A relatively deep-bodied, laterally-compressed fish with a high arched back and a single, short-based dorsal fin placed about midway down the back. The dorsal fin is not rounded like in the Tench. Maximum size in Europe is 450 mm and two kg, but usually much smaller at 150–200 mm. The caudal peduncle is relatively slender, and the tail is distinctly forked. There are no barbels and the mouth is small, with the gape not reaching to the front of the eye. The eye is small and bright red.
There are small axillary processes at the base of the pelvic fins. The scales are of moderate-size. The back is usually an olive-green, the sides silvery and the belly whitish to pale golden or silvery; the fins are red.
Relatively little is known of the ecology of this species in Australia, with most knowledge coming from the study of populations in Lake Eildon and Dandenong Creek catchment in Vic. In its natural habitat in Europe, it swims in schools in ponds, lakes and slow-flowing rivers. It is often found in close association with aquatic plants; however habitat usage patterns vary seasonally, possibly in response to food availability, temperature preferences or predator avoidance.
Roach may live for up to 12 years in Europe, with the average size and age at maturity in Lake Eildon being 161mm and 3.6 years for males and 156mm and 3.3 years for females. Length and age at maturity is substantially larger and older than generally recorded in Europe where males mature at 74–135mm and 1–2 years of age and females at 90–153 mm and 2–3 years. In Lake Eildon females are generally larger than males following maturity and are on average 3 times more abundant than males; the proportion of females increases with fish size.
Overseas the species spawns in shallow water amongst aquatic plants or on hard substrates. In Lake Eildon spawning occurred in September/October when surface water temperatures ranged from 9.4–16.0°C. The eggs are small (~1–1.5 mm diameter), transparent and slightly adhesive. Fecundity in mature females in Europe ranges from 5,000 to 200,000 but is much lower in Australia with mean annual fecundity in Lake Eildon being 4500 eggs (range 2700–7000). Eggs hatch in 4–10 days depending on water temperature, newly hatched larvae are 5–6 mm long. In Europe, juveniles feed predominantly on plankton, and adults on animal and plant material. In lakes in Europe, Roach are known to be efficient predators of molluscs and are important planktivores.
However, in Australian they are mainly benthic feeders, also taking terrestrial insects from the water surface. A study in Dandenong Creek found on average equal proportions of animal and plant material were consumed across all size ranges and seasons, but that larger (200–250 mm fork length) individuals consumed considerably more plant than animal material. Common animal dietary items were beetles and snails, with plant material more important in the warmer months. Roach have been recorded in trout stomachs and are probably eaten by native fish such as Golden perch and Murray cod.
Little is known of the impacts of Roach on native fish species in Australia. It commonly occurs with other alien species but is suspected to compete with Australian native fish for food and habitat. The species may act as a host to the bacterial disease Aeromonas salmonicida which affects salmonids, and other cyprinids (Carp and Goldfish) and possibly Australian natives. Predation of native fishes by Roach is not considered to be an issue in Australia.
Roach is native to Europe and were introduced to Australia in 1861. The species is mainly confined to the coastal waters of Vic around Melbourne but is common in some Victorian waters of the Murray-Darling Basin (parts of the Goulburn River). Previous literature reports from NSW are now thought to be incorrect, as no museum or other records can be located. It is not known from the ACT, Qld or SA.
In the Sustainable Rivers Audit monitoring in the Basin from 2004–2013, a total of 102 Roach were caught (0.05% of the total catch), all from the slopes elevational zone (200–400 m ASL) and only in the Campaspe (78 fish) and Goulburn (28 fish) valleys. The MDB Fish Survey (2014/15–2021/22) captured a total of 25 individuals, with 24 coming from the Campaspe valley and 1 from the Loddon.
The species was originally introduced by acclimatisation societies but is rarely fished for now. The species distribution in Australia is unlikely to expand significantly under global warming as it is already nearing its maximum thermal tolerance. However, upland streams and deeper lakes (that provide some refugia from high summer water temperatures) may provide scope for further expansion.
Baade & Frederich 1998; Bean & Winfield; 1995; Brumley 1996; Cadwallader & Backhouse 1983; Clements 1988; Jepsen & Berg 2002; Kahn et al. 1999; Marcum 2007; Merrick & Schmida 1984; Rowe et al. 2008; Stoessel 2007, 2014; Weatherley and Lake 1967.
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