Introduction

Eastern gambusia (Gambusia holbrooki - also known as plague minnow, gambusia and mosquito fish) are native to North America. They were initially introduced into NSW waters sometime during the 1920s because of their reputation for mosquito control. However, since that time their ability to control mosquito larvae has been shown to be no greater than that of small native fish that feed on insects.

Eastern gambusia have spread widely throughout NSW and thrive in shallow low flowing waterbodies. They can tolerate a wide range of temperatures and water quality. Eastern gambusia breed several times a year throughout the warmer months. Eggs are fertilised inside the female and take 3 or 4 weeks to develop. About 50 young are born live and reach maturity in less than 2 months, which means populations of Eastern gambusia can grow rapidly.

Size: Females to about 60 mm, males to about 35 mm.

Impact on native fish

Eastern gambusia have been associated with the decline of abundance or range of 35 fish species worldwide, including Australian native species such as gudgeon, hardyheads and some rainbow fish.

Competition for food

Eastern gambusia feed on a wide variety of foods. They eat insects such as ants and flies as well as aquatic beetles, bugs and other fauna. Eastern gambusia,,with their high reproductive rate and extended breeding season, may overwhelm suitable habitats with juveniles and deplete food supplies.

Competition

Competition occurs when Eastern gambusia deny other species access to a particular resource, generally through aggressive behaviour. This often involves chasing and fin nipping which can lead to secondary bacterial or fungal infections and potentially death of other fish.

Predation

Eastern gambusia are known to prey upon the eggs and juveniles of other fish species. They have also been linked to the decline of frog species, through the predation of tadpoles and adult frogs.

What can we do?

Some well-established alien species are difficult to control. However, there are some options for their management and improving the odds for native fish.

Environmental management and rehabilitation

Management of issues such as water quality, environmental flows, fish passage and snags can maintain or return conditions to those that best suit native fish. This improves the ability of native fish to compete and creates conditions less suitable for alien species.

Restricting the spread

The spread of alien species has often been associated with their use as bait. For this reason, use of live fish bait is illegal in NSW. Improved education is also needed to avoid the accidental release of these species.

Biological control

Biological control utilises methods such as the introduction of a natural predator or parasite, manipulation of the immune system or genetic material and sterilisation to control or remove an unwanted species. These controls can be effective but are often technically difficult, expensive and require detailed testing to ensure that controls won't impact on other species.

Disease

Diseases such as viruses can effectively manage alien species. However, it can be difficult to find a species specific disease that will not affect other fish.

If you find what you believe is Eastern gambusia (or another species which you suspect is not native to the area), freeze it whole and Report it!

What is I&I NSW doing?

I&I NSW has listed Eastern gambusia (Gambusia holbrooki) as noxious in NSW. Eastern gambusia is listed as a Class 1 noxious species outside the greater Sydney area which means it is illegal to sell or possess it, even in aquaria. In the greater Sydney region it is listed as a class 3 noxious species which allows for sale and possession in this region, however its sale and possession are discouraged. For more detail on the regions covered by this noxious listing, see the noxious list.

References and further reading

Harris, JH and Gehrke, PC (eds). 1997. Fish and Rivers in Stress. The NSW Rivers Survey. NSW Fisheries Office of Conservation and the Cooperative Research Centre for Freshwater Ecology in association with NSW Resource and Conservation Assessment Council.
Rowland, SJ and Ingram, BA. 1991. Diseases of Australian native freshwater fishes with particular emphasis on ectoparasitic and fungal diseases of Murray cod (Macullochella peelii peelii), golden perch (Macquaria ambigua) and silver perch (Bidyanus bidyanus). Fisheries Bulletin 4. NSW Fisheries.
Young, WJ. (eds) 2001. Rivers as Ecological Systems: The Murray-Darling Basin. Murray-Darling Basin Commission. Canberra.