RASOPTA has recruited and trained 12 doctoral students. WP1 on water quality involved 3 ESRs, WP2 on off-flavour 4 ESRs, and WP3 on fish health and welfare 5 ESRs.
WP1 advanced understanding of biofilters in recirculating aquaculture systems (RAS) and their role in microbial ecology, water quality, and system stability. Using 16S rRNA and metagenomic sequencing, ammonia-oxidizing prokaryotes in commercial biofilters were quantified. In addition to conventional ammonia-oxidizing bacteria, Nitrosopumilus and comammox Nitrospira represented ~50% of ammonia oxidizers. Biofilters acted as barriers against microbial invasions, while partial disinfection reduced protection. A large-scale survey across Norway and the Faroe Islands confirmed low pathogen levels and provided the first detailed characterization of bacterial, archaeal, and fungal communities in RAS.
Experimental biofilter swaps showed that biofilters directly shape microbial composition. Analyses demonstrated lower bacterial growth in RAS than in flow-through systems. Biofilters stabilize microbial communities, reduce carrying capacity after disturbances, and enhance bacterial competition, supporting fish health and system resilience.
Particle management studies evaluated dietary additives. Guar Gum and black soldier fly larvae had no negative effects on juvenile rainbow trout. Inclusion of 3% cork in feed improved particle removal via faecal flotation, enabling efficient surface skimming and enhancing drum filter performance, while reducing nutrient leaching. WP1 also provided the first comprehensive analysis of particle size distributions in large-scale RAS, guiding improved feeding strategies.
WP2 addressed off-flavour generation and removal through four coordinated sub-projects combining microbiology, chemistry, and aquaculture. Geosmin-producing myxobacteria were isolated, and key environmental and genetic factors controlling production identified, enabling targeted mitigation. Odorant uptake and removal under various depuration conditions, and the effects of feed, RAS compartments, and fish species, were investigated. Several previously unreported odorous compounds in RAS were identified.
A novel setup was developed to study odor impressions of compound mixtures, complementing instrumental profiling and allowing faster, more accurate evaluation. These results support improved off-flavour management via control of geosmin-producing bacteria, optimized depuration, and tailored feeding, while expanding molecular-level understanding of odours in aquaculture products.
WP3 focused on fish health and welfare, developing a Fluidigm® chip to detect pathogens, water quality, and off-flavours using water samples. Controlled trials showed transport, hypoxia, overcrowding, and exposure to nanoplastics or pharmaceuticals significantly elevated stress, measurable non-lethally via blood biomarkers. Nanoplastic exposure also led to tissue accumulation, indicating bioaccumulation risk.
Pathogen surveillance in water and air revealed airborne transmission as an underestimated infection route. Molecular monitoring showed eDNA/eRNA concentrations correlated with disease occurrence and mortality, allowing predictive thresholds to be established. Ectoparasite surveys identified diverse pathogenic and commensal species, including Thaparocleidus vistulensis in European catfish. A molecular assay was developed with purified reference DNA for qPCR testing. The Fluidigm® chip was finalized, with assays validated in eDNA and eRNA formats and tested on samples from Denmark, Hungary, and the Faroe Islands. Comparative analyses across water, air, and tissue generated large datasets supporting studies on co-infections and distinguishing eDNA (dead + live organisms) from eRNA (live only).
