Mosquitofish in Australia

The western mosquitofish (Gambusia affinis) and the eastern mosquitofish (Gambusia holbrooki) were introduced to Australia in 1925. These spread from the northeast coasts to New South Wales, southern Australia, and parts of Western Australia by 1934. Currently, known populations of wild mosquitofish occur in every state and territory except the Northern Territory, and are found in swamps, lakes, billabongs, thermal springs, salt lakes, and ornamental ponds. Mosquitofish are considered a noxious pest, especially in New South Wales and Queensland, and it is illegal to release them into the wild or transport them live into any of the states or territories. Mosquitofish were introduced by military and local councils to control mosquito populations, however there has been no evidence that Gambusia has had any effect in controlling mosquito populations or mosquito-borne diseases. In fact, studies have shown that Gambusia can suffer mortalities if fed only on mosquito larvae, and survivors show poor growth and maturation. Gambusia typically eat zooplankton, beetles, mayflies, caddis flies, mites and other invertebrates; mosquito larvae make up only a small portion of their diet.

Many ichthyologists believe native species are more effective in population control than mosquitofish. These include species such as the western minnow and pygmy perches. Unfortunately, gambusia may have exacerbated the mosquito problem in many areas by outcompeting native invertebrate predators of mosquito larvae. Because of their aggressive nature and high birthrate, mosquitofish can overtake most native species in an area, drastically harming local populations. Even if they were needed for mosquito control, studies have also shown that at least 5,000 fish/ha would be needed for effective control. However, mosquitoes' breeding environments are migratory and unreliable, so regular fish predators can have little effect from year to year.

Mosquitofish have a high tolerance for salinity, low oxygen, extreme temperatures (from 0.5-35°C), and pollutants, and are therefore able to live in many areas where other fish cannot. Certain thermal adaptations have allowed them to live in places from 55° North to 44° South, expanding their natural range. However, cool temperatures can affect their reproduction cycle. They have a resistance to a wide variety of pollutants, including organic waste, heavy metals, insecticides, herbicides, rotenone, phenol, and radiation.Ichthyologists believe the reason for low mosquito levels in areas populated by gambusia is not because of the fish, but rather the insecticide in the water killing the larvae.

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