Eastern Gambusia removal
and recovery of native fish
communities
Bringing Native Fish Back
The Alien fish threat
The Eastern Gambusia project
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 MurrayDarling Basin Authority’s Native Fish Strategy.
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:
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).

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
Physical 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.
Best results with removal were achieved with
targeted, repeated effort in small, isolated sites,
before Eastern Gambusia spawned.
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 (Gambusia holbrooki). Photo MDBA
Eastern Gambusia removal and recovery of native fish communities
column design
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
Several species of small-bodied native fish were in
better condition and displayed increased
population growth in sites with reduced densities
of eastern gambusia.
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.
Below: Fin-nipping of juvenile Southern Pygmy
Perch. Photo Zeb Tonkin
This indicates that reductions of Eastern
Gambusia abundances will result in improvements
to small bodied native fish populations.
Below: Decision support tool for assessing likely
effectiveness of Eastern Gambusia removal
Does the site facilitate permanent immigration and emigration?
Isolated sites
Is the site of high ecological value
(species / habitat)?
Low ecological value
High frequency
of connection
How frequently does the site connect to
adjoining waterbodies?
Low frequency
of connection
High ecological
value
How frequently does the site connect
to adjoining waterbodies?
Low frequency of
connection
How much structural habitat
does the site contain?
High frequency of
connection
How much structural habitat
does the site contain?
High structural
complexity
High structural
complexity
Low structural
complexity
Large surface
area
What is the size of the site?
Small surface
area
Low structural
complexity
Very Low
Permanently connected
sites facilitating constant
immigration of pest fish
Permanently
connected
Large surface
area
What is the size of the site?
Benefits per $ invested
All sites
Low
Isolated sites of low ecological
value and frequent connection
to adjoining habitats facilitates
frequent immigration of pest
fish
Medium
Isolated sites of high ecological
value but frequent connection
to adjoining habitats. Value of
investment increases with
reduced structural habitat
complexity and surface area
High
Isolated sites of high ecological
value and infrequent
connection to adjoining
habitats. Value of investment
increases with reduced
structural habitat complexity
and surface area due to a
reduction in required effort;
increased ability to undertake
active netting methods and
increased negative interaction
between pest and native
species due to a reduction in
habitat niche partitioning.
Small surface
area
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.
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