Invasive Alien Species in Northern Ireland

Salmo salar, Farmed Atlantic salmon

Salmo salar Linnaeus, 1758
Farming of Atlantic salmon for food started in Norway in 1969. Since then the industry has expanded exponentially and global production of farmed Atlantic salmon now exceeds one million tonnes. After trials in the 1970s, salmon farming in Ireland expanded rapidly from the early 1980s. Wild salmon have declined by some 80 per cent in recent decades and the species is now extinct, or in critical condition, in over 27 per cent of rivers, and endangered or vulnerable in a further 30 per cent. The detrimental effects of salmon farming on wild populations, through escapees interbreeding with wild fish and through the spread of parasites, is thus of increasing concern.
Farmed Atlantic salmon are a domesticated form of wild salmon. Farm salmon are genetically different from wild stocks due to non-native origin, founding effects, and as a result of deliberate and accidental selection, and genetic drift, during domestication. Many farm salmon differences can be related to deliberate selection for faster growth and later maturity. As a result of these changes farmed salmon grow faster and mature later than wild salmon but have very much reduced survival under natural conditions. Farmed salmon can often be told apart from wild salmon due to their ragged fins as a result of damage in crowded culture conditions.
Country of origin
The main widely used domesticated strains of farmed salmon are of Norwegian origin, although some strains originate from Scotland and Iceland and some are of mixed origins.
Current distribution
Salmon farming is undertaken in countries bordering the North Atlantic, which is the native range of wild Atlantic salmon, primarily Norway, Scotland, Faeroes, Iceland, Ireland, and eastern Canada and USA. Farming also occurs outside the native range in Chile and western North America.
Location in Ireland
In Northern Ireland, there is only one major Atlantic salmon farming operation, which started in 1989, and is located off the County Antrim coast and on the Glenarm River. However, escaped farm salmon have been found in many rivers as escaped fish move widely at sea. These fish are likely to have come from farms in western Scotland, where there are over 300 farms, or from Donegal, as well as from the local farming operation. In the Republic of Ireland there are many salmon farms located on the west coast from Donegal to Cork and again escaped farm fish and their offspring have been found in numerous rivers.
Life cycle
Farming mimics the natural life of the Atlantic salmon where the juvenile stage is spent in fresh water and the adult stage in the sea. Juveniles are reared in tanks often situated adjacent to rivers from which they take and discharge water. Without adequate screening on the outlet, escapes of juveniles can occur into the river. After about one year in fresh water the juvenile salmon become smolts and are transferred to cages in the sea where they are reared to marketable size. These cages are made of netting attached to a floating collar and are vulnerable to damage as a result of storms, predators, boat collisions and during routine handling operations, and thus escapes from confinement inevitably occur. Mature escaped salmon enter rivers where they breed and interbreed with wild salmon. Some two million Atlantic salmon currently escape each year from fish farms in the North Atlantic, which is equivalent to some 50% of the total number of wild salmon in the area.
Wildlife and habitat impacts
Numerous detrimental impacts of Atlantic salmon farming have been identified:
  • Farmed salmon are fed on fish meal, which is made largely from marine fish. Overexploitation of these marine fish can result in reduced food supply for other fish, seabirds and marine mammals. More recently, Antarctic krill has been exploited for food manufacture with potential overexploitation and consequently detrimental impact on the Antarctic ecosystem
  • Under farming conditions, high levels of diseases and parasites can build up and these can be transferred to wild fish. Spread of sea lice from farms has been implicated in the decline of Atlantic salmon and sea trout. The parasite Gyrodactylus salaris could be potentially introduced with farm salmon as happened in Norway
  • Waste food and faeces from salmon farm can result in local nutrient enrichment in sea bays producing algal blooms and decline in marine invertebrates. Chemical discharges from disease and parasite treatment can be directly toxic to aquatic organisms
  • As farm salmon have been selectively bred for faster growth, offspring of escaped farm salmon and hybrids with wild salmon outcompete the wild juveniles in a river. Since subsequent marine survival of farm and hybrid salmon is much lower, overall fewer adults return to breed, meaning fewer offspring in the next generation. Repeated escapes produce a cumulative reduction in offspring recruitment, which could lead to extinction of endangered wild populations
  • As farm salmon are derived from a few non-native populations, interbreeding with wild salmon results in a change in genetic make-up of these populations and in an overall reduction in genetic variability among populations. These genetic changes are likely to reduce the ability of wild salmon to adapt to changing environmental conditions, for example, global warming and new diseases
  • Escaped farm salmon may enter rivers and attempt to spawn later in the season when wild fish have already spawned. Late-spawning individuals may dig up the eggs of early-spawning fish, and thereby lower the latter's reproductive success.

