Muscle pigmentation in rainbow trout fed novel protein sources from microalgae and crustaceans: the image analysis approach

The pink-red pigmentation of rainbow trout (Oncorhynchus mykiss) fillet determines consumer choice and economic value of the products. The main responsible for salmonid muscle pigmentation is astaxanthin, that is provided with diet, primarily as synthetic astaxanthin. However, the high costs of synthetic pigments and the consumers’ concerns about their use in animal feeds are leading the research for natural carotenoids. The pattern of fillet pigmentation was assessed in rainbow trout (Oncorhynchus mykiss, n = 63, mean weight 260.5 g) fed for 15 weeks with six (in triplicate) iso-proteic (42%) and iso-lipidic (24%) pelleted diets deprived of fish meal where 10% of vegetable protein blend was replaced by microalgae dried biomass (Arthrospira platensis, AP, Tetraselmis suecica, TS, a mix of Tisochrysis lutea and T. suecica, MA) or red swamp crayfish (Procambarus clarkia, RC) meal. A commercial diet (CO, 3 replicates) was used in the trial. The feeding trial was carried out at the Edmund Mach Foundation (San Michele all’Adige, IT). All procedures involving fish manipulation were carried out in accordance with the EU legal framework relating to the protection of animals used for scientific purposes (Directive 2010/63/EU). They were approved by the Animal Welfare Committee of the Istituto Zooprofilattico Sperimentale delle Venezie and authorized by the Italian Ministry of Health (permission n. 530/2018-PR). An automatic, repeatable, free, 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 (L*, a*, b* indexes) and used to validate image analysis results. Fillet carotenoid content was determined, and the pattern of variation was compared to that obtained by image analysis. Image analysis results and colorimetric data consistently indicated that, even if characterized by a significantly higher fillet carotenoid content respect to vegetable control (0.85–2.13 mg kg−1 vs. 0.17 mg kg−1), fillets of trout fed microalgae-based diets displayed a yellowish colour instead of the desired pink-red one. P. clarkii meal was instead confirmed as a promising and sustainable integration in vegetable aquafeed for carnivorous fish, in view of its carotenoid content (0.49 ± 0.07 mg kg−1) and coloring capacity. Further studies are required to evaluate the potential of this ingredient as a natural source of carotenoids, both testing the effects on flesh pigmentation of higher percentages of inclusion and longer administration times, or at different developmental stages.