A newly described species. Bald carp gudgeon is the previously unknown sexual lineage (Hypseleotris species X) of the hemi-clonal, hybrid taxa formerly known as Lake’s carp gudgeon. This species has been recently proposed for listing as Critically Endangered under the EPBC Act, and is listed under the IUCN Red List.
Bald carp gudgeon is a small, laterally compressed fish with two distinct dorsal fins and a rounded caudal fin. The dorsal colour of Bald carp gudgeon is yellowish-grey to greenish-brown, whilst the belly is paler. In a sample of 191 individuals sampled in late November 2019 from Urumwalla Creek, Bald carp gudgeon were recorded from 23–49 mm total length with 88% < 40 mm.
Bald carp gudgeon can be distinguished from all other sexual carp gudgeons in the Murray-Darling Basin by possessing a blunt head with dorsal scales absent as far posterior as the middle of second dorsal fin (i.e. no scales on the nape). This extent of dorsal scale absence is in contrast to the newly described Boofhead (formerly part of Midgley’s) carp gudgeon which always has scales on the head and nape. Bald carp gudgeon also lacks pointed median (second dorsal and anal) fin tips of the Cryptic (formerly Murray-Darling) carp gudgeon and the Boofhead carp gudgeon and possesses a unique median fin coloration in mature males (the distal portion of fin bearing band of pale orange, shading into thin band of white).
Bald carp gudgeon forms widespread hemiclonal lineages with the Boofhead and Cryptic carp gudgeon in the MDB. Both of these hemiclonal lineages (previously referred to as Lake’s carp gudgeon) may also lack dorsal scales, but the degree and frequency of ‘baldness’ is less. Bald x Cryptic hemiclones are >90% females (and so not blunt-headed) with the rarer males also not blunt-headed (more like Cryptic’s), and both sexes may lack head and nape scales, but it is variable. Bald x Boofhead male hemiclones are blunt headed (like both parents) and generally lack head and nape scales, but usually only as far back as the middle of the first dorsal fin.
Due to its recent discovery and description as a species, almost nothing is known of the specific ecology of Bald carp gudgeon. However, its biology is almost certainly quite similar to the recently described (but long recognised) Boofhead carp gudgeon (formerly known as Midgley’s carp gudgeon). It occupies the same habitat types as other carp gudgeons (low-gradient, slow-flowing or still waters, with abundant macrophyte beds).
In Meadow Creek it is abundant in a weir pool, and Urumwalla Creek is a small, shallow, rocky, intermittent stream with scattered deeper (1–2.5 m.) pools that form refugia in drought periods. Some sections have good riparian shading, others are fairly open. Urumwalla Creek has abundant macrophytes including Phragmites, Typha, milfoil, rushes and Valisneria. Meadow Creek is more open but with similar aquatic vegetation.
Co-occurring fish species in Meadow Creek are Western carp gudgeon, Australian smelt, Mountain galaxias, Carp, Redfin Perch, and Eastern gambusia. In Urumwalla Creek sympatric species are Mountain galaxias, Southern pygmy perch, and Northern river blackfish. The alien species Carp and Redfin perch occur in Blakney Creek, immediately downstream of the Urumwalla Creek junction.
There are no specific studies on the ecology of this species alone (as opposed to studies on generic Hypseleotris spp). Dietary ecology is expected to be similar to other carp gudgeons.
In the wild, gravid Bald carp gudgeon females (37-39 mm TL) have been observed in late November. Like other carp gudgeons, Bald carp gudgeon can reach high population abundance in suitable stream habitats and also when translocated into suitable farm dams. In captivity, spawning of bald carp gudgeon occurs when temperatures exceed 20˚C during spring-summer, with 200–400 adhesive eggs being deposited on a hard surface. Parental care occurs in all carp gudgeon with males generally guarding and fanning eggs, which usually hatch in a few days.
Most individuals probably only live for 12–18 months, but maximum life expectancy may reach two to three years in the wild. Sexual maturity is at 12 months.
Carp gudgeons are generally found in lowland or mid-elevation streams (<625 m ASL). Bald carp gudgeon is only known from two localities in the upper Lachlan River catchment of south-eastern NSW. It is found in two small creeks: Meadow Creek, near Gunning (elevation 560 m ASL) and Urumwalla Creek (elevation 605 m ASL, a tributary of Blakney Creek) near Dalton.
The species has been translocated (2017–19) to create insurance populations in two small dams immediately west of Gunning and also at a farm dam adjacent to Urumwalla Creek. The locations with Bald carp gudgeon have no other species from the hemiclonal complex. Western carp gudgeon is present at Meadow Creek, but this species does not hybridise with other carp gudgeon taxa.
Previous confusion over carp gudgeon taxonomy means that this species possibly may occur unrecognised in other locations. However extensive sampling and genetic examination of >600 Lake’s carp gudgeon individuals across their range in south-eastern Australia, indicates that unrecognised occurrences of Bald carp gudgeon are unlikely. The distribution map only shows confirmed genetic records of this species (no hemiclones or unverified records).
Bald carp gudgeon has recently been proposed for threatened species listing under the national Environment Protection and Biodiversity Conservation Act 1999.
The species is restricted to two isolated subpopulations in unconnected tributary streams, which have limited capacity to disperse and recolonise sites from which local extinction has occurred. Climate change is likely to increase frequency and severity of extreme events such as fire, flood, and drought. Both streams are in fire prone landscapes, with post-fire sedimentation of streams a major concern. Post-fire sedimentation can impact waterways 50–80 km downstream of the burnt area and have severe effects on water quality and aquatic species. Flooding can potentially drown out barriers (constructed and natural) that limit upstream invasion of non-native fishes such as common carp and redfin perch.
In the severe drought of 2017-19 in NSW, the extent and quality of aquatic habitat in Urumwalla Creek was severely reduced with 65% of the stream channel dry by mid Nov 2019 with the stream continuing to dry until the drought broke in January 2020. Introduction of other carp gudgeon taxa that could hybridise with Bald carp gudgeon is a high risk, primarily through contaminated recreational fish stocking into farm dams, or dispersal by other downstream carp gudgeon taxa during flooding. Hybridisation with other carp gudgeons could overwhelm the species. Invasion of Urumwalla Creek by Redfin perch is of concern, as this species is expected to prey heavily on carp gudgeons. Artificial instream barriers have been constructed on this stream to prevent or slow the upstream invasion of Redfin perch. A local Action Plan for the co-occurring Southern pygmy perch in Blakney and Urumwalla Creek will have conservation benefits for Bald carp gudgeon.
Bylemans et al. 2016; Lintermans & Pearce 2017; Pearce 2015; Thacker et al. 2022; Unmack & Pearce 2019; Unmack et al. 2019; Ward et al. 2022; M. Lintermans unpubl. data; L. Pearce unpubl. data; P. Unmack unpubl. data.
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