Rainbow Trout

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 rainbow trout can potentially increase the chance 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 this potential.

Rainbow trout can be affected by a range of bacterial and viral pathogens and parasites, the most important of which can cause: Enteric redmouth disease (ERM), Rainbow trout fry syndrome, Furunculosis, Vibriosis, Bacterial kidney disease (BKD), Bacterial gill disease (GD), Infective Pancreatic Necrosis (IPN), Infective Haematopoietic Necrosis (IHN), Viral Haemorrhagic Septicaemia (VHS), Proliferative kidney disease (PDK), White spot, sea lice infestations, Whirling disease, Hexamitaisis Octomitis, Costiasis, Fluke, Trematodal parasite¹. The effects of disease can have major economic impacts on the farm and in the industry as a whole through mortality and reduced growth.

The first line of defence in disease and pathogen management is effective biosecurity and health plans to minimise disease and its spread^{2, 3}. Certification schemes require such plans, as do other initiatives, e.g. Quality Trout UK⁴, The Code of Good Practice for Scottish Finfish Aquaculture⁵.

When needed, there are a range of medicines and chemical treatments available to control rainbow trout 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. Antibiotic use has fallen greatly to low levels across many trout and salmon producing countries due to the availability of effective vaccines⁷. Vaccination plays an important role in large-scale commercial fish farming and the trout industry has adopted mass vaccination to control several diseases. Fish are routinely vaccinated, both in hatcheries to control freshwater diseases, and before transfer to sea. However, this might not be the case in other countries where antibiotics are relied upon to combat significant bacteriological diseases^{8, 9}.

Through 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.

Medicinal treatment (vaccines, antibiotics and chemical treatment) in most trout farming countries is tightly regulated and normally administered by the farm veterinarian. Across Europe, only approved medicines can be used that are authorised for the disease being treated. For example, within the UK industry, supply and use of medicines is regulated by the UK’s Veterinary Medicines Directorate which maintains a list of authorised products that can be used in salmonids. Similar controls exist in Norway, 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^{10, 11, 12}.

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 salmonid skin and can cause lesions, leading to further problems and secondary infections. Although wild salmonids in the marine environment are natural hosts of sea lice, the possible interaction and impacts on wild fish populations of sea lice from farmed fish is a concern^{13, 14, 15} and the management of sea lice is perhaps the most pressing issue facing the Atlantic salmon aquaculture industry¹⁶. Rainbow trout grown-out in the sea similarly become infected with sea-lice, although there are indications that they are more resistant and require fewer treatments than Atlantic salmon^{17, 18, 19}. Where rainbow trout are grown-out in marine net-pens, effective 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 movement of equipment and staff between premises, disinfect key equipment and introduction of fallowing periods between stocking net-pens with a new cohort of fish.

At national and regional levels, governments are increasingly implementing area management plans that regulate how the salmonid industry operates in particular zones alongside developing improved treatment programmes. The Scottish Government for instance, helps the industry to operate within disease management areas²⁰, whilst in Norway sea lice levels on a regional basis will dictate the industry’s growth and production capacity²¹.

Sea lice control represents a significant use of chemical treatments on marine rainbow trout farms¹⁹. There are concerns that these treatments may have environmental impacts if released into 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 this²². There is also concern that resistance to some sea lice chemical treatments is developing²³.

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^{24, 25, 26}. 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 fish and graze on the sea-lice^{27, 28, 29}. A combination of treatments, techniques and technologies will be needed by the marine salmonid 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 juvenile trout in RAS reduces the marine grow-out phase and the opportunity for disease and pathogen loads to develop could also be reduced²⁷.