Pathogen dynamics and disinfection in Atlantic salmon recirculating aquaculture system

Background

Rearing fish in RAS, which has little contact with the surrounding environment, is ideal because fish can be kept better protected from diseases and fluctuating environmental conditions, as one can maintain an enhanced degree of biosecurity compared to that of ambient systems by preventing fish pathogens from entering the system.

However, there are still some potential transmission sources, including the intake of water and infected fish stocks, among others.

Hence, a set of biosecurity measures guarantee the prevention of entry, establishment, infection, and transmission of disease-causing agents in a production facility.

Several salmonid pathogens have been identified to pose a severe problem in RAS-based farming. These pathogens compromise the health and welfare of fish and, in worst-case scenarios, cause mass mortalities that carry significant financial loss to the producers.

The low water exchange in RAS provides a unique environment that generally contains greater concentrations of waterborne metabolites, nutrients, organic materials, and dissolved metals. The levels and dynamics of these compounds in the system can potentially trigger the proliferation of opportunistic pathogens or, when there is a pathogen breach, provide an environment that supports the growth and survival of unwanted microorganisms.

One of the key issues that remained to be explored is the critical hotspots for pathogen establishment and proliferation in RAS.

Disinfection kills, inactivates and/or eliminates microorganisms and is regarded as an important component that safeguards biosecurity in RAS.

The strategy includes three main aspects:

• i) disinfection to ensure that pathogens are prevented from entering the system (i.e., disinfection of intake water and ancillary materials);
• ii) disinfection to ensure that the rearing environment does not allow pathogens and opportunistic microorganisms to proliferate and cause a disease outbreak (i.e., disinfection of RAS re-used water);
• iii) disinfection to ensure that pathogens are not present in the system for the next production cycle and disease is not transmitted between facilities (i.e., all system disinfection).

Several chemical disinfectants are available on the market, and their active components may include hydrogen peroxide, peracetic acid, chlorine, or glutaraldehyde.

The effectiveness of a given disinfectant depends on the concentration of disinfectant, contact time, half-life, temperature, organic particulate concentration, specific microorganisms and if they are present in a biofilm or not (pathogens which are present in biofilm would be more difficult to kill).

The use of disinfectants in Norwegian aquaculture is regulated by the Norwegian Food Safety Authority (Mattilsynet), where approval of the trade product is sought from either the Norwegian Environment Agency 

Goal

The overarching aim of PathoRAS is to ensure top-notch biosecurity measures in Atlantic salmon RAS through a better understanding of how pathogens proliferate, establish, and survive in a complex ecosystem of the RAS environment. 

Subgoals

Specifically, PathoRAS will:

1. Simulate a pathogen biosecurity breach in RAS and determine the environmental factors affecting pathogen establishment in the different parts of the system
2. Determine the duration of pathogen survival in the system and the effectiveness of system disinfection following a disease outbreak
3. Determine how the pathogen breach affects fish health, system performance and microbial communities in RAS
4. Investigate the biofilm formation in vitro and in situ of different aquaculture-relevant pathogens on RAS surfaces and how these pathogens are susceptible to disinfectants
5. Collate historical and empirical data to draft a recommended code of practice for RAS biosecurity 

What we do

The expected project outputs are multifaceted and relevant, both nationally and internationally.

The results are beneficial to the different sectors of aquaculture, specifically salmon farming, including researchers and academics, fish health service providers, farmers and technology suppliers, and national and international regulatory and legislative agencies (e.g., Mattilsynet, Miljødirektoratet, Statens legemiddelverk).

PathoRAS will identify the areas in RAS where the pathogens thrive best and survive. We know that despite the different biosecurity measures, there are still instances that pathogens can enter the system through different vectors, including the fish, the intake water, or the equipment used.

Therefore, answering the questions on how and where they thrive and survive will provide crucial information in developing disinfection strategies that take into account critical points requiring thorough considerations.

This information can also be used by technology providers in building new RAS farms, especially by considering these pathogen hotspots when accounting for disinfection technology, hydrodynamics, water treatment, and system flow, among a few others.

PathoRAS will exploit experiential knowledge and generate empirical evidence. We believe that a good approach to address questions on pathogen dynamics and disinfection is by combining our understanding of what happened in the past and exploring these questions through empirical research.

This information will allow industry partners to revisit their routines, reassess their risk management plans and eventually re-design the approaches based on the recommendations from PathoRAS.

By identifying some factors contributing to disease outbreaks, we will promote more enhanced proactive and site-specific measures.

PathoRAS will drive the establishment of empirically backed disinfection measures in RAS. Current disinfection routines are based on direct adoption from the food industry and many of the aquaculture-approved disinfectants have not been experimentally tested against key pathogens in a setting close to a normal production scenario.

The PathoRAS project will address this knowledge gap by tank-based and bench-scale trials using the disinfectants currently approved for aquaculture use in Norway.

This will equip the industry with knowledge on how to perform pathogen-specific and/or routine general disinfection.

PathoRAS will create a recommended code of practice on disinfection in RAS. We aim to provide the industry with practical knowledge on disinfection in the best way possible. The recommended code of practice that will be developed at the end of the project will serve as a white paper that can be applied by the industry at their premises.

This will also serve as guides for the authorities for the legislation of approved routines in RAS, especially since such technology is becoming more and more common in Norway. 

Publications

Towards sustainable water disinfection with peracetic acid in aquaculture: A review

Reviews in Aquaculture, 2024 Academic literature review

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