Atlantic Halibut

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 Atlantic halibut can potentially increase the risk of disease outbreaks due to the number of individual animals living in close proximity to each other. Such outbreaks can have major economic impacts on the farm and it is in the best interest of the farmer that good husbandry and a pro-active approach to health management is adopted in order to reduce disease risks.

A range of bacterial, viral and parasitic pathogens can affect Atlantic halibut, the most important of which include Vibriosis, Infective Pancreatic Necrosis, Viral Nervous Necrosis, parasites such as Trichodina and Entobdela species, as well as fat cell necrosis from the effects of direct sunlight¹, ²:

In general, halibut farmers report that disease problems are most likely to occur at the hatchery, during the very early life stages, particularly the long and fragile yolk-sac larval stage, and are often associated with significant losses due to bacterial and viral pathogens³. Once the animals pass this critical stage and become juveniles, the potential for disease problems is reduced.

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 spread⁴. 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 practices⁵. The development of a written health plan updated annually and approved by an aquatic animal health specialist is recommended and often part of EU regulations⁶, certification requirements, and other initiatives e.g. The Code of Good Practice for Scottish Finfish Aquaculture⁷. 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. Maintenance of good daily records of mortalities facilitates future management by highlighting when in the production cycle disease problems are likely to occur.

Hatchery operators should pay attention to controlling adverse microbial floras in systems for yolk-sac and first feeding halibut larvae. Mitigations include microbial managed water systems and addition of probiotics or fine clay suspensions to the rearing tanks⁸. More work is needed in this area.

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 pathogens⁹.

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 RAS⁹.

When needed, there are a range of medicines and chemical treatments available to control Atlantic halibut 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 them⁴. 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 environment⁶.

Within the UK aquaculture industry, supply and control of use of medicines is regulated by the UK’s Veterinary Medicines Directorate which maintains an up-to-date list of authorised products that can be used in food fish species. As most of the products that are presently marketed only have authorisations for a major food species (e.g. Atlantic salmon), such treatments may need to be prescribed under the veterinary cascade¹⁰. Similar controls exist in Norway.

Atlantic halibut 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 used¹¹:

• Antibiotics critical for human medicine, as categorized by the World Health Organisation¹²
• 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 Atlantic halibut 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.