A small, semi-transparent, slender fish with
a small, barely protrusible mouth, thin lips, large silvery eye and long gill rakers (> half the diameter of the pupil). Maximum size 107 mm; commonly <80 mm. The two small dorsal fins are short-based, with the second above or slightly behind the origin of the anal fin. The tail is forked, the pectoral fins are positioned high on the body and the anal fin has 7–12 rays. The midlateral scale count is 36–41 and the transverse scale count is 7.
Body colour is normally silver but can be green or green-brown above the midlateral stripe, and light green, yellow, white or silvery below. The opercula are bright silver. Males develop intense, bright-orange lateral stripes during the spawning season.
The Small-mouthed hardyhead occurs in estuaries but has a very wide salinity tolerance and is thus capable of living in inland habitats such as lakes at the lower end of freshwater rivers. It has been recorded in the wild at salinities more than four times sea water, making it one of the most salt-tolerant fish species known.
It is normally found in brackish lakes, lagoons and estuaries in still or slow flowing habitats, and adjacent marine habitats. It is common in estuarine eel-grass habitats. In the Lower Lakes (Alexandrina and Albert) it is recorded in edge habitats (e.g. near reeds), but there is little sampling effort for small-bodied fish in the middle of lakes, so this may be a sampling artefact. In the Coorong it was the most abundant forage fish species recorded in 2013-14 (67% of the total catch) where it was particularly abundant in the more saline South Lagoon.
It is largely an annual species, breeding in spring and early summer, with the number of larger fish then declining in early to mid-summer as a result of post-spawning mortality. It reaches sexual maturity at 45 mm length and is a multiple batch spawner with an extended spawning season of four months (Sept–Dec). During reproduction only one ovary develops. Recruitment in the Coorong varied between years during the Millennium Drought, with greater recruitment recorded in years with lower salinity (i.e. <~100 ppt), with most recruitment occurring at salinities of 80–100 ppt. The eggs are large (1.6–2.5 mm diameter) and adhesive which could be attached to macrophytes. Spawning in estuaries may be cued by freshwater inflows.
Small-mouthed hardyhead diet in the Coorong is dominated by microcrustaceans, particularly copepods, ostracods (seed shrimps) and amphipods (commonly known as land hoppers or beach hoppers) with these three groups occurring in 65%, 58% and 53% of fish respectively. Other common prey items include chironomids (midge larvae) (32%) and other Diptera (21%).
Small-mouthed hardyhead is often found in the Lower Lakes with other estuarine species such as Lagoon goby, Western blue-spot goby, the diadromous Common galaxias, and the freshwater species Australian smelt and alien Redfin perch. The species is an important food item for many estuarine fish and bird species, including the vulnerable south-eastern fairy tern Sternula nereis nereis and two waterfowl species (Chestnut teal Anas castanea and Grey teal Anas gracilis). Small-mouthed hardyhead were the 3rd most important identifiable fish species in the diet of small Mulloway (<400 mm) and 2nd most important in diet of Australian salmon in the Coorong.
The Small-mouthed hardyhead is a common and widespread species in coastal streams of southeast Australia. In the Basin, it is only known from the Coorong and Lower Lakes of SA and a small distance upstream along the main channel of the Murray and associated wetlands.
During the later stages of the Millennium Drought abundances were high in the Lower Lakes but then declined dramatically following the breaking of the drought as salinity decreased, In the North Lagoon of the Coorong it was the most abundant and widespread fish species during the latter half of the Millennium Drought. No individuals were recorded from the Sustainable Rivers Audit (2004–2013) or the MDB Fish Survey (2014/15–2021/22).
Prolonged loss of freshwater inflows to the Coorong during the Millennium Drought resulted in salinities exceeding 120 ppt for four consecutive years (2007–2010) and a total loss of Small-mouthed hardyhead from >50% of its range. However, once freshwater flows re-established, recovery of the population was rapid (2 years).
Allen et al. 2002; Bice et al. 2016a; Cadwallader & Backhouse 1983; Giatas & Ye 2015; Giatas et al. 2022; Higham et al. 2005; Hossain et al. 2016, 2017; Ivantsoff & Crowley 1996; Molsher et al. 1994; Potter et al. 1986; Smith et al. 2009; Wedderburn & Barnes 2016: Wedderburn & Hammer 2003; Wedderburn & Suitor 2012; Wedderburn et al. 2007, 2014, 2016; Ye et al. 2011, 2022.
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