| Summary: | Today, commercial diets for large farmed Atlantic salmon (Salmo salar L.) commonly contain 30-35 % protein and 35-39 % lipids, i.e. a ratio of protein-to-lipids below 1. Such energy dense diets have generally been shown to improve feed utilization and growth. However, reducing the dietary protein-to-lipid ratio may lead to increased deposition of fat in the muscle and visceral cavity. There is evidence that high levels of body lipids may reduce feed intake and growth in salmonids, and is often referred to as lipostatic regulation. Thus, there is a risk of lowered growth prior to and during periods with increased feed intake and high fat accumulation when feeding high-fat diets. The present thesis test the hypothesis that increased dietary protein-to-lipid ratio, and the possible involvement of lipostatic regulation on body fat levels can be utilized to significantly improve fitness-related traits including growth, survival and nutrient deposition of farmed Atlantic salmon. Paper I describes the effects of isoenergetic diets with different protein-to-lipid (P/L) ratio on growth, feed intake, feed conversion, biometrics, nutrient retention and deposition in S1 Atlantic salmon post-smolt. The study was conducted during the early seawater phase from April to September. Significantly lower muscle fat, whole body lipid, and energy level was observed in the post-smolt fed high compared to low P/L ratio in July, approximately three months after the trial started. Reduced level of muscle fat/body fat in July significantly improved feed intake, growth and weight gain compared to fish fed low P/L ratio from July to September. The high P/L ratio group had also a significantly lower feed conversion ratio based on gutted weight (FCRg). In line with this, the visceral somatic index (VSI) of the group fed the high dietary P/L ratio was relatively stable during the experiment, whereas the VSI of the group fed the low dietary P/L ratio increased gradually, resulting in significantly higher VSI at the end of the trial. The ...
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