A small, oval fish with a laterally compressed body, moderately large oblique mouth and gape reaching just past the front of the very large eye. Maximum size 68 mm; commonly <50 mm. The tail is forked and the single dorsal fin is deeply notched between the spinous front section and the rayed rear section. The anal fin is directly below the rayed rear section of the dorsal fin. The body is olive to semitransparent and the scales have brownish margins, giving it a reticulated appearance. The fins are transparent.
Olive perchlet inhabits the vegetated edges of lakes, creeks, swamps, wetlands and rivers, and is often associated with aquatic vegetation and woody habitat in areas with little/no flow, particularly backwaters. In the Macintyre River in northern NSW, it was rare in the river, but relatively common in some associated lagoons, after a flow event connected them with the river. Olfactory cues may be used to detect connection events between lagoon and riverine habitats. The species forms small, almost stationary, schools during the day in areas close to instream cover, which disperse during darkness.
Both males and females mature at 1 year old, and live for 2–4 years, and females live longer than males. The reproductive ecology of MDB populations and coastal populations differ somewhat, but a study in the mid Murrumbidgee reported spawning occurred once or twice a year generally from November to early January, when water temperature reached 19–27°C, ceasing when water temperatures >27°C, and then breeding again in February when water temperatures dropped <27°C.
Fecundity is usually 380–9966 eggs but as a batch spawner eggs are often present as two distinct size classes in an ovary. In the mid Murrumbidgee there were on average 2500 small eggs (range 622–7222) and ~1390 larger eggs (487–2607) per female. Eggs are small (0.7 mm diam.), demersal and slightly adhesive. Hatching in coastal populations occurs in 5–7 days at 22°C, but is thought to be much quicker (~24 hrs) in Basin populations. Basin larvae are <2 mm long at hatching. In a Macintyre River lagoon, juveniles of 12–18 mm TL were captured in early autumn, with lagoons considered important spawning/nursery habitat. Juveniles reach ~25 mm TL after 80 days.
Olive perchlet is carnivorous, feeds mainly during daylight hours, eating a range of microcrustaceans (copepods and cladocera), aquatic and terrestrial insects including mosquito wrigglers and small arachnids and, occasionally, small fish. Olive perchlet was the third commonest native fish species in the diet of cormorants in the Lachlan catchment in the 1970s.
This species is still present in coastal streams from the Clarence River in northern NSW to north Qld but has significantly declined in most of the Basin. Habitat in the Border Rivers of the MDB was burnt in late 2019 and 700 Olive perchlet were salvaged to establish a captive insurance population.
It is now extinct in SA with the last confirmed record from the Basin drainage in SA in 1983. It was thought extinct in Vic until a 2022 record from Mullaroo Creek. The species historic range only just extended to Vic where it was formerly present in an irrigation channel near Mildura, (last recorded in 1922).
In NSW it is largely only known from Darling-Baaka River tributaries (Bogan and the mid to upper Macintyre and Dumaresq rivers). Apparent recolonisation of the Gingham Watercourse (lower Gwydir catchment) was noted after two large 2011/12 floods, possibly through connection with parts of the Border Rivers floodplain. In Qld it is abundant in some tributaries and lagoons of the mid/upper Condamine-Balonne system and is in low numbers in the Condamine River. It is also present in the Nebine and Warrego catchments.
Formerly, it was widespread in the lower Lachlan, Murrumbidgee and lower Murray rivers and throughout the Darling-Baaka drainage in NSW. Since the 1960s it had not been recorded in any survey of the NSW lower Murray and only a single individual has been recorded (in 2011) in the lower Darling-Baaka below Bourke. A remnant population was rediscovered (after a 33 year absence) in the Lachlan near Lake Brewster in 2007. Recent records in Mullaroo Creek and Menindee Lakes probably represent displacement during high flows in 2021/22. The species was stocked into the Lake Cargelligo weirpool, Washpen Creek (Euston) and Thegoa Lagoon near Wentworth in 2010/11, but these stockings failed. Recent stockings have occurred in the Gwydir and Macquarie catchments, with a surrogate site release planned for late 2022 near Lake Victoria.
A total of 50 individuals were recorded in the Sustainable Rivers Audit (2004–2013) from 3 river valleys (Border Rivers, Condamine, Darling), with 40 of these from lowland altitudinal zones. The MDB Fish Survey (2014/15–2021/22) recorded 29 individuals from 3 river valleys (Border Rivers, Condamine, Macquarie), with 24 from the Macquarie lowlands.
Precise reasons for the decline of this species are unknown, but cold-water pollution that restricts spawning; habitat degradation; river regulation and related decline in wetland condition (e.g. loss of macrophyte beds in SA); and predation by alien species (particularly Eastern gambusia and Redfin perch) are thought to be significant. The species is particularly susceptible to entrainment into irrigation pumps. Connection of lagoons to rivers is important for reproduction (especially when macrophytes are absent from rivers), and subsequent dispersal of juveniles in the northern Basin, and such connection events are adversely impacted by river regulation. High summer flows in regulated reaches may also impact spawning. Increased turbidity has probably led to a loss of macrophyte spawning habitat. Drought and debris flows following bushfires also threaten small, isolated populations.
Allen 1996; Allen & Burgess 1990; Cadwallader & Backhouse 1983; Hutchison et al. 2008; Koehn et al. 2020a; Llewellyn 2008; McNeil et al. 2008; Medeiros 2004; Milton & Arthington 1985; Moffat & Voller 2002; NSW FSC 2001. Shelley et al. 2021; Whiterod et al. 2019; Zukowski et al. 2021; G. Wilson, pers. comm.; M. Hutchison unpubl. data.
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