Invasive Alien Species in Northern Ireland

Gyrodactylus salaries, Monogean salmon parasite

Gyrodactylus salaries Malmberg, 1957
Gyrodactylus salaris is a parasite which infects the skin, gills and fins of salmon, trout and some other types of fish in fresh water. Less than half a millimeter in size it can cause serious damage in some strains of Atlantic Salmon. The effects of the disease are so serious that salmon stocks have now been lost completely from more than 20 Norwegian rivers, with the particular races of salmon in the affected rivers being lost forever. G. salaris does not occur in UK or Irish rivers but experiments have shown that our particular race of salmon, like those of Norway, is killed by the parasite. It is therefore essential that this worm is not introduced to our waters.
Gyrodactylus are small (less than 0.5mm in length), leech-like parasites belonging to a group of worms known as Monegenean Trematodes. Their body is dorso-ventrally flattened, bilaterally symmetrical, without a definite anus and lacking a body cavity. Their organs are embedded in specialized connective tissue known as parenchyma and they lack a respiratory or circulatory system. G. salaris is an ectoparasite bearing as adhesive organs a large posterior disk or haptor and an anterior adhesive region.
Country of origin
Gyrodactylus salaris occurs naturally in the Baltic rivers of Finland and Russia and possibly eastern Sweden.
Current distribution
G. salaris has been recorded from Norway, Finland, Sweden, Russia, Germany, France, Spain and Portugal. Its status throughout many other European countries is unknown.
Location in Ireland
G. salaris does not occur in UK or Irish rivers.
Life cycle
Only one host is involved in the life cycle (a direct life cycle) of G. salaris and these parasites are remarkable in that the hermaphroditic parents give birth to live young which attach directly to the fish host The daughter parasite is the same size as the mother, and inside this daughter there is already a developing granddaughter, in a 'Russian doll' arrangement. Under optimal conditions reproduction can occur at an alarmingly fast rate which contributes greatly to the pathogenicity of this worm.
Wildlife and habitat impacts
G. salaris has potentially devastating impacts both on wildlife and habitats as a direct result of its highly pathogenic effects on salmon populations. To eliminate G. salaris from affected rivers, all types of fish capable of harbouring the worm must be removed, so restoration of salmon stocks in affected Norwegian rivers has involved poisoning whole catchments. Such remedial work is destructive, difficult, very expensive and likely to take many years. In some instances it has not been successful.
Human impacts
This parasite poses no direct threat to human health as its host is a fish. However, it does pose a threat to the economic value of the commercially valuable salmonid fisheries that become infected with it, given its effects as a highly infectious serious fish pathogen.
Key vectors
Movement of live infected salmonid hosts has been the most common vector implicated in the introduction of G. salaris to new locations. This parasite was accidentally transferred for the first time to some rivers of the west coast of Sweden, to Norway and more recently to some rivers in northern Finland and northern Russia.
The parasite is very hardy and may be inadvertently introduced by fishermen. It is capable of surviving for several days in damp conditions such as plastic bags, wet angling equipment and the wet surface of dead fish. The parasite can also survive on other fish species on which it causes no harm. Care needs to be taken to ensure the movement of these other species takes place in accordance with statutory fish health requirements.
What you can do as an individual
Few people are involved in the commercial movement of fish so it is unlikely that people in general will be involved in spreading this parasite. However, prior to arrival in the UK, anglers travelling from areas which are not designated as free of G. salaris, and in particular from those areas known to be infected, such as Scandinavia, should take the following precautions to ensure that their equipment is not contaminated.
All fishing equipment should be thoroughly cleaned and then treated to kill any parasites by:
  1. drying at a minimum temp of 20°C for at least two days
  2. heating for at least one hour at a temp above 60°C
  3. deep freezing for at least one day.

    Advice to specific groups relevant to this species
    Anglers and Fish Farmers
    The infection is a listed notifiable disease and legislation is in place to prevent the transfer of live salmon and trout to British waters. This has been further supplemented by EU legislation that recognises the special status of the UK as being proven free of the parasite. Additional advice specifically aimed at anglers is outlined above.
    Management Measures
    The management measures in place to prevent the spread or introduction of G. salaris are those outlined above. No biological or pharmaceutical anthelminthic controls exist for this parasite in the wild. Affected rivers are normally cleared of all fish species and restocked.
    Management Groups
    The Environment and Heritage Service and Department of Agriculture and Rural Development (DARDNI) Fish Health Unit have worked closely on coordinating and producing a strategy for the prevention of G. salaris introduction to our waters.
    Further information
    See the Invasive Species Ireland web site for further information -
    Text written by:
    Dr Derek Evans