Invasive Alien Species in Northern Ireland

Pseudorasbora parva, Topmouth gudgeon

Pseudorasbora parva (Temminck & Schlegel, 1846)
The topmouth gudgeon (also known as the stone moroko, false harlequin, Japanese minnow, sharpnose gudgeon, or clicker barb to aquarists with whom it is a popular species) is an invasive Asian alien species, which poses major threats to UK fish species. While it has not been found in Ireland, it is now present in at least 25 locations in England and Wales, most of which have been colonised in the past decade. It is the most invasive fish species present in Europe and is one of the most damaging alien aquatic species that could potentially be introduced to Northern Ireland.
The topmouth gudgeon is a small inconspicuous freshwater fish of the carp family, which is bluish-grey on the back grading to silvery on the sides, often with an iridescent violet sheen, and white on the belly. There is a continuous dark band of uniform width along the side (lateral line) of the body. It rarely grows to more than about 8cm and is a short-lived species rarely living more than three years. Superficially it resembles the common gudgeon (Gobio gobio), which is widely distributed in Britain and Ireland. However, the topmouth gudgeon differs in that its mouth is angled upwards (lower jaw upturned) and barbels are absent, whereas in the common gudgeon the mouth is angled downwards and there are two barbels adjacent to it. For a detailed key for identification of the topmouth gudgeon and other cyprinids see Maitland (2004). Pinder (2005) provides a key for the identification of the topmouth gudgeon at various early developmental stages.
Country of origin
The native range is Asia including China, Japan, Korea, Taiwan and the Amur River basin. It was introduced into Romanian ponds in 1960 along with commercial carp species.
Current distribution
From Romania it has spread to the Danube and Rhine systems and most parts of Europe, having been deliberately introduced into several countries, supposedly as food for other fish. It is also present in parts of North Africa. It was introduced into England in 1985, from the German Danube, as a contaminant in ornamental fish, and is now well established at this original location in Hampshire. It was not widely reported elsewhere until the late 1990s but by 2005, topmouth gudgeon was recorded at 25 sites throughout England and Wales. Many of these sites are directly connected to major rivers systems potentially allowing for easy further dispersal.
Location in Ireland
The topmouth gudgeon has not been found in Northern Ireland or the Republic of Ireland to date.
Life cycle
The topmouth gudgeon matures at one year old and can breed up to four times a year. The eggs are spawned in batches over the spring and summer period and are firmly glued to stones, branches, leaves, or artificial structures where they are guarded by the male until they hatch about a week later. Topmouth gudgeon are opportunistic feeders and eat a wide variety of items such as algae, bottom-dwelling invertebrates, snails, and the eggs and larvae of other fish. They will also feed on terrestrial insects on the water surface, such feeding often being accompanied by a clicking noise, hence the alternative name of clicker barb. Although the species favours lakes, it can occur in slower running waters. The life cycle, together with broad environmental tolerance limits, results in very rapid increases in numbers and also facilitates its spread throughout water systems.
Wildlife and habitat impacts
The rapid rate of population increase of the topmouth gudgeon results in it outcompeting native species for food and it also eats their eggs and larvae. It carries a parasite that can infect many other species causing mortality and inhibition of spawning, although the carrier topmouth gudgeon remain apparently healthy. This parasite is genetically very similar to a ‘rosette agent’ (Sphaerothecum destruens), which is known to cause high mortality in salmonid fishes in North America. If introduced to Ireland this parasite could be highly detrimental to the native salmonid populations. Topmouth gudgeon has been shown to be a carrier for several other parasites that infect native fish resulting in increased mortality.
Human impacts
The topmouth gudgeon has several adverse impacts on native fish species which are an important economic resource for tourist and recreational angling. It also has potentially severe implications for commercial salmon farming.
Key vectors
The species has been deliberately introduced (legally) in some cases as part of the ornamental fish trade. As the fry are minute and not readily distinguishable from other cyprinids, they can be spread accidentally and the spread in England and Wales is likely to be due to artificial stocking of angling waters with other species. Since its eggs are stuck to any available surface it is readily passively spread.
What you can do as an individual
Anglers should not bring in live cyprinids as bait from outside Ireland. Although this is illegal, it still takes place. Anglers should also ensure that all angling equipment including waders are thoroughly cleaned in hot soapy water and dried in a warm place after they have been used in waters outside of Ireland. Various commercial sterilants are available from angling suppliers. If these are used, attention should be given to guidelines for safe use and disposal. (Cleaning of angling equipment is in any case a responsible precaution when moving among different water systems within Ireland as well to avoid spread of other alien species already here. A clean line casts better anyway!) Any possible topmouth gudgeon found in Irish waters should be retained and expert identification sought (see management groups below).
Advice to specific groups relevant to this species
Angling groups and statutory bodies responsible for the import of cyprinids (for example, carp) for angling or the ornamental trade should ensure that all imports are from reliable sources and have been thoroughly screened for the presence of topmouth minnow. A contingency plan should be prepared by the management groups to deal immediately with topmouth gudgeon should it be found in Ireland.
Management measures
Management measures should be directed towards prevention of its introduction to Northern Ireland and the Republic of Ireland through closer controls on both legal and illegal introductions of live cyprinids. An assessment of the likely methods of introduction should be carried out and legislation changed if necessary. A publicity campaign is required to alert users of the aquatic environment to potential passive transfer. Once introduced, eradication is very difficult. In enclosed waters other species can be temporarily removed and the water treated with rotenone (plant-derived piscicide) to kill all the topmouth gudgeon. This has been successfully undertaken in a lake in Cumbria. In rivers, the number of topmouth gudgeon has been reduced through removal by electrofishing. These control measures can only be undertaken by authorised and qualified personnel. Full eradication is highly dependent on early detection.
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 Northern Ireland
Loughs Agency
Department of Communications, Marine and Natural Resources Republic of Ireland
Central and Regional Fisheries Boards Republic of Ireland
Further information
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 —
Pinder (2005). Larval taxonomy and early identification of topmouth gudgeon Pseudorasbora parva: a screening tool to reduce risks of unintentional species translocation. Fisheries Management and Ecology 12: 99-104.
Gozlan, R.E., St-Hilaire, S., Feist, S.W., Martin, P. & Kent, M.L. (2005). Biodiversity: Disease threat to European fish. Nature 435: 1046.
Britton, J.R. and Brazier, M. (2005). Eradicating the invasive topmouth gudgeon, Pseudorasbora parva, from a recreational fishery in Northern England. Fisheries Management and Ecology 13: 329-335.
Allen, Y., Kirby, S., Copp, G.H. and Brazier, M. (2005). Toxicity of rotenone to topmouth gudgeon for eradication of this non-native species from a tarn in Cumbria, England. Fisheries Management and Ecology 13: 337-340.
See the Invasive Species Ireland web site for further information -
Text written by:
Prof. Andy Ferguson