Diseases and parasites in salmon

Ulcerative dermal necrosis (UDN) of Atlantic salmon (Salmo salar) was identified as a major cause of disease and death in adult salmon returning to freshwater to spawn in the 1970s and 1980s. It was especially common in the UK but also seen in a number of European countries. The cause of UDN is still unclear although Fusarium infections have been proposed. Infected fish presented with multiple skin lesions which were almost inevitably suffering overgrows of Saprolegnia fungus.^[1] Mortality was high with many fish dying before they were able to spawn.

Cnidarian parasites

Henneguya and other parasites in the myxosporean group have a complex life cycle where the salmon is one of two hosts. The fish releases the spores after spawning. In the Henneguya case, the spores enter a second host, an invertebrate, in the spawning stream. When juvenile salmon migrate to the Pacific Ocean, the second host releases a stage infective to salmon. The parasite is then carried in the salmon until the next spawning cycle. The myxosporean parasite that causes whirling disease in trout has a similar life cycle.[2] However, as opposed to whirling disease, the Henneguya infestation does not appear to cause significant incapacitation of the host salmon — even heavily infected fish tend to return to spawn successfully.

Work on Henneguya salminicola at the Pacific Biological Station in Nanaimo^[3] noted that "the fish that have the longest freshwater residence time as juveniles have the most noticeable infections. Hence in order of prevalence coho are most infected followed by sockeye, chinook, chum and pink." As well, the report says that, at the time the studies were conducted, stocks from the middle and upper reaches of large river systems in British Columbia such as Fraser, Skeena, Nass and from mainland coastal streams in the southern half of B.C. "are more likely to have a low prevalence of infection." The report also states "It should be stressed that Henneguya, economically deleterious though it is, is harmless from the view of public health. It is strictly a fish parasite that cannot live in or affect warm blooded animals, including man".

Sea lice are

In 1972, Gyrodactylus, a monogenean parasite, spread from Norwegian hatcheries to wild salmon, and devastated some wild salmon populations.^[12]

Bacteria

The bacteria Piscirickettsia salmonis causes the disease piscirickettsiosis, which has a mortality rate as high as 90% in certain salmonid fishes and is ubiquitous on Chilean salmon farms. Infected fish may or may not display external symptoms of infection, but they frequently display ulcers on the liver and kidney and often develop anemia. The disease was first reported in 1989 as coho salmon syndrome.^[14]

Viral infectious diseases

In 1984,

Another infectious virus in salmon is Piscine orthoreovirus. It was first discovered in 2010, present in Atlantic Salmon farms exhibiting high levels of Heart and Skeletal Muscular Inflammation (HSMI) and Cardiomyopathy syndrome (CMS).^[17] Since then it has been observed in areas across the globe, such as Great Britain, Chile and across the Pacific Northwest.^[18][19][20] Whether or not it causes HSMI has been through extensive study. Injection of PRV from into healthy Atlantic Salmon has caused HSMI in Norway, but these results haven't been reproducible on the same species of salmon in farms in British Columbia, despite HSMI being directly linked to PRV in one BC farm.^[21][22][23] Because of this, numerous researchers have suggested that PRV may be necessary for HSMI, but not sufficient.^[22]

Interaction with humans

In the wild, diseases and parasites are normally at low levels, and kept in check by natural predation on weakened individuals. In crowded net pens they can become epidemics. Diseases and parasites also transfer from farmed to wild salmon populations. A recent study in

The European Commission (2002) concluded “The reduction of wild salmonid abundance is also linked to other factors but there is more and more scientific evidence establishing a direct link between the number of lice-infested wild fish and the presence of cages in the same estuary.”^[25] It is reported that wild salmon on the west coast of Canada are being driven to extinction by sea lice from nearby salmon farms.^[11] Antibiotics and pesticides are often used to control the diseases and parasites, as well as lasers.^[26][27]

These predictions have been disputed by other scientists^[28] and there is much debate on whether the correlation between sea lice infestation and declining wild salmon stocks is driven by causal factors.

Other conditions

Gas bubble disease is caused by contact with supersaturated water.^[29]

Gallery

• 
  furunculosis

  in marine and freshwater fish.
• 
  Streptococcus iniae, a Gram-positive, sphere-shaped bacteria caused losses in farmed marine and freshwater finfish of US$100 million in 1997.^[30]
• 
  Myxobolus cerebralis, a myxosporean parasite, causes whirling disease in farmed salmon and trout and also in wild fish populations.
• 
  Henneguya salminicola

  , a protozoan parasite commonly found in the flesh of salmonids on the West Coast of Canada. Coho salmon
• 
  Columnaris in the gill of a chinook salmon
• 
  The parasite Henneguya zschokkei in salmon beard
• 
  Sockeye salmon with gas bubble disease
• 
  Male and female Lepeophtheirus salmonis