Ecology, population genetics and risk assessment of the exotic mosquitofish, Gambusia holbrooki, in Tasmania
Creators: name: Lynch, KA
Publisher: name: University Of Tasmania
Year: 2023

description: The Poeciliid fish, Gambusia holbrooki, a native of North America, has been deliberately introduced into many countries for the purpose of mosquito control. Extensively introduced on mainland Australia since the early 1900's, the fish was absent from the island state of Tasmania until it was illegally introduced in 1992. Since that time, the fish has spread throughout the Tamar Estuary in northern Tasmania. Surveys conducted from 2004-2006 revealed that the fish have spread naturally at the modest rate of less than 2 km per year. Populations showed synchronous birth of live young occurred in mid-November each year, and mean abundance was highest in the summer months (`~60 fish``/0.5 m^2`). Gambusia fed predominantly on micro-crustaceans throughout the year, while mosquitoes and amphibians made up a very minor proportion of the diet (&lt;2% in any season). Gambusia appeared to negatively impact tadpole communities with abundance of tadpoles significantly increasing following eradication of the fish. Population genetic analysis using microsatellite markers revealed that Australian Gambusia populations are characterized by low diversity and Tasmanian Gambusia were possibly derived from southeast Queensland. Although significant differentiation existed among most populations, Tasmanian Gambusia populations were generally genetically similar, indicating a single introduction event. The genetic pattern of relationships among Tamar populations indicated that the site, TIWR, is driving the spread of Gambusia throughout the estuary. In May 2005, an attempt was made to eradicate Gambusia from two enclosed water bodies in the Tamar region. Water was pumped from both sites prior to application of hydrated lime (`Ca(OH)_2`). Both eradications proved unsuccessful, and populations were monitored and samples taken to assess the population recovery and genetic consequences of the eradication attempts. The Riverside population recovered quickest with live fish sighted 17 days after the initial lime treatment compared to 8 months at LD2. Estimates of effective population size (`N,`) indicated that less than ten individuals survived the eradication attempt at Riverside [6 (95% CI=2-14)], and less than five at LD2 [1 (95% CI=0-3)]. Allele frequencies varied significantly at both sites following the eradication attempt, and some alleles were not detected post-eradication. However, there was no statistically significant loss of allelic diversity at either of the sites. The rapid recovery of both populations combined with the maintenance of genetic diversity and minimal changes in allele frequencies, indicate that Gambusia are particularly resistant to the negative genetic effects of bottleneck events that dramatically decrease population size. A site-based risk assessment (RA) protocol was developed from a survey of 27 sites within and around the current distribution of Gambusia in the Tamar ‚Äöv¢‚Ä¢30 (all currently support large Gambusia populations). Over 85% of sites returned a risk score from 1-20, with all sites where the fish has never been encountered or failed to establish returning scores of ‚Äöv¢¬ß20. The RA could be utilized by managers of Gambusia in Tasmania to rapidly assess new survey sites in the Tamar region and to guide monitoring considerations in the future.