A small fish with a forked tail and small pectoral fins. Maximum size is 146 mm; rarely exceeds 100 mm. The anal fin origin is almost directly below the dorsal fin and it has a longer fin base than the dorsal fin. The back and sides are olive-green and the belly silvery. The fins are colourless but pigment spots are often present at the base of the fin rays. The top of the head is distinctly flattened; the jaws are equal or the lower jaw is slightly protruding; and the gill covers are silvery. The mouth is very large with the gape extending to well below the eye. The distribution of the Flat- headed galaxias overlaps with the Obscure galaxias, and misidentification is possible. Flat-headed galaxias can be distinguished by its long and flat head, and the position of the origin of its anal fin, which is almost directly below the dorsal fin origin, whereas in the Obscure galaxias, the anal fin origin is distinctly posterior to the dorsal fin origin.
Little is known of the ecology of Flat-headed galaxias other than aspects of its reproduction and response to hypoxia. Historically, it was collected from a variety of habitats including billabongs, lakes, swamps and rivers, usually in still or slow-flowing waters. Like several other small native fish species, it responds to declining oxygen levels (related to high temperatures, input of organic matter, stratification etc.) by coming to the surface to utilise the oxygen-rich boundary layer at the water surface (usually to top 1 mm of water). It is a schooling species that congregates in mid-water. It spawns August–September when water temperatures are above 10.5°C. Fecundity increases with increasing fish length: an 86 mm fish has 2,300 eggs and a 136 mm fish 7,000 eggs. The eggs are round, demersal and slightly adhesive, with egg diameters between 1.3 and 1.6 mm. Eggs are spawned randomly and settle on the bottom, hatching in 8–9 days. At hatching, the larvae are 5.7–8.1 mm in length. Individuals probably mature in their first year, at lengths around 80 mm. The reproductive organs of ripe individuals are large, and can make up about 20–40% of total body weight.
The diet is predominantly aquatic insects with some microcrustaceans. Movement requirements are unknown, but there is a suggestion they may school and move upstream in November–December.
The Flat-headed galaxias is only known from the southern Murray–Darling Basin where it has been recorded patchily. It was known in NSW from the billabongs and main channel of the upper Murray (near Albury) and a variety of habitats in the mid to lower Murrumbidgee River. It was also recorded from a lagoon near Bathurst in the late 1800s, and a single record from near Lake Brewster on the Lachlan in 1972. The population at Albury was last recorded in 2003, and in the Murrumbidgee in 1995. The species was listed as critically endangered in NSW in 2008 and critically endangered nationally in 2016. There is an old record from the Lower Murray in SA. In Vic the species has been recorded from the Kerang Lakes, the Broken, Goulburn, Ovens, Kiewa and upper-Murray catchments and as far west as Avoca River. The species was not collected during the NSW Rivers Survey of 80 sites across the State between 1994 and 1996. Only 2 individuals were collected in the Sustainable Rivers Audit across the Basin (2004–2013) with a further 2 collected by the MDB Fish Survey (2014-2021), all from the Goulburn catchment in Vic. Even at sites where it was known to be historically common, it was a difficult fish to sample adequately, but there is no doubt that it has declined across its range. It is presumed locally extinct in the Lachlan, Murrumbidgee and lower Murray catchments in New South Wales and the lower Murray River in South Australia. It does not occur in the ACT or Qld. A recent targeted survey for the species of 60 sites in Vic detected them at 4 sites: all in the Goulburn or Upper Murray catchments. New approaches to detecting the species utilising eDNA will hopefully shed new insights into the current distribution of this elusive species.
Possibly competition or predation from introduced species such as Redfin perch, trout and Eastern gambusia. River regulation (coldwater pollution and altered flow regimes) and reduced fish passage (alienation of floodplain habitats) may impact this species. The effects of the Climbing galaxias—which has been transferred to inland waters via the Snowy Mountains Scheme—on natural galaxiid populations is unknown, but competition or displacement of the remnant Albury population is possible. Competition and habitat degradation from Carp (e.g. loss of aquatic vegetation) also threaten this species.
Allen et al. 2002; Gilligan et al. 2019b; Kennard et al. 2001; Koehn & O’Connor 1990; Llewellyn 2005; McDowall & Fulton 1996; McNeil & Closs 2007; Merrick & Schmida 1984; Pearce et al. 2018; Raadik 2008b; TSSC 2016; D Stoessel et al. unpubl. data.
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