The Alien fish threat

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Eastern Gambusia removal and recovery of native fish communities

Bringing Native Fish Back

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).

    Eastern Gambusia (Gambusia holbrooki). Photo MDBA

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

© The State of Victoria Department of Sustainability and Environment 2011. This publication is copyright. No part may be reproduced by any process except in accordance with the provisions of the Copyright Act 1968. Authorised by the Victorian Government, 8 Nicholson Street, East Melbourne.

Printed on 100% Recycled paper ISBN 978-1-74287-131-8 online

For more information contact the DSE Customer Service Centre 136 186 or Zeb Tonkin 03 9450 8660.


This publication may be of assistance to you but the State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.

If you would like to receive this publication in an accessible format, such as large print or audio, please telephone 136 186, or through the National Relay Service (NRS) using a modem or textphone/teletypewriter (TTY) by dialling 1800 555 677, or email

This document is also available in PDF format on the internet at

Below: Fin-nipping of juvenile Southern Pygmy Perch. Photo Zeb Tonkin

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