| Sumario: | The biomass of several microalgae species represents one of many single cell-derived products with great potential as dietary ingredients for the aquaculture feed industry. The present study compares the assimilation of dietary nitrogen and total dry matter supplied by fish meal and biomass from Spirulina (Arthrospira platensis) and Nannochloropsis oculata in postlarval Pacific white shrimp, Litopenaeus vannamei. The natural isotopic signatures of the ingredients were used as biomarkers to assess their respective nutritional contributions. Three control diets were manufactured with each of the main ingredients to establish shrimps’ isotopic control values (initial mean weight 51 mg). Four mixed diets were formulated with varying proportions of ingredients on a dietary nitrogen basis (33:33:33 and permutated 50:25:25 proportions). Diets were supplied ad libitum four times daily for 22 days. Shrimp reared under most dietary treatments showed high growth rates. Final mean weight was significantly higher (268 ± 64 mg) in shrimp grown under diets containing only fish meal and those formulated with 50% fish meal (239 ± 64 mg) and 50% Spirulina (234 ± 57 mg). Isotopic assessments of ingredients and shrimp indicated significantly different nutrient assimilations. A higher incorporation of dietary nitrogen (43 to 52%) and dietary carbon (44 to 57%) from Spirulina was observed, while N. oculata and fish meal supplied lower proportions of dietary nitrogen to shrimp growth (4 to 33% and 25 to 51%, respectively). Results indicate a fast digestion and assimilation of Spirulina-derived nutrients, while N. oculata did perform poorly as a fish meal replacement ingredient
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