Fecal metagenomic and metabolomic analyses reveal non-invasive biomarkers of infection in ayu ().

Unlabelled: With the rapid growth of inland aquaculture worldwide, side effects such as the discharge of nutrients and antibiotics pose a threat to the global environments. A sustainable future for aquaculture requires an effective management system, including the early detection of disease through the monitoring of specific biomarkers in aquaculture tanks. To this end, we investigated whether fish feces in aquaculture tanks could be used for non-invasive health monitoring using ayu () infected with , which causes bacterial cold-water disease worldwide. Feces that were subsequently produced in the tanks were used for metagenomic and metabolomic analyses. The relative abundances of the genera (0.6% ± 1.0%, 0.1% ± 0.2%), (11.2% ± 10.0%, 6.2% ± 5.9%), and (0.5% ± 1.0%, 0.0% ± 0.0%) were significantly higher in the feces of the infection challenge test tanks than in those of the control tanks. The abundances of cortisol, glucose, and acetate in the feces of the infection challenge test tanks were 2.4, 2.4, and 1.3 times higher, respectively, than those of the control tanks. Metagenome analysis suggested that acetate was produced by microbes such as . The abundances of indicated microbes or metabolites increased after day 4 of infection at the earliest, and were thus considered possible biomarkers. Our results suggest that feces produced in aquaculture tanks can potentially be used for non-invasive and holistic monitoring of fish diseases in aquaculture systems.

Importance: The aquaculture industry is rapidly growing, yet sustainability remains a challenge. One crucial task is to reduce losses due to diseases. Monitoring fish health and detecting diseases early are key to establishing sustainable aquaculture. Using metagenomic and metabolomic analyses, we found that feces of ayu infected with contain various specific biomarkers that increased 4 days post-challenge, at the earliest. Our findings are the first step in establishing a novel, non-invasive, and holistic monitoring method for fish diseases in aquaculture systems, especially in ayu, which is an important freshwater fish species in Asia, promoting a sustainable future.