A small, laterally compressed fish with a small protrusible mouth, large silvery eye and short, blunt gill rakers. Maximum size 95 mm, commonly 30–65 mm. The two dorsal fins are small and short-based, and the second is directly above the anal fin. The tail is forked and the pectoral fins are positioned high on the body. The mid-lateral scale count is 31–35 and the transverse scale count is 10–12.
The scales on top of the head are large and irregular in shape. The back is generally silver, occasionally golden, and the ventral surface is generally paler with a silvery sheen. There is a silvery-black stripe mid-laterally, and the opercula are bright silver. The scales on the back are round with dark pigment around the margins giving a reticulated appearance. This distinguishes Murray hardyhead from the sympatric Unspecked hardyhead for which the scales appear more diamond-shaped, are arranged in neat rows and (frequently) contain pigment spots throughout the scale rather than just around the margins. The body and fins may develop a golden-orange sheen during the spawning period.
Inhabit lakes, wetlands, backwaters and billabongs. Murray hardyhead prefers slow flowing or still habitats, with sand or silt substrates. Often associated with dense aquatic vegetation (e.g. Ruppia, Myriophyllum or Vallesnaria ) but can persist and breed in habitats without vegetation (possibly using submerged terrestrial structure for cover and breeding substrate). The species appears to thrive in ephemeral systems and can tolerate saline environments (as high as 98,000 EC, seawater is ~55,000 EC) where relatively few other freshwater fish species are found. Murray hardyhead have been recorded in waters with temperatures from to 5–33°C and with dissolved oxygen concentrations as low as 2.5 mg/L.
Mature males have been recorded as small as 27 mm caudal fork length and females at 41mm with fish maturing in their first year of life. Adults may spawn from September–March although breeding usually occurs in spring–early summer. The species is a batch spawner, with documentation of 10-100 adhesive eggs per clutch deposited on submerged structure (although larger batches are likely). Hatching takes 5-10 days after fertilisation in waters above 20 °C.
The Murray hardyhead is a largely annual species, although some individuals survive into their second year. In ideal conditions, individuals spawned early in a breeding season appear able to reach maturity in the same season and may breed themselves as late as March. It is omnivorous, eating primarily microcrustaceans but also aquatic insects and algae, gape permitting. Flooding of off-channel habitats can result in large zooplankton blooms, augmenting recruitment success. It is usually found in schools of distinct size classes. Little is known of its movements although exiting of saline habitats into fresher inflows has been observed, which may indicate historic dispersal behaviour.
Murray hardyhead can be locally abundant under favourable conditions, particularly after freshwater flows in spring-early summer deliver increased productivity, and habitat availability to facilitate successful spawning and recruitment. In exceptional circumstances, catches of >200,000 individuals have been made in one night, when increased salinity deletes competitors like Carp.
The species persists only in the lowland floodplain areas of the southern Basin: in the mid to lower Murray River from its mouth as far upstream as the Kerang Lakes region. Formerly considered widespread and abundant in the Murray and Murrumbidgee river systems in southern New South Wales and northern Victoria, it has suffered a significant reduction in distribution and in 2012 was nationally listed as endangered. It is probably extinct in the Murrumbidgee River system and aside from a translocated population, since 2000 only a single individual has been collected in NSW despite extensive surveys.
Intermittently abundant at some locations, it is still present in a small number of lakes near Swan Hill and Mildura in Vic, several wetlands/saline basins in the South Australian Riverland near Renmark, and in the Lower Lakes of SA.
The species was impacted by the Millennium Drought, with several isolated populations extirpated by declining surface water and increasing salinity. Murray hardyhead has been successfully translocated to several managed locations in NSW, Vic and SA to establish new populations, which generally require management intervention (in the form of water delivery) to secure viability in dry years.
Only a single individual was recorded from the Sustainable Rivers Audit (2004–2013) (Lower Murray catchment) with none recorded from the MDB Fish Survey (2014/15–2021/22).
The precise reasons for its dramatic decline are not known, but likely include a combination of changed salinity (both increases and decreases), habitat degradation, altered flow regimes (decreasing connectivity with floodplain lakes), drought, changes to irrigation practices and impacts of alien species such as Eastern gambusia.
Adams et al. 2011; Crowley & Ivantsoff 1990; Ebner et al. 2003; Ellis 2005; Ellis et al. 2013, 2020, 2022; Ellis & Kavanagh 2014; Gilligan 2005a; Ivantsoff & Crowley 1996; Koehn et al. 2020a; Lintermans et al. 2015; Lyon & Ryan 2005; Stoessel et al. 2019, 2020a; Thiele et al. 2020; Unmack & Dowling 2010; Wedderburn & Hammer 2003; Wedderburn et al. 2007, 2008, 2013; Whiterod et al. 2019; Whiterod & Huntley 2022; Zukowski et al. 2021.
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