Water disinfection strategies to improve Atlantic salmon parr production in freshwater recirculating aquaculture systems

The 2020-2022 was marked by a high inclusion of students in the project, with 8 MSc students, 1 PhD student and 1 pos-hoc conducting their full or partially thesis within RASHealth activities. In 2022 alone, the project results and their application were disseminated through 9 oral talks. Also, the main results were shared in media news articles and partners (e.g., landbasedaq.no, NCE Aquatech CLUSTER, thefishsite.com, Nofima.no). Finally, five peer-reviewed scientific articles with the project results were open-access published in 2022. The project activities, results, and their potential application from project start-up to the reporting date are briefly summarized below per project objective: 1. To evaluate the impact of two water disinfection strategies on biofilter performance, biofilm and water from FW-RAS. In September 2022, a commercial farm in Denmark that was applying peracetic acid (PAA) to control a pathogen was visited and sampled. Preliminary results show that the tanks that received PAA addition had a higher microorganism’s diversity. Moreover, 16s metagenomics analysis identified genus and species related to both nitrification and denitrification, bacteria involved in PAA degradation and bacteria with a high variety of ecological niches. More results in 2023. An ongoing trial at Tromsø Aquaculture Research Station is evaluating the effect of PAA and ozone (O3) on biofilter performance, biofilm and water microbiome. Results in 2023. 2. To develop and optimize an assay to quantify disinfection efficacy on biofilm activity The scientific article “Estimation of nitrifying and heterotrophic bacterial activity in biofilm formed on RAS biofilter carriers by respirometry” published in Aquaculture reported a new method to quantify biofilm activity in RAS biofilters. This method can be applied in-situ to assess the effect of disinfection on biofilm activity. A second scientific article “A novel method to estimate biofilm activity based on enzymatic oxygen release from hydrogen peroxide decomposition” is in preparation. Here it was developed a new method to assess the activity of biofilm formed on biocarriers from a RAS biofilter using hydrogen peroxide. This method is expected to be practical, simple, fast and cheap to estimate biofilm viability and thus to quantify biofilter performance in commercial RAS farms. 3. To assess the effect of PAA water disinfection on Atlantic salmon parr health, welfare and growth The scientific article “Acute dose-response exposure of a peracetic acid-based disinfectant to Atlantic salmon parr reared in recirculating aquaculture systems” published in Aquaculture reported that survival, swimming behaviour and mucosal health were not affected below 1.6 mg PAA/L and that acute mortality, damaged skin and gill necrosis were observed in > 3.2 mg PAA/L. The scientific article “Mode of Application of Peracetic Acid-Based Disinfectants has a Minimal Influence on the Antioxidant Defences and Mucosal Structures of Atlantic Salmon Parr” published in Frontiers in Physiology reported that the mode of application of PAA, continuous or pulses, did not impose a strong influence and it reinforced that minimal effects of PAA application on the indicators of health and welfare underscore the potential use of PAA as a routine disinfectant in RAS. The scientific article “Determination of the Oxidative Stress Biomarkers of 8-Hydroxydeoxyguanosine and Dityrosine in the Gills, Skin, Dorsal Fin, and Liver Tissue of Atlantic Salmon Parr” published in Toxics reported a new chromatography method for determination of two oxidative stress biomarkers, 8-hydroxydeoxyguanosine (8OHDG) and dityrosine (DIY), in the gills, skin, dorsal fin, and liver tissue of Atlantic salmon parr. This method is relevant to assess effects of disinfection agents (e.g., PAA and O3) on fish welfare. 4. To evaluate the infection contribution of two pathogen sources in RAS and the subsequent fish infection dynamics A scientific article titled “Mucosal immunity of Atlantic salmon parr and microbial diversity of a freshwater RAS following a simulated Yersinia ruckeri biosecurity breach” is in preparation. In this study it was found that the disease develops faster in RAS than bath challenge in flow-through systems. Microbiota sequencing showed changes in some of the key bacterial families in the water inlet wall, tank wall, and biofilter, though the magnitude of change was not that substantial. The pathogen was detected in the water inlet wall, tank wall and biofilter. It is expected to be available in 2023. 5. To evaluate the efficiency of two disinfection strategies in controlling pathogen outbreaks and determine how routine disinfection impact fish health and biofilter microbiota A benchmark study on PAA and O3 administration to control a Y. ruckeri outbreak in Atlantic salmon parr FW-RAS and fish welfare indicators assessment is ongoing at Tromsø Aquaculture Research Station. Results in 2023.

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Traditional Atlantic salmon production is shifting thousands of tonnes of fish production to recirculating aquaculture systems (RAS). Outbreaks of pathogens had led to severe losses in Norwegian RAS production and a recent survey revealed an uncertainty about the current disinfection protocols in terms of safety and efficacy. Therefore, it is necessary to gather more knowledge on how and where to apply water disinfectants in Atlantic salmon parr production in RAS, without compromising fish health, welfare and growth, biofilter bacteria or water quality. RASHealth project consists of six (6) work packages, four (4) research trials and fourteen (14) research tasks that are conducted by three (3) partners (Nofima, NTNU and DTU) in Norway and Denmark. The RASHealth’s ambition is: i) to develop new water treatment practices for Atlantic salmon parr in freshwater RAS using ozone or peracetic acid (PAA), ii) to improve RAS biosecurity by adopting disinfection strategies considering the pathogen vectors and reservoirs, iii) to document PAA’s effect on Atlantic salmon parr health, welfare and growth and, to evaluate two administration methods, iv) to develop and test a new assay to quantify biofilm activity during routine activities and v) to monitor the production of disinfection by-products and, their accumulation in the water. The project results and outputs will be disseminated by: i) conferences and workshop presentations, ii) teaching and lecturing, iii) on-site RAS facility workshops, iv) association with ongoing industry-based projects, v) scientific manuscripts in journals and vi) social and specialized media articles and newsletters. It is anticipated that the RASHealth will allow a standardized implementation of water disinfection strategies in RAS with substantial impact on the future planning, design, management practice and performance of RAS.