Growth and body composition of Atlantic salmon ( Salmo salar L.) parr and smolt fed diets varying in protein and lipid contents

Abstract

The aim of the study was to elucidate if dietary protein to energy ratio influenced the smoltification process in Atlantic salmon. Groups of juveniles first fed in February were reared under continuous light until 15 September when the day length was reduced to LD12:12 (short day). After 6 weeks of rearing on a short day, the fish were split into two photoperiod groups. One group remained on LD12:12 (SHORT), while the other group were brought back onto continuous light (WINTER). The two groups were reared on these two photoperiods for a further 10 weeks. During the total 16 weeks of the experiment (from the reduction in day length), the groups were fed one of three different diets (50% protein/31% lipid=LP, 54% protein/27% lipid=MP, 58% protein/24% lipid=HP, protein-to-lipid ratios are given as percent of the dietary dry-matter content). The WINTER photoperiod fish developed a skin silvering typical for smolts, a low condition factor ( K) and low plasma chloride values after a seawater challenge test, important characteristics associated with parr–smolt transformation. These changes were not seen in the fish reared under the SHORT photoperiod. The WINTER group fed high-protein (W-HP) diet gained some more weight and the SHORT group fed the high-protein diet had lower final weight compared to all other groups. In the December sampling, the SHORT groups and W-HP retained a significantly higher level of muscle protein, compared to smolts fed medium- and low-protein diets, indicating that high-protein inclusion level in the diet may be more important than high-lipid inclusion during the smoltification period. Smoltification did not influence whole-body or muscle lipid levels at the end of the experiment, but increased liver lipid levels. Increased plasma glucose and plasma TAG levels indicated an energy release to plasma in the smolt. A depletion of glycogen in the liver of smolts occurred simultaneously as muscle glycogen increased, which may indicate a rearrangement of this energy store. The variations in protein-to-energy ratio did not affect time of smolting or smolt quality, when comparing plasma chloride count and K. Increasing dietary lipid level increased K and HSI, but all smolt groups had significantly lower values than parr groups.