Globally, aquaculture is the fastest-growing animal food sector and today almost half of the World´s consumption of all fish products originates from aquaculture. However, high water consumption and nutrient release from fish farms cause environmental and public concern. This has led to development of fish breeding in systems with water recirculation, named RAS. RAS technology reduces the water consumption, but reuse of the water causes problems with respect to maintaining a high water quality, even when various water treatment approaches are used. In the RASOPTA project, new technologies for improvement of the water quality by reducing nutrients, controlling abundance of harmful bacteria and parasites, will be developed. To ensure a high welfare of the fish, stress level of the fish will be monitored, and for biosecurity reasons, potential spreading of pathogenic virus and bacteria from RAS farms will be examined. A major concern in RAS is microbial production of earthy off-flavours that accumulate in fish and reduce their consumer attractiveness. Occurrence of these microbes will be analysed to develop mitigation procedures. To ensure marketing of tasty fish with RAS technology, sensory and instrumental methods will be applied to determine ideal rearing conditions for optimum taste. For prediction of water quality and health issues in RAS, a DNA-based chip will be developed to allow an early warning of emerging problems in operating RAS. These research goals will be accomplished through interdisciplinary and university-industry driven research training by 8 universities, 3 research institutes and 8 companies. By combining new approaches within microbial ecology, molecular biology, bioinformatics and analytical technologies, the project will deliver solutions to remove barriers that restrict expansion of the innovative and highly efficient RAS fish production.