Atlantic Salmon

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 salmon can potentially increase the risk of disease outbreaks due to the number of individual animals living in close proximity to each other. 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.

Atlantic salmon can be affected by a range of viral and bacterial pathogens, the most important of which cause: Pancreas Disease (PD), Salmonid Rickettsial Septicaemia (SRS), Infectious Pancreatic Necrosis (IPN), Heart and Skeletal Muscle Inflammation (HSMI), Infectious Salmon Anaemia (ISA), and Gill Disease (GD)¹. The effects of disease can have major economic impacts on the industry, for example the ISA outbreak in Chile².

The first line of defence in disease and pathogen management is effective biosecurity and health plans to minimise disease and its spread^{3, 4}. Certification schemes require such plans, as do other initiatives, e.g. The Code of Good Practice for Scottish Finfish Aquaculture⁵.

When needed, there are a range of medicines and chemical treatments available to control Atlantic salmon disease and pathogens, including antibiotics. Antibiotics are used strictly as therapeutants by the industry; they are not used as growth promoters. 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 them⁶. Generally only limited amounts of antibiotics are used rearing Atlantic salmon due to the availability and efficacy of vaccines⁷.

Across many Atlantic salmon producing countries antibiotic use has fallen significantly^{1, 7} as shown in the graph, whilst in some producer countries this has not been the case, e.g. in Chile^{8, 9, 10, 11, 12}. However, reductions in Chilean usage have recently been reported¹³.

Strict adherence to medicine withdrawal periods at farm level, and through pre and post-harvest testing regimes, it is possible to assure seafood buyers and consumers that any residual levels of antibiotics in the edible parts of the fish fall within legal tolerances acceptable in human food.

Vaccination plays a very important role in Atlantic salmon farming, and has been a key reason behind the success of salmon aquaculture. The industry has adopted mass vaccination to control a number of diseases found in freshwater and marine environments, and in many cases smolts are injected with vaccines that provide broad protection against a range of diseases^{14, 15}.

Medicinal treatment (vaccines, antibiotics and chemical treatments) used by Atlantic salmon producers is tightly regulated and normally administered by a veterinarian. Typically, only approved medicines that are authorised to treat a particular disease can be used. For example, in the UK industry, supply and control of medicines is regulated by the Veterinary Medicines Directorate¹⁶ which maintains an up-to date list of authorised products that can be used in salmon as well as other food species. Similar controls exist in Norway, the US, Canada and to a large extent Chile. Regulators ensure that treatments are effective and safe to the target animal and the environment; consumer safety is ensured by specifying withdrawal periods after treatment and before fish are harvested.

Functional aquafeeds include a range of additives used to improve growth and feed utilisation, but also to support the health and stress resistance. Additives, such as probiotics, prebiotics, phytogenics, and immune-stimulants may help improve disease resistance and reduce the intensity of sea lice infection^{17, 18, 19}.

Good husbandry and strict adherence to the principles of biosecurity are also an 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.

Sea Lice

Sea lice (a parasitic copepod) feed on salmon skin and can cause lesions, leading to further problems and secondary infections. Although wild Atlantic salmon are natural hosts of sea lice the possible interaction and impacts on wild fish populations of sea lice from farmed fish is a concern^{20, 21, 22} and the management of sea lice is perhaps the most pressing issue facing the Atlantic salmon aquaculture industry. Biosecurity plans to minimise sea lice are central to health management and are now widely implemented. These include processes to separate different stocks and year-classes of fish, control of the movement of equipment and staff between premises, disinfection of key equipment, and introduction of fallowing periods between stocking net-pen sites with new cohorts of fish.

At national and regional levels, governments are increasingly implementing area management plans that regulate how the salmon industry operates in particular zones alongside developing improved treatment programmes. The Scottish Government for instance, helps the industry to operate within disease management areas²³, whilst in Norway sea lice levels on a regional basis will dictate the industry’s growth and production capacity²⁴.

Treatments for sea lice control constitute the greatest use of any of the range of medicines and chemical treatments used to control Atlantic salmon disease and pathogens. There are concerns that these treatments may have environmental impacts if released in to the water column. In Scotland, Norway, and increasingly in other countries, the recently developed practice of treating fish within well-boats (vessels with wells or tanks for the storage or transport of live fish) prevents this²⁵. There is also concern that resistance to some sea lice chemical treatments is developing²⁶.

Administration of sea lice medicinal treatment is tightly regulated to minimise potential environmental impact and can normally only be done by a veterinarian. There are significant efforts to identify and use alternative non-medicinal delousing treatments: breeding programmes to increase resistance; better monitoring and management procedures; functional feeds; and engineering solutions such as hydrolicing, which uses low-pressure water jets to dislodge sea lice, and thermolicing, whereby fish are bathed in warmer water to separate the lice from the fish. There is an increasing interest in the use of biological delousing approaches; so-called cleaner fish (wrasse species and lumpsucker) that are introduced into the net-pens with the reared salmon and graze on the sea-lice^{27, 28}. A combination of treatments, techniques and technologies will be needed by the Atlantic salmon industry to manage and combat sea lice.

RAS Facilities

One of the advantages of RAS technology is the improved level of biosecurity provided, with the opportunity to reduce disease outbreaks and eliminate some diseases altogether. RAS can control environmental conditions leading to more stability and favours reduced disease outbreaks. 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 of and reluctance by the system manager to disinfect biological filters. Producing larger, more robust smolts in RAS reduces the marine grow-out phase and the opportunity for disease and pathogen loads to develop could also be reduced.