Modern salmon diets are low in finite marine resources and dominated by plant-based ingredients. This results in reduced availability of n-3 long-chain polyunsaturated fatty acids (LC-PUFA) such as EPA and DHA for the fish. To maintain health-promoting aspects of aquafeeds , enriching feeds with essential fatty acids of non-marine origin becomes increasingly important. Here, we investigated whether diets supplemented with the DHA- rich microalgae Schizochytrium can improve production performance and the response to a commonly used disinfectant that causes oxidative stress . Atlantic salmon post smolts (~126 g) held in triplicate tanks were fed diets enriched with Schizochytrium limacinuum at 2% (SL2) or 14% (SL14) inclusion or a control diet (CD). Following eight weeks of restricted feeding at 1% of their biomass, the fish were exposed to peracetic acid (Wofasteril classic, Kesla, Germany, at 2.5 µl/l), a commonly used disinfectant in RAS. Fish were sampled before exposure, 1 hour, and 18 hours after exposure, reflecting an acute response.
Including Schizochytrium in the diet improved feed conversion, growth, and protein retention in Atlantic salmon, most notably in fish receiving the SL 14 diet (Figure 1). A trend of increasing lipid content with SL inclusion was apparent in whole-body and muscle samples. Fatty acids in whole-b ody and muscle samples reflected dietary levels, with enrichment of DHA but decreasing concentrations of the pro-inflammatory C20:3n6 in fish fed SL diets . The diet significantly influenced bacterial ß-diversity in the digesta but not in the mucosa of the hindgut. Most abun dant taxa in the digesta were Floricoccus , Vibrio and Lactococcus and additionally Streptococcus in the mucosa. Overall exposure to PAA resulted in a classic stress response with an increase in plasma glucose and cortisol levels . However, glucose levels in fish fed SL14 did not increase following PAA exposure. Catalase protein levels in the liver increased following exposure to PAA by 2- fold except for fish fed SL2, while levels of SOD1 remained unchanged. Feeding SL2 reduced lipid peroxidation (MDA concentration) in the liver, while exposure to the stressor had no effect on lipid peroxidation . Overall, our results highlight that including Schizochytrium in the diet can improve growth performance and stress resilience of Atlantic salmon. This is an important step towards developing health promoting aquafeeds for Atlantic salmon.