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A small, slender fish with moderately thick lips, a small protrusible mouth and short, blunt gill rakers. Maximum size 78 mm; usually 50–60 mm. The two small dorsal fins are short-based and the second is directly above the anal fin. The tail is forked, the pectoral fins are positioned high on the body and the anal fin has 6–9 rays. The midlateral scale count is generally 32–35 and the transverse scale count is 7–7.5. The scales on top of the head are large and irregular in shape.
Body colour varies between localities and seasons. There is a dusky stripe from the snout, through the eye and operculum, extending to the base of the caudal fin, although this stripe may be black, golden or silvery. Males may be bright golden-yellow during the spawning season. This species was often previously referred to as the Flyspecked hardyhead. However, this name is not applicable to the subspecies that occurs in the Murray-Darling Basin, which generally lacks the dark spots of the northern, coastal subspecies.
Misidentification with the sympatric Murray hardyhead is common. Unspecked hardyhead generally have a more acute angular snout, than Murray hardyhead. The dorsal scales of Unspecked hardyhead appear diamond-shaped and are arranged in uniform rows along the shoulder and sides, and generally exhibit pigmentation throughout the scale. Murray hardyhead scales appear more rounded and are arranged in a haphazard pattern on the shoulder posterior of the first dorsal fin and pigment is generally limited to the outer margins of Murray hardyhead scales.
The Unspecked hardyhead is often collected around the margins of slow-flowing, lowland rivers, and in lakes, backwaters and billabongs. It prefers slow-flowing or still habitats with aquatic vegetation and woody debris, and sand, gravel or mud substrates. Populations are found in larger, deeper, permanent wetlands in the Murray River in SA, but whether this indicates preference or persistence is unknown. Males mature at ~ 37 mm and females at ~24 mm (SL) with both sexes likely reaching maturity in their first year of life.
It spawns from September to March, with a peak in spring when water temperatures are above 24°C and is capable of multiple “batch” spawning events (eggs at different stages of development are typical in the ovary of ripe females). Fecundity is low, with only 20–107 eggs laid per batch. The eggs are transparent and demersal or attached to aquatic vegetation higher in the water column, with filamentous adhesive strands. Eggs are approximately 1.3–1.7 mm diameter and larvae are 3–5 mm TL on hatching, after 4–7 days incubation.
A schooling species, it is often detected moving between main river channels and off-channel wetlands in the Lower Murray during periods of lateral connectivity. Little is known of its movements longitudinally up and down rivers although fish >40 mm have been detected moving upstream through fishways in the Murray and Murrumbidgee rivers with most movement in the afternoon or dusk. Low light levels (e.g. in road culverts) could be a barrier to movement by this species.
The species is carnivorous, eating small insects such as mosquito larvae, and microcrustaceans (plankton). In turn, it is eaten by birds and larger fish such as Golden perch, forming an important link in aquatic food webs. Lifespan is poorly known, but in aquaria can live for ~ 5 years. In the wild, few fish probably exceed two years of age.
Unspecked hardyhead is distributed throughout lowland river and wetland habitats of the Basin and is abundant and widely distributed in wetlands and main channel habitats of the Murray River in SA and wetlands in the mid-Murray. The species was the second most abundant fish captured (~16,000 individuals, 12 % of the total catch) in wetland monitoring in SA from 2004-2007 and occurred at 79% of all wetlands sampled. In the Sustainable Rivers Audit (2004 – 2013) it was the 6th most abundant native species overall (1.9% of total fish catch) with 4154 individuals caught across 13 river valleys and 72% and 22% coming from lowlands and slopes respectively. 7762 individuals were caught across 11 river valleys in the MDB Fish Survey (2014/15–2021/22). Formerly intermittently abundant, the species appears to have suffered a reduction in distribution. Temporal patterns in hydrology and habitat can, however, contribute to substantial variation in relative abundance.
In the southern MDB, populations are present from the Lower Lakes (Alexandrina and Albert) extending upstream in the Murray River to Albury, in the Murrumbidgee upstream to Wagga, and the Lachlan upstream to Parkes. They are also present in many river valleys of the northern Basin. The species is known from some Vic tributaries (e.g. Broken Creek, Loddon and Goulbourn rivers), but it is rare or uncommon. Pre-1913 collections of ‘hardyheads’ by David Stead from upland sites (~780 m; the Cudegong above Rawden; the Murrumbidgee near Cooma) may also have been of this species, but they were likely misidentifications.
The reasons for decline in abundance or range are likely to include cumulative impacts of salinisation (which affects food and habitat availability), isolation and degradation of habitat, localised impacts of obstructed fish passage, and impacts of alien species such as Carp, Eastern gambusia and Redfin perch. Large aggregations below weirs without fishways can be subject to significant bird predation.
Allen et al. 2002; Barrett 2008; Baumgartner 2003;
Conallin et al. 2011; Harris & Gehrke 1997; Higham et al. 2005; Hutchison et al. 2020; Ivantsoff & Crowley 1996; Jones et al. 2017; Llewellyn 1979; McCasker 2009; McCasker et al. 2014; MDBC 2004b; Moy et al. 2020b; Semple 1985; Smith et al. 2009; Wedderburn & Hammer 2003; Wedderburn & Suitor 2012; Wedderburn et al. 2007, 2008; Wilson & Ellison 2010.
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