Turbot

In common with all other animal farming systems in which animals are raised in greater numbers than they would be found in nature, the farming of turbot can potentially increase the risk of disease outbreaks due to the number of individual animals living in close proximity to each other¹. It is essential that good husbandry and a pro-active approach to health management is adopted at each farm location in order to minimise and mitigate these risks.

The main pathogens potentially threatening farmed turbot are bacterial and parasitic. Among the most common bacterial diseases are Vibriosis, Flexibacteriosis, Furunculosis, Streptococcosis and Edwardsiellosis; the most common parasitic diseases are Trichodiniasis, Scuticociliatosis, Microsporidiosis and perhaps the disease of most concern is Enteromyxosis².

The first line of defence in disease and pathogen management is the continued maintenance of optimal health conditions (e.g. ensuring that maximum stocking densities are not exceeded at various stages of the production cycle), as is the development of effective biosecurity and health plans at individual farm and area level to minimise disease and its spread³. The key elements of biosecurity include: practical and appropriate legislative controls, adequate diagnostic and detection methods for infectious diseases, disinfection and pathogen eradication methods, reliable high-quality sources of stock; and best management practices⁴. The development of a written health plan updated annually and approved by an aquatic animal health specialist is recommended and often part of EU regulations⁵ and certification requirements. The farmer should follow the instructions of aquatic animal health specialists about who to inform and how to stop the spread of disease. Regular health checks and screening allows for rapid action to be taken if problems begin to develop. Some certification schemes set targets for maximum average real percentage mortality rates. Maintenance of good daily records of mortalities facilitates future management by highlighting when in the production cycle disease problems are likely to occur.

One of the advantages of RAS technology is the improved level of biosecurity it provides. RAS can control environmental conditions leading to higher system stability which can reduce the risk of a disease outbreak. Biosecurity in RAS needs to be extremely tight; introduced parasites or pathogens in RAS systems can be very hard to control due to the difficulty and reluctance to disinfect biological filters. Inferior RAS designs may inadvertently provide favourable conditions for disease outbreaks or the reproduction of opportunistic pathogens⁶.

In PAS there is less control over environmental conditions compared to RAS, and therefore potentially weaker biosecurity. PAS can be exposed to more variable conditions leading to more system instability which can increase the risk of a disease outbreak. Whilst implementing biosecurity in PAS might be more challenging than in RAS, it still provides for better control of pathogens than an open water net-pens, for instance. PAS physically separate stocks from wild fish (and disease they may carry) and by pumping inflow water and through sand filters reduces the risk of pathogen introduction. In addition, the lack of bio-filtration means that treating pathogens (once in the system) is less problematic than in RAS⁶.

When needed, there are a range of medicines and chemical treatments available to control turbot disease and pathogens, including antibiotics. Overuse of antibiotics in farming or for human medical treatment speeds up the development of antibiotic resistance, which is when bacteria change and become resistant to the antibiotics used to treat them⁷. In Europe, medicines and chemicals used during the farming of fish destined for human consumption are tightly regulated to minimise impacts to the target animal, consumer and environment⁵. In regions such as Asia-Pacific, whilst regulations have been created or tightened, the most important development has been the increasing uptake of Best Management Practices (BMPs), codes of conduct or practices, and certification schemes which help in tackling disease issues³.

Turbot farmers routinely vaccinate their stocks against some of the key diseases particularly vibriosis and Flexibacteriosis and industry research and development is prioritising vaccine development as well as genomics applied to breeding for better resistance to pathogens^{2, 8}. A nodavirus vaccine developed for sea bass is being adapted for other commercially important species, including turbot⁹.

Turbot farmers should only use veterinary medicines and chemicals that are approved by national authorities and these should be prescribed by an aquatic animal health specialist. Farmers should follow the instructions of the aquatic animal health specialist about storage, and the medicines or chemicals should be used as per directions. Stock should not be harvested before completion of the withdrawal period specified for the medicine. Records of medicine and chemical stocks and their usage should be kept. The following veterinary medicines should not be used¹⁰:

• Antibiotics critical for human medicine, as categorized by the World Health Organisation¹¹
• Veterinary medicines (excluding vaccines) used prophylactically prior to evidence of a specific disease problem
• Veterinary medicines (excluding vaccines) to serve as growth promoters

These prohibitions are frequently part of regulation and specified in turbot certification programmes.

Good husbandry and strict adherence to the principles of biosecurity are also important aspect of managing the movement of eggs and live fish between sites, including in some cases internationally, where there is an opportunity to spread pathogens between locations.

The rarity of escapes from land-based facilities (especially from RAS) and wastewater treatment by both RAS & PAS help to reduce/eliminate the release of diseases and pathogens into the environment³.