Taking a fish out of water: How it can assist conservation efforts (Baan)

By Rhys Coleman, Tarmo Raadik, Vin Pettigrove and Ary Hoffmann

Dwarf Galaxias

Dwarf galaxias.Credit: Rhys Coleman and Andrew Weeks.

The dwarf galaxias, Galaxiella pusilla, is a small (usually 40 mm long), freshwater fish of national conservation significance in Australia. They are typically found in shallow, still or slow flowing water, with dense submerged and/or emergent aquatic vegetation such as wetlands, swamps, billabongs and small streams. As well as habitat loss, invasive fish are major threat to this species – most notably eastern gambusia, Gambusia holbrooki. Since its introduction to Australia in the early 1900s for the purpose mosquito control (and the associated risk of mosquito-borne diseases), eastern gambusia has become widespread and is now classified as noxious. Impacts from eastern gambusia on dwarf galaxias include predation on eggs or fry, fin nipping, and competition for food and habitat.

Efforts to control eastern gambusia tend to involve manual removal or the application of fish poison.

Efforts to control eastern gambusia in Australia are rare, and when they occur, tend to involve manual removal or the application of fish poison (Ayres and Clunie 2010). Manual removal can be time-consuming and expensive, while fish poisons (e.g. rotenone) can result in unintended impacts to non-target fauna. In a recently accepted article in Marine and Freshwater Research (Coleman et al., 2016) we demonstrated that, as an alternative to physical or chemical removal, management actions to control eastern gambusia could take advantage of local adaptations in dwarf galaxias.

Most habitats occupied by dwarf galaxias have a seasonally variable and unpredictable hydrologic regime, where water levels substantially contract during dry periods and expand during wet periods. Consistent with these habitat characteristics, and in contrast to eastern gambusia, this research found that dwarf galaxias have adaptations to habitat drying by being able to persist without surface water for more than 12 days. When surface water was absent, a key driver of survival duration was the moisture retention capacity of various habitats (sediment only, vegetation, detritus, artificial cray burrows). Physiological adaptations to habitat drying that were observed include air breathing and higher respiration rates in air compared to eastern gambusia. These same adaptations were also observed during concurrent experiments with little galaxias (Galaxiella toourtkoort), a new species recently described by Coleman et al. (2015) as part of the same CAPIM PhD project.

Dwarf galaxias have adapted to persist without surface water for more than 12 days.

The findings indicate that periodic drying of dwarf galaxias habitats provides a way of protecting dwarf galaxias in water bodies where eastern gambusia has invaded. Moreover, there was very low survival of eastern gambusia and high survival of dwarf galaxias in habitats with abundant leaf litter and other organic matter, that suggests targeted revegetation of fringing habitat could further disadvantage eastern gambusia. The research also indicates that water level manipulation to create short periods of habitat drying may provide the additional benefit of encouraging the presence of invertebrate food sources that dominate recently wetted habitats, whilst minimising invertebrates that are potential predators of small fish (e.g. dragonflies, beetle larvae) that tend to dominate habitats inundated for extended periods.

Periodic drying of dwarf galaxias habitats provides a way of protecting dwarf galaxias where eastern gambusia has invaded.

Regardless of the management intervention, any eradication attempt must consider that the original pathway of invasion and the risk of re-invasion of eastern gambusia (Ayres and Clunie 2010). Where the likelihood of eastern gambusia re-invasion is high, retaining a subset of isolated dwarf galaxias habitats is recommended as ‘insurance populations’ to manage the risk of system-wide extinctions due to eastern gambusia spread. Given similar adaptations to habitat drying were also observed in little galaxias, this species is also expected to benefit from a management strategy that involves habitat drying to control eastern gambusia.

For further information please contact: Rhys Coleman (Waterways and Wetlands Research Manager, Melbourne Water) rhys.coleman@melbournewater.com.


Ayres R, and Clunie P. (2010). Management of freshwater fish incursions: a review. PestSmart Toolkit publication, Invasive Animals Cooperative Research Centre, Canberra, Australia. Available at http://www.feral.org.au/wp-content/uploads/2011/08/PestSmart_LitReview.pdf .

Coleman RA, Hoffmann AA, and Raadik TA. (2015). A review of Galaxiella pusilla (Mack) (Teleostei: Galaxiidae) in south-eastern Australia with a description of a new species. Zootaxa 4021(2): 243–281.

Coleman RA, Raadik TA, Pettigrove V, and Hoffmann AA (2016). Taking advantage of adaptations when managing threatened species within variable environments: the case of the dwarf galaxias Galaxiella pusilla (Teleostei, Galaxiidae). Marine and Freshwater Research 68, 175-186.