The Alien fish threat
Alien fish species are recognised as one of eight major threats to native fish in the Murray-Darling Basin (MDB) and the control of these species is one of the key driving actions of the Murray-Darling Basin Authority’s Native Fish Strategy.
Eastern Gambusia (Gambusia holbrooki), a native of the south-east of the United States of America and Mexico, was brought to Australia in 1925 to ostensibly control mosquito larvae (which it fails to do). It has detrimental impacts on native fish fauna globally and is widespread in the MDB. Sixteen of the 37 native freshwater fish of the MDB have major niche overlaps with the Eastern Gambusia, suggesting these species are at high risk of impacts such as predation of eggs and /or larvae, competition for food and habitat, and aggression (including fin nipping). Eastern Gambusia are poor swimmers and prefer still waters to flowing waters, so the smaller native fish occupying the slower, still water habitats of the MDB wetlands are at particularly high risk, including the Ambassids (glassfish), Nannopercids (pygmy perches), Melanotaenids (rainbowfishes), Athernids (hardyheads), Eleotrids (gudgeons) and Retropinnids (smelt).
The Eastern Gambusia project
Given the threat of Eastern Gambusia to native fish communities and the lack of current effective control options, this project was established to explore the feasibility of controlling Eastern Gambusia populations to densities where native fish communities could recover. The project:
Reviewed current knowledge on Eastern Gambusia and its impacts
Studied the responses of native fish communities in natural billabong systems in the MDB to the reduction of Eastern Gambusia
Provided a framework to assess the feasibility and effectiveness of control options, and
Developed a template for evaluating control options for other alien species in the MDB.
Eastern Gambusia removal
Best results with removal were achieved with targeted, repeated effort in small, isolated sites, before Eastern Gambusia spawned.
hysical removal of Eastern Gambusia before the species’ spawning season did result in major reductions in Eastern Gambusia abundance, even resulting in complete eradication at some sites. The degree of success depended on site hydrology, connectivity, climate, habitat and size.
A simple decision support tool was developed (see overleaf) to enable managers to assess the likely effectiveness of physical removal of Eastern Gambusia at specific sites.
If physical removal at a particular site is likely to have low ecological benefit for a given investment, other mitigation strategies such as habitat restoration should still be considered.
Eastern Gambusia colonisation
During the removal and control experiments, Eastern Gambusia displayed an astonishing capacity to rapidly colonise habitats, with just a few individuals establishing population sizes in the thousands in a three to four month period. It has been calculated that 10 adult females could produce a population of 5 million individuals in a six-month period. This rate of increase is far higher than even the most common native fish species in the region, emphasising the species’ ability to out compete native species.
Fish community response
This indicates that reductions of Eastern Gambusia abundances will result in improvements to small bodied native fish populations.
everal species of small-bodied native fish were in better condition and displayed increased population growth in sites with reduced densities of eastern gambusia.
Below: Decision support tool for assessing likely effectiveness of Eastern Gambusia removal
The degree of improvement following such reductions is very species specific, with improvements likely to be greatest for native species with a more restricted trophic niche such as pygmy perch, glassfish and hardyhead species as compared to more common generalist species.
Results also indicated Eastern Gambusia removal may result in small improvements to carp populations, suggesting that Eastern Gambusia removal may also have unexpected benefits to other exotic species. This highlights that site specific ecosystem function must be considered before undertaking a removal program.
Published by the Victorian Government Department of Sustainability and Environment Melbourne, June 2011
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Below: Fin-nipping of juvenile Southern Pygmy Perch. Photo Zeb Tonkin