A small fish with a broad, flat head and large mouth. Maximum size 115 mm; usually 80 mm. The eyes are positioned close together, high on the head. The gill openings are broad, extending forward to, or below the eye. There are two separate dorsal fins and a rounded tail. The dorsal colour is variable: it may be grey, brown, black, yellowish or reddish brown, often with a series of darker blotches on the back, sides and below the dorsal fins. The belly is usually lighter and may be yellowish. The anal and dorsal fins often have a series of faint, grey stripes with orange areas in between. There is often a black blotch at the base of the caudal fin.
The mouth in males is larger, extending to at least below the pupil, whereas in the female it extends to below the front of the eye. Flat-headed gudgeon and Dwarf flat-headed gudgeon are sympatric throughout most of their range, and small Flat-headed gudgeon can be confused with its smaller relative. It can be readily distinguished from the Dwarf flat-headed gudgeon by its larger size; the presence of gill openings on the underside of the head that extend forward to, or below the eye; the eyes close together, and the larger number of pectoral fin rays (16–20, usually 18–19).
This benthic species has generalist habitat requirements, being found in slow-flowing areas in a range of aquatic habitats from lowland rivers, streams, wetlands or lakes, billabongs and dams and is often found in weedy or muddy areas with abundant cover in the form or rocks or logs. In wetlands in the Lower Murray in SA, it was more abundant in permanent (rather than ephemeral) wetlands.
This is a relatively short-lived, quick growing species, maturing at 42–50 mm length at ~1 year old, with females probably living for 2 years and males a bit longer. It breeds from spring to mid-autumn when water temperatures are between 18 and 28°C and rising. Fecundity ranges from 120–2020 eggs which are attached to solid objects such as rocks and wood and aggressively guarded by the male, which fans the eggs with its pectoral fins. The eggs are elongate, 1.2–2.2 mm long and 0.7–0.9 mm wide, and pointed at one end. They hatch after ~3.5–5 days at temperatures of 15.9–22.6 °C and the newly hatched larvae are 3.15–4.3 mm long. This species comprised 96% of the drifting larval fish fauna in the Campaspe River between 1995 and 2001, with the majority of this downstream drift occurring in late spring/early summer. Flow regime appears to play little part in breeding, and the species does not routinely utilise the floodplain for larval development.
The species is more benthic and a weaker swimmer than the pelagic Common galaxias or Australian smelt. Little is known of longitudinal riverine movements, but it is known to move between the main river channel and wetlands in both the Upper and Lower Murray. In coastal streams, mass migrations have been recorded, between estuarine and freshwater reaches across weirs and other barriers to movement. It is a carnivorous ambush predator of aquatic insects, molluscs, tadpoles, crustaceans and small fish. Its large mouth gape allows it to differentiate its diet from other coexisting small fish species, allowing it to consume larger prey.
A recent study in Lake Alexandrina noted predation of flat-headed gudgeon by both Golden perch and Redfin perch, and similarly in the Coorong it was a significant dietary item for larger Congolli suggesting the species may be important in trophic dynamics.
The Flat-headed gudgeon is found across most of the southern half of the MDB, and the Macquarie River in the northern Basin. There are very rare records from the Darling (downstream of Toorale) and Macintyre rivers, but whether these represent remnant populations or translocated fish is unknown. It is common in the main channel, wetlands and tributaries of the Lower Murray; along the edges of the Lower Lakes; and periodically in the Coorong in SA. Indeed, it was the most commonly captured species in the Lower Lakes from 2009–2012. It is largely absent from upland areas and is not present in the ACT or Qld portions of the Basin.
It was recorded from 16 river valleys in the Sustainable Rivers Audit (2004–2013) with only 20 out of 2197 individuals captured found in the uplands (400–700 m elevation), with 54% from the lowlands (0–200 m) and 45% from slopes (200–400 m). 1391 individuals were caught in 13 river valleys in the MDB Fish Survey (2014/15–2021/22) with 82% captured in the slopes elevational zone. It appeared to decline across much of the Basin in NSW during the early 1990s (except in the Murray) but the species appears to be bouncing back in this State, particularly in the Lachlan and Macquarie drainages. In the Murrumbidgee catchment it is found up to Burrinjuck Reservoir and is relatively common in the Murray downstream from Hume Dam. It is common in Vic tributaries from the Mitta Mitta west to the Wimmera River and the lowermost Ovens River. It is also frequently present in coastal streams in SA, Vic, NSW and Qld.
None known. Predation by alien species such as Redfin perch can be significant, but it rarely eliminates the species.
Baumgartner et al. 2008; Becker & Laurenson 2007; Cadwallader & Backhouse 1983; Conallin et al. 2011; Giatas & Ye 2015; Harris & Gehrke 1997; Higham et al. 2005; Humphries et al. 2002, 2008; Kilsby & Walker 2010, 2012; Larson & Hoese 1996a: Lintermans & Phillips 2004; Llewellyn 2007; Lloyd & Walker 1986; Lyon et al. 2010; Pollard 1973; Pusey et al. 2004; Smith et al. 2009; Thacker et al. 2008; Wedderburn & Barnes 2016; Wedderburn et al. 2014. Wilson et al. 2008.
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