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). The species 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). In particular, the smaller native fish occupying the slower, still water habitats of the MDB wetlands are at high risk, including the Ambassids (glassfish), Nannopercids (pygmy perches), Melanotaenids (rainbowfishes), Athernids (hardyheads), Eleotrids (gudgeons) and Retropinnids (smelt).
Eastern Gambusia (Gambusia holbrooki) Photo Tarmo Raadik
Eastern Gambusia project
The Eastern Gambusia removal project explored 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:
With Eastern Gambusia now well established throughout the Murray-Darling Basin and demonstrating an ability to rapidly colonise sites, it is clear that currently, it is not feasible to undertake large scale Eastern Gambusia control or eradication across the MDB.
This does not, however, preclude management actions for this species. Instead, minimising the ecological impact of Eastern Gambusia at a localised scale may be an important conservation strategy, particularly in areas supporting threatened fish fauna, until further control measures are developed.
What to Consider
A response to a presence of Eastern Gambusia at a site might include:
Drain/ dry the site
Poison the site
Remove Eastern Gambusia
Whilst drying or poisoning a site has been shown to be a successful eradication strategy, many regions or specific sites are not suitable to these techniques e.g. fragile ecosystems or the presence of threatened fish species. It may then be worth considering physical removal, which will minimise impacts on non-target biota.
Physical removal of Eastern Gambusia before the species’ spawning season can result in major reductions in Eastern Gambusia abundance, even resulting in complete eradication at some sites. The degree of success depends on site hydrology and hydrological connectivity, ecological value, habitat complexity, habitat size and climate.
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
A decision to remove fish will consider the likely effectiveness of the required effort and any negative impacts, including a possible increase in other exotic species (such as carp) as a result of removal actions.
A simple decision tool has been developed (see page 3) which should assist in assessing the likely effectiveness of removal actions at a particular site.
Best results with removal are achieved with targeted, repeated effort in isolated, small sites, before Eastern Gambusia spawn.
The success of removal activities (as assessed by the proportion of fish removed and ecological benefits) will depend on:
Best results are achieved by targeting Eastern Gambusia pre-spawning, in winter-early spring, with water temperatures <16°C and photoperiod <12 hours.
Consideration should be given to the capacity for sustained effort: unless complete eradication is achieved, suppression of Eastern Gambusia population size lasts little more than a month, therefore repeat removal must be undertaken at intervals such that removal is > the population growth rate.
If a site is permanently or frequently connected to adjoining habitats, repeated re-invasion will render any removal benefits as very short lived.
Sites supporting small threatened species (e.g. pygmy perch or glassfish) will yield greater ecological benefits from removal than effort at a site with lesser-impacted, more common species.
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. Site specific ecosystem function must be considered before undertaking a removal program.
Site Size and Features
Eastern Gambusia are strongly attracted to light and heat – it is important to identify and concentrate removal activities at areas of a site which are shallow and most exposed to direct sunlight.
Presence of structural habitat/ woody debris/ dense macrophytes can reduce the effectiveness of netting, but bait trapping may be quite effective in these sites. It should be noted that sites with abundant structural habitat may also support more cover and more separation between Eastern Gambusia and native fish fauna, and may therefore be in less need of Eastern Gambusia control.
Complete eradication is possible in very small sites (surface area <3m² ) but in the current study eradication was also achieved in sites up to 600m².
What to do
Undertake removal during winter and early spring.
Target the shallowest, brightest-lit area of water.
Targeted seine netting is by far the most effective method, especially for initial visits.
Set bait traps with the trap entry area at the surface, in an area of water receiving the most direct sunlight.
Sustained use of even dip nets can be useful if seine netting is not possible.
Repeat targeted effort over several weeks.
Light weight fine seine net
Bait traps, possibly dip nets
Polypropylene barrier fencing across inflow points may help minimise re-colonisation if sites become connected.
Thorough understanding of localised hydrology.
Regulations specific to each state may also restrict the activities which may be used by non-permit holders. E.g. the type and number of nets, water bodies where they can be used and requirements for euthanizing and disposal of fish. Please contact your specific state body for details on removal activities and requirements for euthanizing and disposal of fish. Details of specific state regulations can be found at
www.dpi.vic.gov.au for Victoria;
www.dpi.nsw.gov.au/fisheries/recreational/regulations for NSW;
www.dpi.qld.gov.au for Queensland and;
www.pir.sa.gov.au/fisheries for SA.
Published by the Victorian Government Department of Sustainability and Environment
Melbourne, June 2011. © The State of Victoria Department of Sustainability and Environment 2011
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Above: Demonstrating Eastern Gambusia removal to school students. Photo Glenys Avard