Darling River hardyhead

Identification

A small, laterally compressed fish with a small protrusible mouth, thin lips, large silvery eye and moderately short, blunt gill rakers. Maximum size 80-90 mm TL. There are two small, short-based dorsal fins, with the second directly above the anal fin. The tail is forked, the pectoral fins are positioned high on the body and the anal fin has 5–8 rays. The scales are small and rarely overlap; the mid-lateral scale count is 37–38 and the transverse count 14–18. There are usually no scales on top of head, but if present they are small and circular.

The back is dusky gold, the mid-lateral stripe is dark and silvery with silvery gold below, and the ventral surface is always paler, with a silvery sheen. The opercula are bright silver. The Darling River hardyhead is easily distinguished from other hardyheads by its large number (up to 17) of transverse scales.

Biology and Habitat

The Darling River hardyhead is found in slow-flowing, clear, shallow waters or adjacent to  aquatic vegetation at the edge of such waters. It has also been recorded from the edges of faster-flowing habitats, such as runs at the head of pools. Little is known of its life history.

A study in the Macintyre River found the preferred habitat of pre-spawning adults was wide, deep, slow flowing pools with sandy or cobble bottoms. Fish were mainly found near the bank or out to about 30% of the river width, with an average water depth of 500 mm. Riffle habitats (high velocity, shallow, narrow width) were not selected, and overhanging exotic riparian vegetation and instream woody debris were non-preferred. During the day fish were usually observed at the bottom of large, open pools, and/or hiding under rocks, but at night occupy shallower habitats, at a range of depths. In contrast, another NSW dataset suggests that presence of this species is more often associated with flowing waters (riffles and rapids) and the presence of filamentous algae.  

In the Macintyre River small subadult specimens (9–24mm) first appear in November, often coinciding with the disappearance or significant reduction of adult fish, which were always present up until September/October. There appears to be a short, relatively predictable breeding season (late Sept-early Oct) followed by adult die-off or decline, suggesting that this species might generally have a lifespan of only 1 year (like Murray hardyhead). However, the presence of larger individuals suggests that some may live for 2-3 years, and they have been kept in in captivity for 5 years. Spawning corresponds with increasing water temperature and daylength.

Nothing is known of critical nursery habitats for either larval or juvenile fish. In aquaria courtship behaviour has been observed with both sexes briefly turning bright gold with the male following the female around, nudging the rear of her body, and sometimes then darting in front of the female to perform a small ‘dance’. Spawning is similar to rainbowfish with both sexes side by side releasing eggs and milt as they move through/across the spawning substrate.

The species can be found singly or in small or large schools (50+ fish). Major dietary items are algae (48% diet composition), dipteran larvae (34%) with stonefly larvae (3%) and other small insects (15%). In the Macintyre River, diet varied between upstream and downstream sites, with diet diversity increasing in a downstream direction; upstream diet was dominated by aquatic macroinvertebrates (midge larvae and mayfly nymphs) and downstream diet was dominated by detritus and ostracods.  

Distribution and Abundance

This hardyhead is found in the northern Basin in the upper tributaries of the Darling River in NSW (Namoi, Gwydir, Macintyre catchments). It is also known from parts of the Hunter system in coastal NSW but is extremely rare or possibly now extinct. The species was collected at 11% of >250 sites sampled above 200 m elevation in the Border Rivers, Gwydir and Namoi catchments between 2002 and 2011. In the Macintyre River, Darling River hardyhead is abundant at sites where riparian cover is moderate to good, while appearing to avoid part of the channel where the riparian cover is either poor or dominated by willows and other exotic species.

The 2017-19 drought resulted in drying of many small upland tributaries where this species occurs, and populations declined or were lost, with the upper Namoi and Gwydir populations severely impacted.

Generic river health sampling is not particularly effective for this species with the NSW Rivers Survey in the mid-1990s failing to record any individuals, as did the Pilot Sustainable Rivers Audit (SRA) sampling in the early 2000s of ‘best available’ sites in the northern Basin.  In the subsequent 11 years of sampling for the SRA, a total of 144 individuals were recorded: 70 from the Border Rivers (Macintyre, Dumaresq, Severn), 64 from the Namoi and 10 from the Gwydir. The MDB Fish Survey (2014/15–2021/22) recorded 60 individuals, 41 from the Gwydir and 19 from the Border Rivers. In NSW the species has been recorded between about 250–1030 m elevation.

Potential Threats

The relatively recent recognition of this species means that any historic declines have not been documented. Frequent, prolonged, or severe drought and water abstraction may be an issue in the smaller creeks that it occupies, and protection of deep pools (preferred habitat) as refuges is important. Sedimentation is likely a significant threat as it will reduce pool depth and remove hiding spaces under rocks. Spread of exotic riparian vegetation may also reduce the local distribution of the species. Other hardyheads are potentially threatened by habitat degradation, altered flow regimes and impacts of alien species such as Eastern gambusia.

General References

Adams et al. 2011; Crowley & Ivantsoff 1990; Gehrke & Harris 2004; Gilligan & Moy 2019; Harris & Gehrke 1997; Ivantsoff & Crowley 1996; Moy 2013; Moy et al. 2018, 2020a; NSW FSC 2014; Unmack & Dowling 2010; Wilson & Ellison 2010; D. Gilligan, unpubl. data; K. Moy unpubl. data and aquaria observations.  

See complete reference list