1. Overview
    Cnidarian parasites  Sea lice  Flatworms  Bacteria  Viral infectious diseases 

2. Interaction with humans

3. Other conditions

4. Gallery

5. Notes

6. References

7. Further reading

8. External links

This article is about diseases and parasites in salmon, trout and other salmon-like fishes of the Salmonidae family.

Overview

Cnidarian parasites

According to Canadian biologist Dorothy Kieser, the myxozoan parasite Henneguya salminicola is commonly found in the flesh of salmonids. It has been recorded in the field samples of salmon returning to the Queen Charlotte Islands. The fish responds by walling off the parasitic infection into a number of cysts that contain milky fluid. This fluid is an accumulation of a large number of 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.^[1] 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.

According to Dr. Kieser, a lot of work on Henneguya salminicola was done by scientists at the Pacific Biological Station in Nanaimo in the mid-1980s, in particular, an overview report^[2] which states that "the fish that have the longest fresh water 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".

According to Klaus Schallie, Molluscan Shellfish Program Specialist with the Canadian Food Inspection Agency, "Henneguya salminicola is found in southern B.C. also and in all species of salmon. I have previously examined smoked chum salmon sides that were riddled with cysts and some sockeye runs in Barkley Sound (southern B.C., west coast of Vancouver Island) are noted for their high incidence of infestation."

Sea lice

Sea lice are ectoparasites which feed on mucus, blood, and skin, and migrate and latch onto the skin of wild salmon during free-swimming, planktonic nauplii and copepodid larval stages, which can persist for several days.^[5][6][7] Large numbers of highly populated, open-net salmon farms can create exceptionally large concentrations of sea lice; when exposed in river estuaries containing large numbers of open-net farms, many young wild salmon are infected, and do not survive as a result.^[8][9] Adult salmon may survive otherwise critical numbers of sea lice, but small, thin-skinned juvenile salmon migrating to sea are highly vulnerable. On the Pacific coast of Canada, the louse-induced mortality of pink salmon in some regions is commonly over 80%.^[10]

Flatworms

In 1972, Gyrodactylus, a monogenean parasite, spread from Norwegian hatcheries to wild salmon, and devastated some wild salmon populations.^[11]

Bacteria

Enteric redmouth disease is a bacterial infection of freshwater and marine fish caused by the pathogen {{nowrap|Yersinia ruckeri}}. It is primarily found in rainbow trout and other cultured salmonids. The disease is characterized by subcutaneous hemorrhaging of the mouth, fins, and eyes. It is most commonly seen in fish farms with poor water quality. Redmouth disease was first discovered in Idaho rainbow trout in the 1950s.^[12]

Viral infectious diseases

In 1984, infectious salmon anemia (ISAv) was discovered in Norway in an Atlantic salmon hatchery. Eighty percent of the fish in the outbreak died. ISAv, a viral disease, is now a major threat to the viability of Atlantic salmon farming. It is now the first of the diseases classified on List One of the European Commission’s fish health regime. Amongst other measures, this requires the total eradication of the entire fish stock should an outbreak of the disease be confirmed on any farm. ISAv seriously affects salmon farms in Chile, Norway, Scotland and Canada, causing major economic losses to infected farms.^[13] As the name implies, it causes severe anemia of infected fish. Unlike mammals, the red blood cells of fish have DNA, and can become infected with viruses. The fish develop pale gills, and may swim close to the water surface, gulping for air. However, the disease can also develop without the fish showing any external signs of illness, the fish maintain a normal appetite, and then they suddenly die. The disease can progress slowly throughout an infected farm and, in the worst cases, death rates may approach 100 percent. It is also a threat to the dwindling stocks of wild salmon. Management strategies include developing a vaccine and improving genetic resistance to the disease.^[14]

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 British Columbia links the spread of parasitic sea lice from river salmon farms to wild pink salmon in the same river."^[15]

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.”^[16] It is reported that wild salmon on the west coast of Canada are being driven to extinction by sea lice from nearby salmon farms.^[17] Antibiotics and pesticides are often used to control the diseases and parasites, as well as lasers.^[18][19]

These predictions have been disputed by other scientists^[20] 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

Gallery

Notes

References

• {{cite book |author=Axelrod HR and Untergasser D |title=Handbook of fish diseases |publisher=T.F.H. Publications |location=Neptune, NJ |year=1989 |pages= |isbn=978-0-866227032 |oclc= |doi=}}
• {{cite book |author=Andrews C|title=The Manual of Fish Health |publisher=Voyageur Press |location=Stillwater, MN |year= 1988|pages= |isbn=978-1-56465-160-0 |oclc= |doi=}}
• {{cite book |author=Fairfield, T |title=A commonsense guide to fish health |publisher=Barron's Educational Series |location=Woodbury, N.Y |year=2000 |pages= |isbn=978-0-7641-1338-3 |oclc= |doi=}}

Further reading

• [https://web.archive.org/web/20101214173552/http://farmedanddangerous.org/uploads/File/Reports/SeaLice_FullReport.pdf Sea Lice and Salmon: Elevating the dialogue on the farmed-wild salmon story] Watershed Watch Salmon Society, 2004.
• Martin Krkoek et al. (2007) "Declining Wild Salmon Populations in Relation to Parasites from Farm Salmon" Science, Report 318(5857):1772-1775.

External links

• Help with Stress & Disease
• [https://web.archive.org/web/20080906111849/http://www.blackwellpublishing.com/journals/jfd/ Journal of Fish Diseases]
• [https://web.archive.org/web/20120204193830/http://europa.eu/scadplus/leg/en/lvb/l12022.htm The European Union puts in place a framework of measures to combat certain fish diseases effectively and to prevent their spread.]
• Watershed Watch Salmon Society A British Columbia advocacy group for wild salmon
• Wild Salmon in Trouble: The Link Between Farmed Salmon, Sea Lice and Wild Salmon - Watershed Watch Salmon Society. Animated short video based on peer-reviewed scientific research, with subject background article Watching out for Wild Salmon.
• Aquacultural Revolution: The scientific case for changing salmon farming - Watershed Watch Salmon Society. Short video documentary. Prominent scientists and First Nation representatives speak their minds about the salmon farming industry and the effects of sea lice infestations on wild salmon populations.
• [https://web.archive.org/web/20100103205837/http://www.farmedanddangerous.org/page/sealice Sea Lice] Coastal Alliance for Aquaculture Reform. An overview of farmed- to wild-salmon interactive effects.
• [https://web.archive.org/web/20090501102114/http://www.farmedanddangerous.org/page/salmon-farming-problems Salmon Farming Problems] Coastal Alliance for Aquaculture Reform. An overview of environmental impacts of salmon farming.
• Fish farms drive wild salmon populations toward extinction Biology News Net. December 13, 2007.
• [https://web.archive.org/web/20110605075255/http://www.st-andrews.ac.uk/~merg/salmonidparasites.htm Salmonid parasites] University of St Andrews Marine Ecology Research Group.

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