A small, laterally compressed fish with a deeply notched single dorsal fin. Maximum size 75 mm; usually less than 65 mm. The tail is slightly rounded, and the mouth is small, reaching to just below the front of the eye. The lateral line is discontinuous. The lower, free edge of preorbital bone is straight, with the edge strongly serrated (compare with Southern pygmy perch).
The body colour is gold to dusky brown-grey, paler on the belly, with dark spots or blotches in a row along the midline. The pelvic fins turn black in breeding males. The black pupil of the eye forms an imperfect circle (slightly notched compared with circular in Southern pygmy perch) and there is a black band from the snout through the eye.
In the Basin, the Yarra pygmy perch was restricted to Lake Alexandrina in slow-flowing or still waters, wetland or drainage channel habitats with abundant submerged aquatic vegetation. It was found in small groups, often cohabiting with Southern pygmy perch. It breeds in spring, at water temperatures between 16 and 24°C. The diet includes microcrustaceans, molluscs and aquatic insects, such as mosquito larvae.
In the Basin, the Yarra pygmy perch was found only in Lake Alexandrina in the lower Murray, with this population genetically distinct from those in other catchments. It is also found in coastal streams in western Vic and patchily throughout southeast SA, and was once locally common within suitable habitats in Lake Alexandrina. Although only discovered in the MDB in the early 2000s, the MDB population was extirpated during the latter stages of the Millennium Drought (post 2008) as the declining lake water levels isolated their fringing reedbed refuge habitat exposing them to predators like Redfin perch. Emergency rescue and the establishment of surrogate dams allowed for reintroductions following the breaking of the drought, but these reintroductions back into Lake Alexandrina were ultimately unsuccessful. and the species is now extinct in the wild in the MDB.
Some pygmy perch are still maintained as backup populations and can hopefully form the basis for future successful reintroductions. As a species that only occurred in Lake Alexandrine, no individuals were recorded from the stream fish monitoring programs the Sustainable Rivers Audit (2004–2013) or the MDB Fish Survey (2014/15–2021/22).
A combination of habitat degradation (loss of fringing macrophyte predation refuge) through drought-related lowered water levels in Lake Alexandrina, coupled with abundant alien predator Redfin perch resulted in the extinction from the wild of the MDB population.
Outside of the Basin, Yarra pygmy perch populations have been substantially fragmented and depleted historically by wetland drainage and modification (for agriculture, urban and industrial development). The main cause of wetland loss and degradation in Victoria has been total or partial drainage, with shallow wetlands particularly impacted. Reduced flooding and likely poor dispersal capacity of the species means that recolonisation after population loss is unlikely. Unrestricted stock access in wetlands on private property results in disturbance and removal of instream and riparian habitat. Alteration to flow regime disrupts natural replenishment of shallow freshwater habitats and reduces wetland connectivity to deeper and more permanent water bodies, particularly during drought or extended dry spells.
Predation by Redfin perch and trout; competition/aggression from Eastern gambusia; and habitat disturbance by Carp may also threaten critical pygmy perch habitats.
Allen et al. 2002; Cadwallader & Backhouse. 1983; Hammer 2002b; Hammer et al. 2010, 2013; Higham et al. 2005; Saddlier et al. 2013;
Wedderburn et al. 2012, 2014; Wedderburn et al. 2019, 2022; Whiterod et al. 2019; Woodward & Malone 2002.
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