Muscle pigmentation in rainbow trout (Oncorhynchus mykiss) fed diets rich in natural carotenoids from microalgae and crustaceans

The amount of ingredients of marine origin used in salmonid feed industry is rapidly decreasing and fish meal and fish oil have been largely replaced by vegetable ingredients. New potential protein rich ingredients for aquafeed formulations should therefore be alternative to vegetable ones. The pattern of fillet pigmentation was thus assessed in rainbow trout (Oncorhynchus mykiss) fed for 15 weeks isoproteic (42%) and isolipidic (24%) diets deprived of fish meal where 10% of protein from a blend of vegetable ingredients was replaced by cyanobacteria and microalgae dried biomass (Arthrospira platensis, Tisochrysis lutea and Tetraselmis suecica) or Louisiana red swamp crayfish (Procambarus clarkii) meal. An automatic, repeatable, and objective image analysis tool for the integrated determination and direct visualization of fillet colour was applied to digital images. Colour of fillets was also measured with a chromameter, recording data as L*, a*, b* indexes and used to validate image analysis results. Fillet carotenoid and lipid contents were determined, and their pattern of variation was compared to that obtained by image analysis. Fillet colouring capacity of crayfish meal was also compared to that of commercial synthetic astaxanthin included in rainbow trout feed. To this last aim, test diets were administered for 12 additional weeks. The image analysis and colorimetric data consistently indicated that, even if characterized by a significantly higher carotenoid content respect to fish fed a vegetable diet, fillets of trout fed the cyanobacteria and microalgae including diets displayed an undesirable yellowish colour. P. clarkii meal instead resulted a promising functional ingredient to supplement plant protein-based diets for trout, in view of its lipid composition and astaxanthin content. In fact, despite a relatively low dietary carotenoid level due to low percentage of inclusion, a desirable pink pigmentation was clearly detectable in fillets of trout fed the crayfish meal-including diet, which resulted in colour differences which were less obvious relative to those of fish given the astaxanthin-supplemented feeds at the end of the additional 12 weeks.