    Human impacts
    As wild Atlantic salmon and sea trout populations are important economic resources for tourism and recreation, decline in wild salmon and sea trout, in which salmon farming has undoubtedly played a part, has resulted in the loss of employment due to closure of fisheries and hotels reliant on angling clients. Concerns have been expressed as to possible adverse health effects of eating farm salmon but the data on which these are based are disputed by the industry.
    Key vectors
    Farmed salmon have been deliberately introduced for commercial farming operations. Escape of such fish to the wild occurs as a result of damage to cages due to storms, boat collisions, predators and during routine handling operations. Deliberate release has also occurred in some situations either to dispose of surplus juveniles or in a misguided attempt to ‘enhance’ wild populations.
    What you can do as an individual
    Individuals may wish to avoid the purchase of farmed Atlantic salmon and only consume wild salmon. While wild Atlantic salmon is of limited and seasonal availability, wild Alaskan salmon is now readily available in good supermarkets and comes from a Marine Stewardship Council certified sustainable fishery.
    Advice to specific groups relevant to this species
    Anglers should kill any farm salmon caught in a river (within legal limits) rather than practice catch-and-release as for wild salmon. Angling groups should lobby for stricter management controls on salmon farming.
    Management measures
    • Alternative food sources (for example, plant) should be developed as food for farmed salmon
    • The NASCO Guidelines in Resolution CNL(06)45 (Williamsburg Resolution) on farming salmon should be the minimum standard for the construction and operation of fish farms
    • Smolt rearing units should not outflow into salmon rivers and cages should not be placed in lakes in such catchments
    • Marine cages should not be situated within 30km of major salmon rivers
    • Where escapes occur, appropriate recovery plans and resources should be available for immediate deployment
    • Sterile triploids should be used until full containment is possible although further investigations into the use of triploids and other bioconfinement methods are urgently required.

      Management groups
      Department of Agriculture and Rural Development Northern Ireland
      Department of Culture Arts and Leisure Northern Ireland
      Fisheries Conservancy Board Northern Ireland
      Loughs Agency
      Department of Communications, Marine and Natural Resources Republic of Ireland
      Central and Regional Fisheries Boards Republic of Ireland
      North Atlantic Salmon Conservation Organisation (NASCO)
      Further information
      NASCO (2001) The Williamsburg Resolution. Available at:
      Staniford, D. (2002). Sea cage fish farming: an evaluation of environmental and public health aspects (the five fundamental flaws of sea cage fish farming). Available at: (Gyrodactylus salaris page)
      Crozier, W.W. (1993). Evidence of genetic interaction between escaped farmed salmon and wild Atlantic salmon (Salmo salar L.) in a Northern Irish River. Aquaculture 113: 19-29.
      Crozier, W.W. (2000). Escaped farmed salmon, Salmo salar L., in the Glenarm River, Northern Ireland: genetic status of the wild population 7 years on. Fisheries Management and Ecology 7: 437-446.
      Ferguson, A., Fleming, I., Hindar, K., Skaala, Ø., McGinnity, P., Cross, T. and Prodöhl, P. (2007). Farm Escapes. In The Atlantic Salmon: Genetics, Conservation and Management. (Verspoor, E., Stradmeyer, L. and Nielsen, J., eds.). Chapter 12. Blackwell, Oxford. (To be published March 2007)
      Hindar, K., Fleming, I.A., McGinnity, P. and Diserud, O. (2006). Genetic and ecological effects of salmon farming on wild populations: modelling from experimental results. ICES Journal of Marine Science 63: 1234-1247.
      McGinnity, P., Prodöhl, P., Ferguson, A., Hynes, R., Ó Maoiléidigh, N., Baker, N., Cotter, D., O’Hea, B., Cooke, D., Rogan, G., Taggart, J. and Cross, T. (2003). Fitness reduction and potential extinction of wild populations of Atlantic salmon Salmo salar as a result of interactions with escaped farm salmon. Proceedings of the Royal Society of London Series B 270: 2443-2450.
      Text written by:
      Prof. Andy Ferguson