Effects of dietary protein and lipid levels and DP DE−1 ratio on growth, feed utilization and hepatosomatic index of juvenile haddock, Melanogrammus aeglefinus L.

Juvenile haddock, Melanogrammus aeglefinus L. (initial weight, 13.5 ± 0.1 g) were fed practical diets containing digestible protein to digestible energy (DP DE^?1) ratios of 25–30 g DP MJ DE^?1as‐fed using three protein levels (450, 500 and 550 g kg^?1) each at two lipid levels (110 and 160 g kg^?1) for 63 days. The results showed mean weight gain and feed conversion ratio were highest for diets containing 28.5 and 30.2 g DP MJ DE^?1. DP DE^?1 ratio had no significant effect on protein efficiency ratio except at the lowest level (24.7 g DP MJ DE^?1) indicating a protein sparing effect of higher lipid when dietary protein is below the requirement. Haddock appears to preferentially use protein as the prime source of DE. DP DE^?1 ratio had little effect on apparent digestibility (AD) of protein while AD of lipid was significantly affected. Significant differences in AD of energy and organic matter were found to be inversely related to the carbohydrate level of the diet. DP DE^?1 ratios of 28.5 g DP MJ DE^?1 or lower resulted in significantly higher hepatosomatic indexes. The highest whole‐body nitrogen gains and energy retention efficiencies were achieved at 28.5 and 30.2 g DP MJ DE^?1, whereas only slight differences in nitrogen retention efficiencies were observed. The highest levels of energy retained in the form of protein were achieved at 28.5 and 30.2 g DP MJ DE^?1. The diet that provided the best growth, feed utilization and digestibility with minimal HSI contained 546 g kg^?1 protein (513 g kg^?1 DP), 114 g kg^?1 lipid, 164 g kg^?1 carbohydrate, 17.0 MJ kg DE^?1 and a DP DE^?1 ratio of 30.2 g DP MJ DE^?1.