Diseases and parasites in salmon

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Sample of pink salmon infected with Henneguya salminicola, caught off the Queen Charlotte Islands, Western Canada in 2009

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

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, most likely 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, particularly Lepeophtheirus salmonis and various Caligus species, including C. clemensi and C. rogercresseyi, can cause deadly infestations of both farm-grown and wild salmon.[3][4] 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]

Some background[edit]

Sea lice on the back of an Atlantic salmon

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

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.[12] 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.[13]

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.[14] 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.”[15] It is reported that wild salmon on the west coast of Canada are being driven to extinction by sea lice from nearby salmon farms.[16] These predictions have been disputed by other scientists[17] and recent harvests have indicated that the predictions were in error. Antibiotics and pesticides are often used to control the diseases and parasites.

Wild salmon[edit]

The myxosporean parasite Ceratomyxa shasta infects salmonid fish on the Pacific coast of North America

According to Canadian biologist Dorothy Kieser, protozoan 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 lifecycle 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, most likely an invertebrate, in the spawning stream. When juvenile salmon out-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 lifecycle.[18] However, as opposed to whirling disease, the Henneguya infestation does not appear to cause disease in 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[19] 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, particularly Lepeophtheirus salmonis and a variety of Caligus species, including Caligus clemensi and Caligus rogercresseyi, can cause deadly infestations of both farm-grown and wild salmon.[20][21] Sea lice are ectoparasites which feed on mucous, blood, and skin, and migrate and latch onto the skin of wild salmon during free-swimming, planktonic naupli and copepodid larval stages, which can persist for several days.[22][23][24] 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.[25][26] 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%.[27]

Farmed salmon[edit]

Atlantic salmon

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

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.[28] 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.[29]

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."[14] 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.”[30] It is reported that wild salmon on the west coast of Canada are being driven to extinction by sea lice from nearby salmon farms.[16] Antibiotics and pesticides are often used to control the diseases and parasites.

Notes[edit]

