Dietary inclusion of the red seaweed Asparagopsis taxiformis boosts production, stimulates immune response and modulates gut microbiota in Atlantic salmon, Salmo salar

Salmonids represent US$23 billion of global aquaculture value, yet Atlantic salmon farms lose approximately 10% of their production to diseases and parasites every year. New approaches to minimise such losses are ur- gently needed because current treatments (e.g. antibiotics) have environmental and human health impacts with increasing sea temperatures predicted to further exacerbate the impacts of disease. Immunostimulants that boost fish resistance to disease without negative environmental or human health impacts are currently being assessed. Seaweeds and their extracts are used as immunostimulants for land animals and are increasingly being inves- tigated for use in finfish aquaculture, including for Atlantic salmon. Here we show that when the red seaweed Asparagopsis taxiformis and its extract were incorporated in Atlantic salmon feed, fish growth rates were enhanced up to 33%, feed intake was enhanced up to 13%, FCR were reduced, and innate immune responses were enhanced up to 58% compared to fish fed unsupplemented control diets over 4 weeks. Overall, fish fed the methanolic extract of A. taxiformis (at an inclusion of ~1% on a dry weight basis, D:D of feed) had the best combination of enhanced growth rate, feed intake and immune response. Fish fed the immunostimulant lipo- polysaccharide (LPS) derived from Escherichia coli had the highest innate immune response in our trial, however LPS had no enhanced effect on growth or feed intake. Additionally, we provide evidence that the seaweed and LPS supplements modulated the expression of immune and stress-related genes in both the liver and head kid- neys. More specifically, the fish fed the supplemented diets showed increased expression of the HSP70 gene in both their liver and head kidney after 2 weeks of treatment. At 4 weeks high HSP70 and lysozyme gene expression was observed in the fish fed the two seaweed methanolic extract diets. The seaweed diets also enhanced the diversity of bacterial communities within the hindgut of Atlantic salmon while the LPS treatment appeared to have the opposite effect. Whole A. taxiformis or its methanolic extracts could therefore be used as functional feed ingredients that boost the immune response and enhance the growth rate of Atlantic salmon without affecting feed efficiencies.