Effects of dietary protein and lipid levels on growth, protein digestibility, feed utilization and body composition of Atlantic cod ( Gadus morhua)

A study was conducted to obtain more information about optimal macronutrient ratios in diets for Atlantic cod ( Gadus morhua). Fourteen extruded diets based on fish meal, fish oil and maize starch were fed to cod (initial weight 77 g) in 150-L tanks ( n = 2–4) in an incomplete factorial design. The diets contained approximately 49, 54, 58 or 63% crude protein and 11, 16, 20, 23 or 28% crude lipid (6–23% starch). The growth trial was stopped during the third week because of a parasite attack, but the fish continued to receive their respective experimental diets. Seven weeks after the original start, the growth trial was re-started. Over the following 8 weeks, the fish grew from a mean weight of 121 g to final weights ranging from 196 to 261 g, but there was no significant effect of dietary protein or lipid levels on growth rate. The feed efficiency ratio (FER) was negatively correlated with the dietary starch concentration and positively correlated with the dietary lipid concentration. FER was significantly higher for cod fed the diets containing 20% or more lipid. Protein digestibility was slightly higher in the 63% protein diets than in those containing 58% protein and higher in the diets containing 23 or 28% lipid than in those containing 11% lipid ( P < 0.05). The hepatosomatic index (HSI) increased during the total 15-week period from an initial level of 6.4% to a range of 7.4 to 13.2% and was significantly affected by an interaction between dietary protein and lipid concentrations. The dietary protein:energy ratio accounted for 69% of the variation in HSI. Nitrogen retention was significantly affected by the interaction between dietary protein and lipid levels. To remove the effect of the liver growth in the cod, the growth rate of the carcass was calculated. In the cod fed the diets containing 58% protein, carcass growth rate was greater than in those fed the 49% protein diets. At a dietary lipid level of 20%, which was optimal for FER, a comparison of the diets containing 49 or 58% protein showed no significant differences in feed intake, whole body or carcass growth rates, protein digestibility or HSI. The 49% protein diet led to greater nitrogen retention in the cod and is less expensive than the 58% protein diet and, thus, is recommended as the most optimal diet for cod under 300 g.