Carp (Cyprinus carpio) as a powerful invader in Australian waterways JOHN D. KOEHN Cooperative Research Centre for Freshwater Ecology, Arthur Rylah Institute for Environmental Research, Heidelberg, Australia SUMMARY 1. The invasion of carp (Cyprinus carpio L.) in Australia illustrates how quickly an introduced fish species can spread and dominate fish communities. This species has become the most abundant large freshwater fish in south-east Australia, now distributed over more than 1 million km 2 . 2. Carp exhibit most of the traits predicted for a successful invasive fish species. In addition, degradation of aquatic environments in south-east Australia has given them a relative advantage over native species. 3. Derivation of relative measures of 13 species-specific attributes allowed a quantitative comparison between carp and abundant native fish species across five major Australian drainage divisions. In four of six geographical regions analysed, carp differed clearly from native species in their behaviour, resource use and population dynamics. 4. Climate matching was used to predict future range expansion of carp in Australia. All Australian surface waters appear to be climatically suitable for carp. 5. This assessment strongly reinforces the need for immediate management of carp in Australia to include targeted control of human-assisted dispersal, such as use of carp as bait by anglers, distribution to new locations by anglers and the use of the ‘Koi’ strain in the aquarium industry. 6. Given their historical spread, dispersal mechanisms and ecological requirements, the expansion of carp across most of the remainder of Australia is to be expected. Keywords: alien species, conservation, freshwater fish, habitat modification, introduced species, invasive species, risk assessment Introduction The movement of species beyond their natural range has been cited as one of the most pervasive and ecologically damaging of human activities (Lodge, 1993; Wilcove et al., 1998). In predicting scenarios for changes in biodiversity for 2010, Sala et al. (2000) concluded that biological exchange was a relatively more important threat to the biodiversity of freshwater ecosystems than to other ecosystems because of the intentional and unintentional release of organisms. The magnitude of this problem is increasing (Lodge, 1993) and the integrity of aquatic ecosystems is being chal- lenged worldwide by invasive species (Moyle & Light, 1996). Australia has a relatively depauperate fresh- water fish fauna by world standards with slightly over 200 species native (Wager & Jackson, 1993: Allen, Midgley & Allen, 2002) and currently 43 species of alien fish are recorded or established in inland fresh waters in Australia (Koehn & Mackenzie, in press). More than 16% of native species are considered to be under serious conservation threat nationally (Australian Soci- ety for Fish Biology, 2001), and detrimental interactions with alien species are considered to be a threat to 77% of these species (Jackson, Koehn & Wager, 1993). Carp (Cyprinus carpio L.) are native to Eastern Europe and central Asia. Carp, which have been bred Correspondence: John D. Koehn, Cooperative Research Centre for Freshwater Ecology, Arthur Rylah Institute for Environ- mental Research, 123 Brown St Heidelberg 3084, Australia. E-mail: John.Koehn@dse.vic.gov.au Freshwater Biology (2004) 49, 882–894 882 Ó 2004 Blackwell Publishing Ltd