Early life stress causes persistent impacts on the microbiome of Atlantic salmon

Intensively farmed fish are commonly stressed, often leading to immune impairment and increased susceptibility to disease. Microbial communities associated with the gut and skin are vital to host immune function, but little is known about how stress affects the fish microbiome, especially during the sensitive early life stages. We compared the effects of two aquaculture-relevant stressors on the gut and skin microbiome of Atlantic salmon fry: an acute cold stress during late embryogenesis, and a chronic environmental stress during the larval stage. Acute cold stress had a lasting effect on the structure of both the gut and the skin microbiome, likely due to disruption of the egg shell microbial communities which seed the initial colonisation of the teleost microbiome upon hatching. In contrast, chronic post hatch stress altered the structure of the gut microbiome, but not that of the skin. Both types of stressors promoted similar Gammaproteobacteria ASVs, particularly within the genera Acinetobacter and Aeromonas which include several important fish pathogens and, in the gut, reduced the abundance of Lactobacillales. This suggests that there may be common signatures of stress in the salmon microbiome, which may represent useful stress biomarkers in aquaculture.