During the early 1900s, the two fish species (G. holbrooki and Gambusia affinis) were distributed internationally from the south eastern United States to biologically control mosquitoes.  Australia received Gambusia stocks in the 1920s. They are currently widespread and abundant in Australian freshwater systems, occurring within all states and territories, and in at least 9 of the 12 drainage divisions. Gambusia holbrooki is classified as a noxious species in multiple states because of their prolific nature and detrimental impact on native fauna.

Surveys of G. holbrooki distribution in Melbourne waterbodies have revealed that this pest is widespread and pervasive. Correlations of their occurrence with water quality have showed that G. holbrooki distribution is not affected by heavy metal and Total Petroleum Hydrocarbons (TPH) concentrations in waterbody sediment. Gambusia holbrooki inhabited a broad spectrum of polluted waterbodies. We used microsatellite markers and DNA sequences from the mitochondrial cytochrome b and cytochrome oxidase 1 regions to investigate genetic differences among G. holbrooki populations from the south east coast of Australia.

Australian populations of G. holbrooki lacked mitochondrial diversity, suggesting that there was a small female founding population.  When compared to previous studies, our sequence information supports that only G. holbrooki were introduced into Australia, and confirms identification by Lloyd and Tomasov (1985).  Levels of nuclear genetic variation were extremely high within and between some of the 80 populations assessed. Several populations were inbreeding and/or had experienced bottleneck events.  Population relatedness generally followed the stream hierarchical model, however sporadic population establishment also occurred. 

The high levels of variation observed between some populations may be attributed to biological characteristics of the species, for example, population reductions over winter period followed by recolonisation/expansion events, sperm storage and multiple insemination ability of females, high fecundity, and multiple broods per season.  This genetic information will assist the species management, for example, by discerning populations suitable for permanent eradication.