Key words: Gambusia holbrooki, pest species control, native fish recovery, Native Fish Strategy.
Threats and Impacts: Alien fish species have been 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 drivers of the Native Fish Strategy. There is growing evidence of detrimental impacts of Eastern Gambusia (Gambusia holbrooki) on native fish fauna globally, and this species has been identified as potentially one of the key alien species contributing to the decline of a number of native fish within the MDB, where it is widespread (Figs 1 and 2). The ecological impacts of the Eastern Gambusia in the MDB remain uncertain and this project addressed these research needs by integrating surveys and experimental work in natural billabong systems throughout the MDB.
Broad aim and specific objectives: The specific objectives of the project were to:
1. Review current knowledge of the impacts of Eastern Gambusia on native fishes of the MDB.
2. Provide information on the response of native fish communities following the reduction of Eastern Gambusia populations.
3. Provide a framework to evaluate the feasibility and effectiveness of such control actions and form a template for evaluating control options for other alien fishes across the MDB.
Figure 1: Mature female Eastern Gambusia (photo courtesy of Tarmo Raadik)
Figure 2: High density of Eastern Gambusia in a shallow backwater environment (Photo courtesy of Tarmo Raadik)
Methods: The project was divided into four phases. The first phase involved a review of current knowledge of the impacts of Eastern Gambusia on native fishes of the MDB. The second phase involved a broad-scale, cross-sectional study of wetland fish communities to develop hypotheses about the effect of Eastern Gambusia on native fish communities in these enclosed systems. The third phase was a field trial of Eastern Gambusia control in small isolated billabongs, to test the hypotheses through density manipulation experiments and to provide information on control options and Eastern Gambusia population dynamics. The fourth phase identified strategies to maximise the level of improvement to the native fish community through Eastern Gambusia control given a fixed budget (benefit maximisation), and to minimise the cost of achieving a defined significant improvement in the native fish community (cost minimisation). Finally, the project provided a template for evaluating control options for other alien fishes across the MDB.
Findings: The review of literature exploring impacts of Eastern Gambusia on native fishes of the MDB identified that 16 of 37 native species have major habitat or diet (or both) overlaps with Eastern Gambusia. The most significant overlaps were with small-bodied species e.g. Glassfish (Ambassidae), Pygmy-perches (Nannopercidae), Rainbowfishes (Melanotaeniidae), Hardyheads (Atherinidae), Gudgeons (Eleotridae) and Smelt (Retropinnidae). The review therefore concluded that Eastern Gambusia is likely to have contributed to the decline (in distribution and/or abundance) of the Olive Perchlet (Ambassis agassizii), Southern Pygmy-perch (Nannoperca australis), Murray-Darling Rainbowfish (Melanotaenia fluviatilis) and Purple-spotted Gudgeon (Mogurnda adspersa).
An assessment of wetland communities throughout the mid-Murray region of the MDB found that Carp Gudgeon (Hypseleotris spp.) and Eastern Gambusia were the dominant species in both abundance and distribution. The results of the survey suggest that Eastern Gambusia do not have a negative influence on abundances of the more common native species (e.g. Carp Gudgeon and Flat-headed Gudgeon (Philypnodon grandiceps) most likely due to the generalist nature of such species enabling co-existence. Gambusia were found to impact on the abundance of juveniles of several native species, and on their general health by ‘fin nipping’
Several small isolated billabongs had Eastern Gambusia removed to observe how native fish would respond. During this trial, astonishingly, a few individual Eastern Gambusia were able to re-establish populations of thousands within three or four months. Most importantly, the results of the removal trial indicate that reductions of Eastern Gambusia abundances will result in some improvements to small-bodied native fish populations, and these effects may be enhanced within billabongs without complex habitat (making Gambusia easier to catch and remove), and containing native species with quite specific diets.
In examining the cost-effectiveness and logistics of Eastern Gambusia removal, this study presents a strategy to determine the feasibility of removal for different scenarios and concluded that the highest benefits per dollar invested were for habitats with low frequency of connection to other Eastern Gambusia populations, low structural complexity and of high ecological value.
Lessons learned and future directions: This project provides fundamental ecological information necessary for management of Eastern Gambusia. This project provides managers with a decision making tool to assess the cost benefit of Eastern Gambusia removal for a range of habitat scenarios. This will result in better targeted action of controlling this pest species and maximise benefits to native fish populations. This project will raise awareness of the impacts of Eastern Gambusia on native fish and what benefits may be obtained for native fish following Eastern Gambusia removal.
Stakeholders and Funding bodies: This project was funded through the Murray-Darling Basin Authority’s Native Fish Strategy.
Contacts: Zeb Tonkin, South Australian Research and Development Institute. Tel: + 61 3 9450 8600, Email: zeb.tonkin@depi.vic.gov.au.
EMR Project Summaries 