| Summary: | New sustainable feed ingredients are a necessity for the salmon aquaculture industry. In this study, we examined the effect of pre-extrusion processing of two microalgae, <i>Nannochloropsis</i> <i>oceanica</i> and <i>Tetraselmis</i> sp., on the growth, fatty acid content in the flesh and health of Atlantic salmon. The fish were fed one of the following five diets for nine weeks: (1) CO: a fish meal-based control (basal) diet, (2) NU: a <i>Nannochloropsis</i> diet, (3) NE: a pre-extruded <i>Nannochloropsis</i> diet, (4) TU: a <i>Tetraselmis</i> diet, and (5) TE: a pre-extruded <i>Tetraselmis</i> diet. The algae-incorporated diets contained 30% of the respective microalgae. Our results showed that the best growth performance was achieved by the CO diet, followed by the NE diets. Feeding of unprocessed <i>Nannochloropsis</i> and <i>Tetraselmis</i> resulted in a significant reduction in enterocyte vacuolization compared to the CO feeding. A significant effect of processing was noted in the fillet fatty acid content, the intestine and liver structure and the expression of selected genes in the liver. The expression of antioxidant genes in both the liver and intestine, and the accumulation of different fatty acids in the fillet and liver of the extruded algae-fed groups, warrants further investigation. In conclusion, based on the short-term study, 30% inclusion of the microalgae <i>Nannochloropsis oceanica</i> and <i>Tetraselmis</i> sp. can be considered in Atlantic salmon feeds.
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