174. Losses in salmon farming are often due to disease. These diseases are classed as parasitic, bacterial, viral or fungal. The high stocking densities of fish in aquaculture render them particularly susceptible to disease and parasites. Moreover, most waters are populated by wild fish which may carry and be susceptible to disease agents, and steps have to be taken to minimise the likelihood of disease outbreaks in the farmed stock.
175. There is a wide variety of diseases which can affect salmon. In fresh water, fungi and protozoan parasites are the greatest threat. Eggs are particularly susceptible to fungal infection so treatment with fungicide is necessary. We are told that malachite green is the agent which provides the most effective control. Similarly, ectoparasites can be controlled with formalin or formalin combined with malachite green (see paragraph 217). Bacterial diseases such as furunculosis, caused by Aeromonas salmonicida and enteric redmouth (ERM), caused by Yersinia ruckeri, are usually treatable with antibiotics. Bacterial kidney disease (BKD), caused by Renibacterium salmoninarum, is rare but at present there are no effective antibiotic treatments and the impact of the disease in individual cases may be serious. Infectious pancreatic necrosis (IPN) is a viral disease and untreatable at present. In sea water the three major problems of concern are (i) pancreas disease, (ii) sea lice and (iii) furunculosis all of which can cause very considerable welfare problems and economic loss. Other problems may include IPN, vibriosis and, in rare cases, BKD.
176. Furunculosis avoidance is a major factor affecting fish management. The introduction of new and efficacious injectable vaccines to prevent furunculosis has shown encouraging signs of success and vaccines have largely superseded antibiotic treatment which was becoming less effective due to the development of resistant strains of the causal agent A. salmonicida. Although oral administration of vaccines is still a relatively new technique, the development of oral vaccines could improve welfare by reducing handling. Vaccines which protect against several diseases are desirable because they further reduce handling.
177. Sea lice, which erode the skin causing tissue damage, have traditionally been controlled using dichlorvos and more recently hydrogen peroxide. However, the development of resistance by some sea lice has reduced the effectiveness of dichlorvos. Concern has also been expressed about possible effects of dichlorvos on the marine environment. Wrasse are small fish which clean parasites off large fish and they are also used to help control sea lice on farmed stock (see paragraphs 208-213). They may be eaten by large fish. The development of an effective vaccine against sea lice could significantly contribute to improving welfare.
178. Pancreas disease is caused by a virus and can cause loss of weight or emaciation. Affected fish may be more susceptible to other diseases and treatment of these diseases may be less effective. At present, control of disease relies upon husbandry practices and the impact can to some extent be alleviated by modifying feeding regimes. The development of an effective vaccine could significantly improve welfare.
179. Salmon farmers should try to minimise stress by avoiding handling and keeping stocking densities down (see paragraphs 121-123). Site selection is equally important and operators should keep year classes on separate sites to avoid cross infection. Leaving sites fallow, entering into management agreements with several operators in a given area, removing all fish before restocking, and exchanging disease information are useful methods of limiting disease. There are also environmental reasons for fallowing. Implementation of such changes has led to an improvement in fish survival and growth.
180. British legislation requires certain diseases to be reported to the responsible authority (MAFF or SOAEFD). There are eight such notifiable diseases and several categories of seriousness are recognised by the European Union. List I diseases are exotic to the EU: the viral disease infectious salmon anaemia (ISA) falls into this category. List II diseases have a limited distribution in the EU. There are two list II diseases at present, infectious haematopoietic necrosis (IHN) and viral haemorrhagic septicaemia (VHS). IHN has never been found in Great Britain and VHS has been found only once, in farmed turbot, in 1994. List III diseases are considered to be less serious than list I or list II diseases or are treatable and endemic in the EU. Five list III diseases are notifiable in Great Britain - BKD, IPN in salmon, furunculosis in salmon, spring viraemia in carp (SVC) and gyrodactylosis (caused by Gyrodactylus salaris). G. salaris has never been found in Britain.
181. Farmers should familiarise themselves with the Fisheries Departments' leaflets "Guide to Importing Fish" and "Combating Fish Disease" which set out import rules and on-site control measures.