Introduced to Australia in 1925 as a potential mosquito control agent, Eastern gambusia (Gambusia holbrooki) is now present in almost every major Australian drainage basin, including the Murray-Darling Basin. Eastern gambusia are extremely aggressive, often harassing, eating and attacking native fishes as well as many amphibians and invertebrates. This project explored in aquaria how the aggressiveness of Eastern gambusia changed according to the relative abundance of two native fish species (Carp Gudgeon, Hypseleotris spp.) and juvenile Golden perch, (Macquaria ambigua ambigua). The project was designed to improve understanding of how Eastern gambusia might behave when it colonises new areas and how behavioural responses might be affected by efforts to control their numbers.
The study found that Eastern gambusia were highly aggressive towards both species of native fish and that aggressiveness increased when they were outnumbered by native fishes. The study also found that the type of aggressive behaviour by Eastern gambusia (e.g. biting or chasing) was specific to the native species it was interacting with. Eastern gambusia were shown to dominate the available habitat within the tank very shortly after introduction. The high aggression and dominance behaviour exhibited by Eastern gambusia when outnumbered may aid invasions of this species into new habitats, and may also provide useful clues on possible approaches for eradication of this invasive species.
Implications for native fish
The increased aggression by Gambusia when outnumbered potentially means that control efforts that reduce the abundance of Gambusia (that don’t fully eradicate them) may not be beneficial to native fish in some cases. The research suggested that aggressive interactions from Gambusia may not decrease as a result of control efforts.
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