A medium-sized fish with a forked tail. Maximum size 1,200 mm (Length to Caudal Fork; LCF) and 60 kg; usually up to 4–5 kg. The mouth is of moderate size, with thick fleshy lips and two pairs of barbels (‘whiskers’) at the corners. The single long, low dorsal fin has a stout serrated spine at the front. The scales are large and thick. Some individuals are only partly scaled, with very large scales in three or four rows ('mirror carp'), and some have no scales (‘leather carp’). The back is usually olive-green, silvery-grey or brownish and the belly is creamy or silvery-yellow.
Carp is usually associated with warm, slow-flowing lowland rivers or lakes (<700 m ASL) but is now also found in clear, cool fast-flowing streams. In off-channel habitats both adults and juveniles are commonly associated with aquatic macrophytes, and in rivers with woody habitats. It is tolerant of a wide range of environmental conditions (salinities up to ~40% of seawater) and able to survive extremely low levels of dissolved oxygen.
Carp can be long-lived, with individuals >30 years old recorded in Australia and >50 years overseas. Males are sexually mature at 2–3 years (300 mm LCF) and females at 3–4 years (350 mm LCF). Spawning usually occurs in spring and summer when water temperatures are 17–25°C. Spawning fish congregate in shallow water and adhesive eggs are laid in clumps on freshwater vegetation, logs, and submerged grass. Eggs are 0.5–1.5 mm in diameter and hatch in 2–8 days, depending on water temperature. A 6 kg female can contain 1,500,000 eggs. Large wetland areas such as the Macquarie Marshes, Namoi, and Gwydir wetlands, and Barmah-Millewa Forest, are significant ‘hotspots’ for Carp breeding, although Carp will readily spawn in riverine habitats without floodplain or wetland habitats. Inundation of floodplains provide conditions suitable for large recruitment pulses that drive populations.
Carp feed by ‘mumbling’ in the sediment on the bottom or banks of water bodies. This involves sucking in sediment, sorting the edible items from the inedible sediment, and expelling the sediment through the gill openings. Diet includes freshwater insect larvae, zooplankton, crustaceans, molluscs and to a lesser extent plant material. Carp are highly mobile having a flexible and complex movement ecology, with some fish recorded moving 650 km in the Murray River but many being relatively sedentary, showing fidelity to spawning and refuge sites. Carp move from rivers into floodplain habitats when flooded, where shallow, warm, spawning habitats and food resources are plentiful. In winter adults’ school in deeper pools, refuge habitats and lakes.
The impacts of Carp are not clear, but its feeding behaviours can increase turbidity levels, undermine riverbanks and damage aquatic vegetation. They can alter zooplankton levels, exacerbate algal blooms and in high abundances are likely competing with native fish for food and space. In dryland rivers, large impacts on native fish biomass from food resource monopolization by Carp have been shown. It is also thought that Carp predation has resulted in the extirpation of an endangered river snail.
Carp is listed in the 100 world's worst invasive species by the IUCN. Records of carp consuming native fish in Australia are extremely rare, but records exist of feeding on Australian smelt (congregated below a weir) and of a subadult Northern river blackfish in carp stomach contents. Carp have also been accused of being egg predators and disturbing Freshwater catfish nests, but impacts have not been quantified.
Carp also potentially alter aquatic food webs through ‘locking up’ large quantities of carbon, with there being few predators of larger adult Carp. Adult Murray cod prey mostly on juvenile Carp (<200 mm LCF) but large cod (>1m) can eat Carp of ~410 mm LCF. Carp carry a number of introduced parasites which infect both native and alien fish, with the Anchor worm (Lernaea cyprinacea) the most obvious.
Carp are native to central Asia and eastern Europe. They were first introduced into Australia in the mid-1800s but remained in two relatively confined locations. These two populations were different strains of the one species. In the late 1950s, a fish farmer illegally introduced a new strain, Boolarra, and since the early 1960s it has rapidly colonised water- courses throughout SE Australia. A fourth strain, Koi, was introduced in the mid-1970s and is present in the upper Murrumbidgee, upper Lachlan, some NSW and SE Qld coastal catchments, wetlands near Perth and one lake in Tas. The wild Koi strain lacks the bright orange, black or white colouration seen in aquarium Koi. Carp are present in the majority of slopes and lowland rivers and creeks, and in upland streams as well, and often comprise 70–90% of fish biomass. In the Sustainable Rivers Audit monitoring (2004–2013) Carp was caught in every river valley and was the second most abundant fish (behind Bony herring) with 40,548 caught (19% of the total catch). 89.8% were caught in lowland, 7.8% in slopes, 2.2% in upland and 0.2% in montane zones. 13,552 were caught in the MDB Fish Survey (2014/15–2021/22), again in every river valley. It was estimated in 2021 there was between 199 to 357 million carp in Australia (for average and wet years respectively) weighing 205,000 to 368,000 tonnes! The proposed release of cyprinid herpesvirus-3 (CHV-3) may significantly reduce Carp num- bers, but it should not be viewed as a ‘silver bullet’ as it is unlikely to permanently reduce abundance, with populations likely to rebound over time. However, CHV-3 could provide a window of opportunity to bolster native fish populations through other actions such as fish passage, habitat rehabilitation, and improved management of regulated flows.
Beesley et al. 2014a; Brown et al. 2005; Brown & Gilligan 2014; Brumley 1996; Conallin et al. 2012; Crook 2004; Davis et al. 1999; Driver et al. 2005; Ebner 2006; Forsyth et al. 2013; Gilligan et al. 2010a; Harris & Gehrke 1997; Haynes et al. 2009; Jones & Stuart 2009; King et al. 2016; Koehn 2004; Koehn et al. 2000, 2016; Koehn & Nicol 2014, 2016; Lowe et al. 2000; Marshall et al. 2019; McColl & Sunarto 2020; Nichols et al. 2019; Nicol et al. 2004; Stuart & Jones 2002, 2006; Stuart & Conallin 2018; Stuart et al. 2021; L. Pearce unpubl. data.
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