DHA-Rich Algae Feed Modulates Atlantic Salmon Health, Microbiota and Stress Response

Reducing the amount of fishmeal and fish oil in salmon feed has resulted in a constant decline in the health-promoting long-chain polyunsaturated fatty acids (LC-PUFAs) EPA and DHA available for the fish. As such, enriching feeds with essential fatty acids of nonfish origin becomes increasingly important. Here, we investigated whether diets supplemented with the DHA-rich microalgae Schizochytrium limacinum can improve production performance and health of Atlantic salmon reared in a recirculating aquaculture system (RAS). Atlantic salmon post-smolts (~126 g) held in triplicate tanks were fed diets enriched with S. limacinum at low (2%; SL2) or high (14%; SL14) inclusion or a control diet (CD). Following 8 weeks of feeding, the fish were exposed to peracetic acid (PAA), a commonly used disinfectant for RAS. Sampling of different health indicators alongside profiling the intestinal bacterial communities associated with the digesta and mucosa was used to evaluate the health-promoting effects of the functional diets. Including Schizochytrium in the diet improved feed conversion, growth, and protein retention in Atlantic salmon, most notably in fish receiving the SL14 diet. Fatty acids in whole-body and muscle samples reflected dietary levels, where DHA levels were particularly increased. 16S rRNA amplicon sequencing revealed a significant effect of the diet on beta-diversity in digesta but not in mucosa. Relative abundance of specific genera was not affected by Schizochytrium inclusion, and most bacteria belonged to either Firmicutes, Proteobacteria, or Actinobacteria. Treating the fish with PAA induced an acute stress response, but the increase in plasma glucose and ion levels was diet dependent. Overall, our results highlight that including Schizochytrium in the diet can improve growth performance and modulate the response of Atlantic salmon to acute oxidative stress.