Short description of Gyrodactylus salaris, Salmon fluke

Minute parasite (<1mm long) attaching to the outer body and gills of fish in the salmon family, damaging the skin, which can lead to infections. There are many related Gyrodactylus species that are difficult to distinguish morphologically.

Impact summary: Gyrodactylus salaris, Salmon fluke

It can be highly pathogenic to non-Baltic strains of Atlantic salmon (Salmo salar) and other salmonids, and has caused catastrophic decline of Atlantic salmon stocks in Norway.

Habitat summary: Gyrodactylus salaris, Salmon fluke

This is an ectoparasite on the skin, gills and fins of Atlantic salmon and other salmonids fishes (e.g. Arctic charr, Rainbow trout, Brown trout, Grayling). It is a cold water species surviving at temperatures 0-20°C and salinities up to 7psu.

Overview table

Environment	Freshwater
Species status	Non-Native
Native range	Russian Far East, Finland, Russia Central, Russia East, Russia North, Russia Northwest, Russia South, Sweden
Functional type	Parasite or parasitoid
Status in England
Status in Scotland
Status in Wales
Location of first record
Date of first record	Unknown

Origin

It is adapted to an isolated evolutionary form of Atlantic salmon in Baltic river catchments in Russia, Sweden, Finland, Latvia and Lithuania. For these Baltic strains of Atlantic salmon, Salmon fluke infections rarely cause clinical disease.

First Record

Not recorded in GB to date.

Pathway and Method

Not recorded in GB to date.

Species Status

It was introduced into Norway with Salmon imported from Sweden in the early 1970s, and decimated wild Salmon populations in 46 Norwegian rivers. Entire river catchments were colonised within 2 years of the parasite first being detected. By 1996, it had spread to the northern rivers of Finland and Russia, to Germany, France, Spain, Portugal and former Yugoslavia. It is now known from 13 European countries, the most recent records originating from Poland and Italy. Surveys conducted in the 1990s and on-going government based surveillance programmes indicate that GB is currently free of Salmon fluke. Nevertheless, there is considerable concern about accidental introductions of this species, particularly since experimental exposure of native British salmon stocks to this parasite demonstrated their susceptibility.

Dispersal Mechanisms

Although unable to swim, detached Salmon flukes can survive for several days and infect fish on contact. Migrating fish can also distribute it, if they do not pass through salt water. It can be distributed to new rivers by stocking of infected fish and accidentally on fishing or other equipment such as boats or boots. Movement of live Rainbow trout was identified as the most important route of possible transmission in England and Wales, with additional local spread by mechanical transmission on equipment.

Reproduction

This long-lived species can reproduce both sexually and asexually. Females produce up to four broods of 2-7 live individuals which are already pregnant themselves. Only a single individual is thus needed to spread the species. Populations rapidly develop at 6.5-13.0°C, but can also grow at 2.5°C (surviving 33 days) to 19°C (4.5 days). It may reproduce on different salmonid species. Reproductive capability and longevity decline at higher salinities.

Known Predators/Herbivores

None known.

Resistant Stages

Detached flukes can survive in air under damp conditions for some days.

Habitat Occupied in GB

Not recorded in GB to date.

It has been introduced into a number of European countries including Norway, Denmark, Germany and France, and extending as far south as Portugal, Spain and Italy.

Environmental Impact

In non-Baltic stocks of Atlantic salmon, this parasite multiplies unchecked by an immune response and death typically results. In Norway, northern Finland and the White Sea area, introductions resulted in reductions in wild Atlantic salmon populations of up to 98% over a period of 5 years. Knock-on effects include reduced freshwater mussel recruitment caused by declines in abundance of their salmonid host fishs.

Health and Social Impact

None known.

Economic Impact

In the late 1990's in Norway, an estimated 250-500 tonnes of production were annually lost from salmon farms. Since the introduction of the parasite to Norway from the Baltic, the economic loss caused by Gyrodactylus salaris amounts to 480 million Euro.

Identification

Shinn, A.P., Collins, C., Garcia-Vasquez, A., et al. (2010) Multi-centre testing and validation of current protocols for the identification of Gyrodactylus salaris (Monogenea). International Journal for Parasitology, 40, 1455-1467.

Shinn, A., Sommerville, C. & Gibson, D. (1995) Distribution and characterization of species of Gyrodactylus Nordmann, 1832 (Monogenea) parasitizing salmonids in the UK, and their discrimination from Gyrodactylus salaris Malmberg, 1957. Journal of Natural History, 29, 1383-1402.

Biology, ecology, spread, vectors

Bakke, T. & MacKenzie, K. (1993) Comparative susceptibility of native Scottish and Norwegian stocks of Atlantic salmon, Salmo salar L., to Gyrodactylus salaris Malmberg - laboratory experiments. Fisheries Research, 17, 69-85.

Bakke, T., Jansen, P. & Hansen, L. (1990) Differences in the host resistance of Atlantic Salmon, Salmo salar L., stocks to the monogenean Gyrodactylus salaris Malmberg, 1957. Journal of Fish Biology, 37, 577-587.

Harris, P.D., Jansen, P.A. & Bakke, T.A. (1994) The population age structure and reproductive biology of Gyrodactylus salaris Malmberg (Monogenea). Parasitology, 108, 167-173.

Johnsen, B.O. & Jensen, A. (1986) The Gyrodactylus story in Norway. Aquaculture, 98, 289-302.

Peeler, E.J., Gardiner, R. & Thrush, M.A. (2004) Qualitative risk assessment of routes of transmission of the exotic fish parasite Gyrodactylus salaris between river catchments in England and Wales. Preventive Veterinary Medicine, 64, 175-189.

Management and impact

Mo, T.A. (1994) Status of Gyrodactylus salaris problems and research in Norway. Parasitic Diseases of Fish (ed J. W. Lewis), pp. 43–48. Samara Publishing, Tresaith, Dyfed, Wales.

Soleng, A., Poleo, A.B.S. & Bakke, T.A. (2005) Toxicity of aqueous aluminium to the ectoparasitic monogenean Gyrodactylus salaris. Aquaculture, 250, 616-620.

General

Johnsen, B.O. & Jensen, A. (1986) The Gyrodactylus story in Norway. Aquaculture, 98, 289-302.

https://www.cabi.org/isc/datasheet/80813