Nutritive value of novel feed protein sources to rainbow trout (O. mykiss) and European sea bass (D. labrax).

The dried biomass of certain microalgae and the meals obtained from arthropods like insects are deserving a growing interest as potential sources of protein in aquafeeds but very little information still exists on their actual nutritive value to fish. The present study investigated the nutrient composition and the in vivo nutrient digestibility in rainbow trout (O. mykiss) and sea bass (D. labrax), of a panel of three microalgae dry biomass (Arthrospira platensis, S, Tetraselmis suecica, Te, Tisochrysis lutea, Ti), two Hermetia illucens larval meals (HM1, 2) and of a meal obtained by processing whole Louisiana red swamp crayfish (RCM). The proximate, amino acid and fatty acid compositions of the test ingredients were determined with standard methods. Their nutrient and energy apparent digestibility coefficients (ADCs) were estimated with the indirect method and by difference relative to those of a reference diet. Six test diets were obtained by mixing each test ingredient and the reference diet at a 30:70 (w:w) ratio. All diets were added with acid insoluble ash (1500 mg/kg) and Yttrium oxide (100 mg/kg) as inert markers before being extrudedinto 3 mm pellets. The ADCs of macronutrients and energy of the reference and test diets were measured in juvenile rainbow trout (47 ± 1.5 g) and sea bass (37 ± 1.0 g) kept in tank-units each composed by three 65-L vessels stocked with 45 fish and fitted with a settling column for faecal collection. Each diet, offered to visual satiety in two daily meals, was evaluated over three independent 10-day faecal collection periods. Despite obvious differences in crude protein content (CP) among test ingredients (range 36–53% as fed), they could be considered valuable sources of feed protein although, apart from RCM, they were limiting in sulphur amino acids relative to fish requirements. The macronutrient and energy ADCs were unaffected by the marker and resulted systematically lower in sea bass relative to those of trout (p<.05). Extremely high CP and energy ADCs were estimated for RCM (>98%, p<.05) and the lowest in case of Te (<82% and <66% for CP and energy respectively, p<.05) with the remaining ingredients having intermediate values.The results of this study provide a first data set of the nutritive value of novel feed protein sources to two important fish species for the European aquaculture.