Nannatherina balstoni
Balston’s Pygmy Perch
Species Summary
The rarest of all the endemic freshwater fishes of south-western Australia, Nannatherina balstoni historically ranged from the Moore River in the north to Two Peoples Bay (Goodga River) in the south, but has disappeared from over half that range and now survives only in highly fragmented populations in the state’s extreme south-west corner.
Distribution & Habitat
Nannatherina balstoni has been lost from the entire northern half of its range, including the Moore River — recent surveys of almost 70 sites across that river and its tributaries between 1996 and 2003 failed to find a single specimen, the river’s main channel now salt-affected from large-scale land clearing and heavily infested with mosquitofish. It fares little better in the Blackwood River catchment, where surveys of 151 sites turned up just one low-salinity location still holding the species. As salinisation has spread through the larger rivers of the south-west, Nannatherina balstoni has been pushed into small tributaries, headwater streams and lakes that remain fresh and largely undisturbed, with its centre of distribution now the small lakes and streams around Windy Harbour in D’Entrecasteaux National Park. It inhabits dark, acidic water (pH as low as 3.0) with large seasonal temperature swings (11–30 °C), and has only ever been recorded in fresh water (generally under 2 ppt), suggesting low salinity tolerance. It co-occurs with the Western Pygmy Perch (Nannoperca vittata) over part of its range, though always in smaller numbers.
Biology
Larvae feed mainly on cladocerans, copepods and midge larvae, while juveniles and adults shift to mostly terrestrial prey. Both sexes reach maturity at the end of their first year of life, at an average total length of about 6.0 cm in males and 6.3 cm in females.
Compiled for Home of the Rainbowfish from ANGFA sources: “Nannatherina balstoni (Balston’s pygmy perch)”, In-Stream 15(1), February 2006, p. 21.