50. The kDP values for diets 49/26 and 60/18 were 0.76 and 0.65, respectively (P < 0.05). Using data from the two lowest ration levels for diet 40/26 and the complete regressions for the other diets, the maintenance requirements for DP for the fish fed diets 40/26, 49/26 and 60/18 were 561, 542 and 651 mg kg−0.7 d−1, respectively. The diminishing returns in protein growth as intake of diet 40/26 increased suggest that the dietary amino acid level was not high enough to elicit further protein gain. Diet 60/18 led to numerically higher growth rates, but diet 49/26 resulted in lower DP maintenance and higher efficiency of utilization of DP for growth above maintenance than diet 60/18. Thus, the results obtained with full-fed fish and those generated using the RRL technique, seem to indicate that diet 49/26 contains more optimal dietary nutrient and energy levels for post-smolts.