A small, elongate, laterally compressed fish. Maximum length 100 mm; commonly 40–60 mm. Slender, silvery and largely transparent apart from a prominent silver-orange to bluish lateral band. The eyes are large, the opercula are silvery and the tail is moderately forked. A small adipose fin is present. The pectoral and pelvic fins of males are larger than in females. Nuptial tubercles are present on the body and head and are larger in males. A cucumber-like odour is apparent to some people in freshly caught individuals. Recent genetic and morphometric studies show that smelt in the lower Basin (downstream of about Mildura) have historically mixed with the Tas smelt species (R. tasmanica) and so are slightly different in fin ray and vertebral counts to the rest of the MDB smelt.
Two species of smelt are currently recognised, but recent genetic investigations suggest that there are as many as five species present in Australia; only one occurs in the Basin. Consequently, some of the information summarised below likely refers to a number of closely related species. Typically, smelt is a schooling, pelagic species in the southern Basin, commonly recorded from slow moving or still water in a variety of habitats (e.g. river channel, wetlands, lakes) where they can be found in large numbers (thousands). They are in highest abundance in lakes or non-flowing environments, but in Queensland it is commonly encountered in riffles or along shorelines in association with fringing vegetation. In Murray River tributaries, larval smelt were collected from a range of habitats, but prefer deeper billabongs. Like several small native fish species, it responds to declining oxygen levels (related to high temperatures, input of organic matter, and stratification) in floodplain or lake environments by utilising the oxygen-rich boundary layer at the water surface (usually the top 1 mm of water). Interestingly, smelt swimming speed varies between reservoir and river populations, with river populations having significantly higher prolonged swimming speed performance. Although smelt have been found to migrate both longitudinally and laterally, these movements are limited, with genetic and otolith studies demonstrating that there is little mixing between subpopulations and movements between catchments are rare. Upstream migrations of juvenile and adults have been recorded during daylight hours in the Murray and Murrumbidgee rivers, with fish as small as 21 mm attempting to migrate and fish of 31 mm successfully moving upstream through fishways. Low light levels (e.g. in road culverts) could be a barrier to smelt movement. Smelt also move laterally, particularly on a rising river level, with fish moving from mainstream into ephemeral off-channel wetland habitats during the day and moving out at night. In the lower Murray, smelt use of off-channel permanent wetlands was also common, and it commonly moved between these habitats and the main river channel. The connection of river and flood- plain habitats provides access to improved food resources and can significantly affect smelt growth rates, but growth varies with the timing and length of connection events.
Both sexes mature towards the end of their first year and may live for two or more years, although most only live for a year. Fecundity ranges from 100 to 1,000 eggs depending on fish size. Smelt are an opportunistic spawner, with a flexible breeding and recruitment strategy that is able to cope with variable flow and temperature regimes. Spawning occurs when water temperatures reach ~11–15°C, generally in spring and early summer in the Basin, but for up to 9 months of the year in the Campaspe River. In the Lower Murray, smelt are multiple batch spawners, and females produce discrete batches of eggs every 3–4 days. The eggs are ~1 mm diameter, demersal and adhesive; sinking and adhering to aquatic vegetation, sediment or debris. Hatching occurs in 9–10 days and the larvae are <5 mm long. Smelt grow quickly, reaching ~15 mm SL after 30 days and 30 mm after 80 days. Smelt are a diurnal carnivore and the diet consists mainly of terrestrial insects and microcrustaceans, although a variety of small aquatic insects are also eaten. Smelt do not tolerate handling well, and considerable care is required to avoid mortality.
Australian smelt is one of the most wide- spread and abundant species at lower and mid altitudes in south-eastern Australia. It is not generally found in upland headwater streams with fast flows in the southern Basin, but occurs in these habitats in the northern parts of its range. In the Murrumbidgee catchment it is rarely recorded above 600 m ASL. In the Basin, it has been recorded from most lowland streams and is also common in coastal streams from central Qld to the Murray mouth in SA. It was the fifth most abundant fish species recorded in wetlands in the lower Murray. In the Sustainable Rivers Audit (2004 – 2013) it was the third most abundant native species overall (behind Bony herring and Carp gudgeons) (4.4% of total fish catch) with 9,640 individuals caught across 22 river valleys (not the Paroo) and 62% and 27% coming from low- lands and slopes respectively. 8,657 individuals were caught across all 23 river valleys in the MDB Fish Survey (2014/15–2021/22).
None known, but barriers to fish passage may fragment populations.
Barrett 2008; Baumgartner 2003; Bice et al. 2016b; Conallin et al. 2011; Ebner et al. 2009c; Hammer et al. 2007; Hughes et al. 2014; Humphries et al. 2002, 2008, 2013; Jones et al. 2017; Keep et al. 2021; King et al. 2016; Leigh 2002; Lyon et al. 2010; Mallen-Cooper 1994; McDowall 1996a; Milton & Arthington 1985; McNeil & Closs 2007; Moffat & Voller 2002; Pusey et al. 2004; Smith et al. 2009; Stuart et al. 2008; Svozil et al. 2020; Tonkin et al. 2011; Woods et al. 2010; P. Unmack unpubl. data.
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