A small fish with a rounded tail, distinctly flattened head and an upturned mouth. Maximum length is ~35 mm Total Length (TL) for males and ~80 mm TL for females, with fish >60 mm rare. The single high, soft-rayed dorsal fin originates well back on the body.
Females are much larger than males and mature females usually have a large, black blotch (known as the ‘gravid spot’) just above the vent. Males have the front rays of the anal fin elongated and modified to form the gonopodium, which is used in breeding.
Colouration is usually olive to brownish on the back, with bluish-grey sides and a silvery belly.
The Eastern gambusia is a schooling fish, often occurring in large aggregations, and commonly found in lakes or still or slow-flowing streams, mostly around the shallow edges or amongst freshwater plants. Maturity can be reached after only 4–6 weeks, at about 17– 20 mm long. Fish generally live for <12 months in the wild, with females born late in the breeding season able to overwinter and breed the following summer. Breeding occurs during the warmer months and a female produces an average of about 30–50 young (varying from 12–375 depending on female age/size) in each brood, and up to nine (but generally 2–5) broods per year. Broods are produced every 4–6 weeks during the reproductive season, with birth controlled by daylength and water temperatures (temperatures over ~16 °C and daylength greater than 12–13 hours required).
The species does not lay eggs, but produces live young, with the gestation period generally being 21– 40 days and births mostly occurring in the morning. Within a clutch, embryos are usually at multiple stages of development, and all females with embryos also contain mature unfertilised eggs. Female Gambusia can store viable sperm for several months after their last mating. The fertilised eggs develop inside the female and the young are a few millimetres long when born. The intensity and size of the gravid spot is a good predictor of the number and developmental stage of the internal embryos.
The Eastern gambusia is a poor swimmer and does not migrate, but displacement can occur during flooding. It tolerates of a wide range of water temperatures, oxygen levels, salinities and turbidities. Because of its ability to breed rapidly, it has assumed plague proportions in many habitats. Generally, Gambusia abundance is greatest in summer and autumn (following breeding) and numbers drop dramatically in winter (particularly in cooler climates).
Often referred to as Mosquitofish, it was introduced into Australia for mosquito control but unfortunately mosquito larvae do not figure prominently in its Australian diet. Consequently, Mosquitofish should not be used as the common name as it implies some environmental or social benefit which is largely incorrect. Eastern gambusia is an opportunistic omnivore but is primarily carnivorous and the diet contains a range of small freshwater invertebrates and windblown terrestrial insects. It is a diurnal visual predator and is cannibalistic.
An aggressive species, Eastern gambusia chase, harass and fin-nip fish much larger than themselves. They also prey on the eggs of native fish and frogs and larval native fish, and significantly reduce growth rates of small native fish. Aquarium studies have shown that individuals are more aggressive when in lower densities than native fish, which has implications for local control efforts. Aggression also increases with water temperatures ≥ 25 °C.
Gambusia is implicated in the decline of more than 35 fish species world-wide, at least 13 of which occur in Australia. It has been listed as a key threatening process for frog populations in NSW and is implicated in the decline of more than 15 species of frogs in Australia. Genetic options to control the species are currently being investigated.
Eastern gambusia is native to rivers draining to the Gulf of Mexico; gambusia, (both western and Eastern species) are the most widespread freshwater fish in the world, established on every continent except Antarctica. Eastern gambusia was introduced into Australia in 1925, firstly to Sydney and shortly afterwards to Brisbane. Health authorities made further introductions in the 1930s and 1940s with the Brisbane City Council establishing a hatchery to breed and release the species. Captive breeding and release for mosquito control continued up to at least the 1990s in some areas.
The species was also distributed to many military camps during World War II. Now widely distributed throughout Australia, it is commonly found in farm dams, slow-flowing waters and shallow wetlands, and is widespread and abundant across the Basin. Eastern gambusia was the most abundant alien fish and the 3rd most abundant species captured overall (37,943 fish: 17.4% of total catch) in the Sustainable Rivers Audit from 2004–2013 and in the MDB Fish Survey (20,248 fish: 11.6%) from 2014/15–2021/22 and captured in all river valleys. Misidentification as a native species and ignorance of the detrimental impacts of Gambusia mean that the species is still being deliberately spread to new water bodies.
Aarn & Ivantsoff 2001; Arthington & Marshall 1999; Beesley et al. 2014a; Howe et al. 1997; Ivantsoff & Aarn 1999; Lawrence et al. 2016; Lloyd et al. 1986; Macdonald & Tonkin 2008; Macdonald et al. 2012; McDowall 1996b; Morgan et al. 2021; Norazmi-Lokman et al. 2016, 2021; NSW NPWS 2003; Pen & Potter 1991; Pink et al. 2011; Pyke 2008; Remon et al. 2016; Rowe et al. 2008; Stoffels & Humphries 2003; Tonkin et al. 2012, 2014b; White & Pyke 2011.
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