  1. ^ Crosier, Danielle M.; Molloy, Daniel P.; Bartholomew, Jerri. "Whirling Disease – Myxobolus cerebralis" (PDF). Retrieved 2007-12-13. 
  2. ^ N.P. Boyce, Z. Kabata and L. Margolis (1985). "Investigation of the Distribution, Detection, and Biology of Henneguya salminicola (Protozoa, Myxozoa), a Parasite of the Flesh of Pacific Salmon". Canadian Technical Report of Fisheries and Aquatic Sciences (1450): 55. 
  3. ^ Sea Lice and Salmon: Elevating the dialogue on the farmed-wild salmon story Watershed Watch Salmon Society, 2004.
  4. ^ Bravo, S. (2003). "Sea lice in Chilean salmon farms". Bull. Eur. Assoc. Fish Pathol. 23, 197–200.
  5. ^ Morton, A., R. Routledge, C. Peet, and A. Ladwig. 2004 Sea lice (Lepeophtheirus salmonis) infection rates on juvenile pink (Oncorhynchus gorbuscha) and chum (Oncorhynchus keta) salmon in the nearshore marine environment of British Columbia, Canada. Canadian Journal of Fisheries and Aquatic Sciences 61:147–157.
  6. ^ Peet, C. R. 2007. Thesis, University of Victoria.
  7. ^ Krkošek, M., A. Gottesfeld, B. Proctor, D. Rolston, C. Carr-Harris, M.A. Lewis. 2007. Effects of host migration, diversity, and aquaculture on disease threats to wild fish populations. Proceedings of the Royal Society of London, Ser. B 274:3141-3149.
  8. ^ Morton, A., R. Routledge, M. Krkošek. 2008. Sea louse infestation in wild juvenile salmon and Pacific herring associated with fish farms off the east-central coast of Vancouver Island, British Columbia. North American Journal of Fisheries Management 28:523-532.
  9. ^ Krkošek, M., M.A. Lewis, A. Morton, L.N. Frazer, J.P. Volpe. 2006. Epizootics of wild fish induced by farm fish. Proceedings of the National Academy of Sciences 103:15506-15510.
  10. ^ Krkošek, Martin, et al. Report: "Declining Wild Salmon Populations in Relation to Parasites from Farm Salmon", Science: Vol. 318. no. 5857, pp. 1772 - 1775, 14 December 2007.
  11. ^ a b Stead, SM and Laird lLM (2002) Handbook of salmon farming, page 348, Birkhäuser. ISBN 978-1-85233-119-1
  12. ^ New Brunswick to help Chile beat disease Fish Information and Services
  13. ^ Fact Sheet - Atlantic Salmon Aquaculture Research Fisheries and Oceans Canada. Retrieved 12 May 2009.
  14. ^ a b Seafood Choices Alliance (2005) It's all about salmon
  15. ^ Scientific Evidence.
  16. ^ a b Krkosek M, Ford JS, Morton A, Lele S, Myers RA and Lewis MA (2007) Declining Wild Salmon Populations in Relation to Parasites from Farm Salmon Science, 318, 5857: 1772.]
  17. ^ Riddell, B. E., R. J. Beamish, et al. (2008). "Comment on "Declining Wild Salmon Populations in Relation to Parasites from Farm Salmon"." Science 322(5909): 1790b [1]
  18. ^ Crosier, Danielle M.; Molloy, Daniel P.; Bartholomew, Jerri. "Whirling Disease – Myxobolus cerebralis" (PDF). Retrieved 2007-12-13. 
  19. ^ N.P. Boyce, Z. Kabata and L. Margolis (1985). "Investigation of the Distribution, Detection, and Biology of Henneguya salminicola (Protozoa, Myxozoa), a Parasite of the Flesh of Pacific Salmon". Canadian Technical Report of Fisheries and Aquatic Sciences (1450): 55. 
  20. ^ Sea Lice and Salmon: Elevating the dialogue on the farmed-wild salmon story Watershed Watch Salmon Society, 2004.
  21. ^ Bravo, S. (2003). "Sea lice in Chilean salmon farms". Bull. Eur. Assoc. Fish Pathol. 23, 197–200.
  22. ^ Morton, A., R. Routledge, C. Peet, and A. Ladwig. 2004 Sea lice (Lepeophtheirus salmonis) infection rates on juvenile pink (Oncorhynchus gorbuscha) and chum (Oncorhynchus keta) salmon in the nearshore marine environment of British Columbia, Canada. Canadian Journal of Fisheries and Aquatic Sciences 61:147–157.
  23. ^ Peet, C. R. 2007. Thesis, University of Victoria.
  24. ^ Krkošek, M., A. Gottesfeld, B. Proctor, D. Rolston, C. Carr-Harris, M.A. Lewis. 2007. Effects of host migration, diversity, and aquaculture on disease threats to wild fish populations. Proceedings of the Royal Society of London, Ser. B 274:3141-3149.
  25. ^ Morton, A., R. Routledge, M. Krkošek. 2008. Sea louse infestation in wild juvenile salmon and Pacific herring associated with fish farms off the east-central coast of Vancouver Island, British Columbia. North American Journal of Fisheries Management 28:523-532.
  26. ^ Krkošek, M., M.A. Lewis, A. Morton, L.N. Frazer, J.P. Volpe. 2006. Epizootics of wild fish induced by farm fish. Proceedings of the National Academy of Sciences 103:15506-15510.
  27. ^ Krkošek, Martin, et al. Report: "Declining Wild Salmon Populations in Relation to Parasites from Farm Salmon", Science: Vol. 318. no. 5857, pp. 1772 - 1775, 14 December 2007.
  28. ^ New Brunswick to help Chile beat disease Fish Information and Services
  29. ^ Fact Sheet - Atlantic Salmon Aquaculture Research Fisheries and Oceans Canada. Retrieved 12 May 2009.
  30. ^ Scientific Evidence.
  31. ^ Agnew W, Barnes AC (May 2007). "Streptococcus iniae: an aquatic pathogen of global veterinary significance and a challenging candidate for reliable vaccination". Vet Microbiol 122 (1–2): 1–15. doi:10.1016/j.vetmic.2007.03.002. PMID 17418985. 

References[edit]

  • Axelrod HR and Untergasser D (1989). Handbook of fish diseases. Neptune, NJ: T.F.H. Publications. ISBN 0-866227032. 
  • Andrews C. The Manual of Fish Health. Stillwater, MN: Voyageur Press. ISBN 1-56465-160-6. 
  • Fairfield, T (2000). A commonsense guide to fish health. Woodbury, N.Y: Barron's Educational Series. ISBN 0-7641-1338-0. 

Further reading[edit]

External links[edit]