Title: Effect of protein levels on nutrient and energy digestibility in diet of Arctic charr (Salvelinus aplinus)

Alternative protein sources and inclusion levels need to be optimized in aquafeeds to make aquaculture production efficient and cost-effective. As feed costs contribute the most to operational costs, the nutrient input and utilization need to be balanced more, especially proteins, because it contributes the highest cost in aquafeeds. This study was done to estimate nutrient digestibility in aquafeeds, prepared at various protein inclusion levels, on Arctic charr (Salvelinus alpinus). The four experimental diets were prepared to be iso-energetic and contain 30%, 34%, 38% and 42% protein levels. The fish (647g) were fed in quadruplicates for 7 weeks under partial recirculation. Digestibility of protein, lipid, energy, organic matter, inorganic matter, phosphorus and zinc was estimated at the end of the feeding period following chemical and calorimetric methods against yttrium oxide as an inert marker. The protein ADC was significantly high (>89%) in all treatments and was positively correlated to protein levels in diet. Both lipid and energy ADC followed the same trend of being significantly high in higher protein diets. The overall ADC of organic matter was significantly high (>89%) and positively correlated to protein level in diet. The ADC of both summed and individual minerals were positively correlated to protein content in diet. Protein content in feeds for Arctic charr should be kept between 36% and 39% to achieve optimal nutrient and energy digestibility; however proteins can be reduced to 36% without hampering nutrient digestibility.