Feed optimization in atlantic cod ( Gadus morhua): Fat versus protein content in the feed

Two feeding experiments were performed with cod. One involved 40-g fish fed for 26 days and the other 180-g fish fed for 60 days. Feeds based on squid mantle, capelin oil and dextrinized potato starch were made up to give from 75% to 27% of available protein energy, balanced with from 11% to 61% fat energy, all with 12–15% carbohydrate energy. In the second experiment, three groups of fish were fed each day, and three groups every third day, whereas in the first experiment all fish were fed five days a week. Relative growth, feed conversion, digestibilities and retentions of protein and fat were determined. Protein retentions were calculated as PER (live weight gain over protein intake) and PPV (protein gain over protein intake). The apparent digestibilities for protein and fat were above 90%, except for a diet with 61% available energy from fat. The protein-sparing effect of fat seemed to be low in cod, and 26% to 29% of protein was retained, as compared with 50% to 60% of fat, except for groups on a very low-fat diet or on reduced feeding frequency. Dietary lipid was deposited as liver fat, and a linear relation between the liver index and the fat intake was demonstrated. It was concluded that PPV is a more appropriate index for growth than PER in studies on a lean fish such as cod with a low-fat fillet and a fatty liver. A reduced feeding frequency had no effect on feed conversion and PER whereas the weight gain and PPV were reduced. Based on these experiments optimal growth for cod may be achieved with a feed giving available energy concentrations of 60%, 25% and 15% from protein, fat and carbohydrate, respectively, fed every day.