Invasive Alien Species in Northern Ireland

Cyprinus carpio, Common carp

Cyprinus carpio Linnaeus, 1758
Since, unlike Great Britain, Ireland had no freshwater connection to the rest of Europe at the end of the last ice age, all purely freshwater fish (that is, fish that cannot live in sea water) including those of the Cyprinid family (common carp, rudd, common bream, roach, gudgeon, tench, dace and minnow), and others such as perch and pike, have been artificially introduced to Ireland. As the common carp rarely breeds naturally in Irish waters, except in some years in the southern part, it has been introduced from England, or elsewhere in Europe, on a regular basis over many centuries, originally as a source of food and more recently as a sport fish, its potentially large size and reluctance to take a bait making it especially valued by specialist anglers.
The common carp is a deep–bellied freshwater fish with two barbels, one short and one longer, at each side of its ventral mouth. The colour is very variable but normally the back is olive brown, sometimes with a greenish tinge, the sides have a golden hue and the belly is yellow. Many colour forms (koi carp – red, white, gold, black, variegated, etc.) have been bred for ornamental ponds. In the ‘wild’ common carp the scales are large and completely cover the body. Other forms have been selectively bred. Mirror carp has irregular rows of large mirror–like scales. Leather (or naked) carp is almost devoid of scales. Line carp have a single lateral line row of scales. There are also many variants within each of these types. Common carp can grow to over 1 metre in length and a mass of 25kg+. They are long–lived with fish of 50 years being recorded, although 20–30 years is more typical.
Country of origin
Common carp is an Asian species but is possibly also native to eastern Europe, including the Danube, although human movements of common carp for more than 2000 years make this difficult to ascertain.
Current distribution
Widely introduced from the Danube to most parts of western Europe, from fourteenth century onwards, including southern England where it breeds naturally. They are now found throughout much of England, Wales and southern Scotland as a result of ongoing introductions. Carp farming for food is widely practised in many European countries, as well as throughout the world.
Location in Ireland
In Northern Ireland, after a policy change in 2000 allowed the introduction of the species, carp fisheries have been set up in a number of lakes including at Dromore and Newtownards, County Down, and Carnagh Forest lakes Loughgall, County Armagh. There are sporadic records of the occurrence of the species in Northern Ireland prior to 2000. For example, one was found in Lurgan Park Lake, County Armagh in 1989 and at Willis’ Lake in Belfast. In the Republic of Ireland it is widely distributed and found in many small enriched ponds, lakes and canals. Some of these populations have been established from breeding stocks in Ireland whereas others have been derived from English or European sources. Ornamental koi carp are widely kept throughout Ireland.
Life cycle
Breeding only takes place in Ireland in some years as a temperature of at least 18°C is required and most carp populations are sustained by regular stocking from English or continental European farms. Spawning takes place in June or July in shallow weedy areas. The female produces large numbers (250,000 per kg) of small (c.1mm) eggs. Feeding is similar to many other cyprinids with the young feeding on plankton and graduating to a wide range of bottom–living invertebrates and plant material. Common carp hybridises readily with both the Crucian carp (Carassius carassius) and feral goldfish (Carassius auratus) producing fertile hybrids, although these species are not found in the wild in Ireland.
Wildlife and habitat impacts
As the carp rarely breeds in Ireland it is restricted in its distribution and thus limited in its impacts. Adult common carp uproot submerged aquatic plants. They also stir up the mud on the bottom resulting in increased turbidity and decreased plant growth, as well as further enrichment of the water due to the release of nutrients. Benthic invertebrate numbers are also reduced directly and indirectly. These changes can result in reduced food for other fishes and aquatic birds. In North America, Africa and Australia it has become a pest species in some waters due to its ability to colonise extensively and alter ecosystems by its feeding habits. The main potential impact in Northern Ireland is due to the continual restocking and the possibility of alien fish, such as the topmouth gudgeon being introduced as contaminants. There is also the possibility of introduction of new diseases and parasites as not all are screened for in current health checks. Similar problems also potentially occur with ornamental koi carp if these are introduced to non–confined waters or unwanted fish are ‘dumped’ in the wild. Climatic warming could result in carp breeding successfully in Ireland, which would significantly increase the threat posed by this alien species.
Human impacts
Angling for carp is increasingly popular in Ireland and it is an economic resource for angling tourism and recreation. Keeping of koi carp is popular among aquarists. Worldwide common carp is one of the most economically important freshwater fish for food farming.
Key vectors
All existing populations are due to deliberate introductions to establish angling stocks or as ornamental fish. Although such introductions are tightly controlled illegal introductions still occur. For example, earlier this year two pike anglers from England were stopped with containers of live roach and carp as they boarded the ferry at Holyhead.
What you can do as an individual
Only buy common carp (including koi) from a reputable supplier with appropriate health certification. Do not buy illegally imported fish. Do not introduce carp into natural systems without proper authorisation. Ornamental carp should not be introduced into the wild or open ponds (that is, connected to river systems).
Advice to specific groups relevant to this species
Angling and management groups should carry out a full risk assessment prior to stocking of carp in new or existing waters. This should include possibilities of contamination with other alien species and of diseases and parasites. Carp should only be obtained from reputable dealers with appropriate health certification. Suspect diseased fish should be notified immediately to the appropriate authorities.
Management measures
Review requirements for licensing of carp introductions including procedures for eliminating the risk of co–introductions of other alien species and of diseases and parasites.
Management groups
Environment and Heritage Service Northern Ireland
Department of Culture Arts and Leisure Northern Ireland
Department of Agriculture and Rural Development Northern Ireland
Fisheries Conservancy Board
Loughs Agency
Central and Regional Fisheries Boards, Republic of Ireland
Further information
Keep Fish Diseases Out. Available at:
Keep Fish Diseases Out –  A guide to protecting freshwater fish stocks from Spring Viraemia of Carp. Available at: (Topmouth gudgeon page).
Maitland, P.S. (2004). Keys to the freshwater fish of Britain and Ireland, with notes on their distribution and ecology. Freshwater Biological Association Scientific Publication No. 62. (available from the FBA Publications Department –
Text written by:
Prof. Andy Ferguson