Gambuisa affinis can be distinguished from G. holbrooki by the number of dorsal and anal fin elements and the shape of the gonopodium on males. Boschung and Mayden (2004) suggest the following combination of characters:

Walters and Freeman (2000) recommend the follow combination of characters (where each element is counted individually):

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental	Socioeconomic	Beneficial

Gambusia affinis is a host to the tapeworm Bothriocephalus acheilognathi which can infect over 200 species of fishes, including several valuable to commercial hatcheries (McAllister et al. 2015). However, the tapeworm was originally introduced by Ctenopharyngodon idella in North America.

According to Courtenay and Meffe (1989), G. affinis has had the greatest ecological impact by far of any of the introduced poeciliids. Although widely introduced as mosquito control agents, recent critical reviews of the world literature on mosquito control have not supported the view that G. affinis is particularly effective in reducing mosquito populations or in reducing the incidence of mosquito-borne diseases (Arthington and Lloyd 1989; Courtenay and Meffe 1989; Azevedo-Santos et al. 2017).
Gambusia affinis is considered a strong competitor, predator, and invader due to its rapid learning ability, behavioral flexibility, environmental tolerance, high reproductive potential and functional response, and fast growth rate (Magellan et al. 2019; Mofu et al. 2019; Rehage et al. 2020). Because of its aggressive and predatory behavior, G. affinis may negatively affect populations of small fishes through predation and competition (Myers 1967; Courtenay and Meffe 1989), and could even benefit mosquitoes by decreasing competitive pressure from zooplankton and predation pressure from predatory invertebrates (Hoy et al. 1972; Bence 1988; Blaustein and Karban 1990).
The high functional response and feeding rates of G. affinis can have significant impacts on zooplankton populations and in some cases leads to trophic cascades. In a previously fishless experiment pond, G. affinis decimated populations of some macroinvertebrates that were not adapted to fish predation (dytiscid beetles, baetid Ephemeroptera, corixid and gerrid hemipterans, libellulid odonates)(Harmon and Smith 2021). In an experimental pond, introductions of G. affinis impacted the pelagic food web and reduced the abundance of Daphnia, eventually leading to a trophic cascade as phytoplankton populations increased dramatically (Fryxell et al. 2016). Similarly, experimental pond abundances of Daphnia, mid-sized cladocerans, and total zooplankton were lowered by the introduction of G. affinis (Geyer et al. 2016). In mesocosm experiments designed to mimic local community structure, G. affinis directly reduced abundances of zooplankton and three native amphibian species. The resulting trophic cascades indirectly increased phytoplankton, periphyton, and freshwater snail biomass (Preston et al. 2018). In a field study, G. affinis altered the zooplankton community composition and size distribution, likely through size-selective predation. Consequently, G. affinis can indirectly slow leaf-litter decomposition rates due to predation on detritivores. These reductions may lead to trophic cascades and indirect impacts on ecosystem functions (Hinchliffe et al. 2017).
Introduction of G. affinis can also precipitate algal blooms via heightened consumption of zooplankton grazers (Hurlbert et al. 1972). The reduction in zooplankton abundance and subsequent trophic cascade caused by G. affinis led to increased chlorophyll concentrations in mesocosms (Rettig and Smith 2021). In contrast, the introduction of G. affinis in a natural wetland was associated with a 50% decline in relative phytoplankton fluorescence and total phosphorus, and sharp increases in N:P ratios in the water column. These impacts may be due to the assimilation of fish biomass acting as sink for nutrients (Preston et al. 2017).

Gambusia affinis often attacks, shreds fins of, and sometimes kills other fish species. Gambusia affinis is known to prey on eggs, larvae, and juveniles of various fishes, including those of largemouth bass (Micropterus salmoides) and common carp (Cyprinus carpio); it is also known to prey on adults of smaller species (Meffe 1985; Courtenay and Meffe 1989). In some habitats, introduced G. affinis reportedly displaced select native fish species regarded as better or more efficient mosquito control agents (Danielsen 1968; Courtenay and Meffe 1989). Courtenay and Meffe (1989) and Pyke (2008) have reviewed impacts on a variety of native fishes and organisms. Smith and Harmon (2019) found that in mesocosm ponds, G. affinis had a negative effect on successful colonization of gray tree frogs (Hyla versicolor) through direct physical interactions.
Introduced G. affinis has been particularly destructive in the American West where they have contributed to the elimination or decline of populations of federally endangered and threatened species (Courtenay and Meffe 1989). Specific examples of their negative effects include a habitat shift and a reduction in numbers of the threatened Railroad Valley springfish (Crenichthys baileyi) in springs in Nevada (Deacon et al. 1964) and the local elimination of the endangered Sonoran topminnow (Poeciliopsis occidentalis) in Arizona (Moyle 1976; Meffe et al. 1983; Meffe 1985). Introduced fishes, including G. affinis, are likely at least partially responsible for the decline of the Chiricahua leopard frog (Rana chiricahuensis) in southeastern Arizona (Rosen et al. 1995). In California, Gambusia affinis has been documented to prey heavily on California newt (Taricha torosa) larvae (Gamradt and Kats 1996) and Pacific treefrog (Hyla regilla) tadpoles (Goodsell and Kats 1999). Gambusia affinis use the same habitat as the plains topminnow (Fundulus sciadicus) and have displaced the topminnow and other species with their aggressive behavior (Whitmore 1997). But did not appear to compete for food with them from sites in the Central Great Plains (Nebraska) (Thiessen et al. 2018).  Gambusia affinis is also thought to contribute to the failed stocking of barrens topminnows (Fundulus julisia) and their long term population decline in Tennessee (Ennen et al. 2021). Gambusia affinis is also responsible for the elimination of the least chub (Iotichthys phlegethontis) in several areas of Utah (Whitmore 1997). Meffe (1983, 1985) found that G. affinis are very aggressive, even towards larger fishes.
Gambusia affinis, and other introduced poeciliids, have also been implicated in the decline of native damselflies on Oahu, Hawaii. Often the distributions of the damselflies and introduced fishes were found to be mutually exclusive, probably resulting from predation of the fishes on the insects (Englund 1999). In mesocosm experiments, introduced Gambusia affinis had extreme negative mortality effects on Salamandra larvae (salamanders) (Blaustein et al. 2017). Gambusia affinis can hybridize with G. holbrooki, which is native in parts of the south-east region of the United States (Wilk and Horth 2016). However, there are no native Gambusia spp. in the Great Lakes region.

There is little or no evidence to support that Gambusia affinis has significant socio-economic impacts in the Great Lakes.

There is little or no evidence to support that Gambusia affinis has significant beneficial impacts in the Great Lakes.

Realized:
Anecdotal observations in the early 20th century spurred the reputation of G. affinis as a successful control agent of mosquito populations via consumption of their larvae (Pyke 2008). Since these times, many studies on the success of G. affinis as a mosquito control agent have been completed and have often led to different outcomes (Pyke 2008). It remains unclear if G. affinis controls mosquitoes at a rate that is any higher than other indigenous species (Pyke 2008). In a set of field experiments, mosquitoes avoided ovipositing where G. affinis is present (Silberbush and Resetarits 2017). In a review by Azevedo-Santos et al. (2017), the use of G. affinis as mosquito control is ungrounded, ecologically damaging and other methods should be prioritized. Several studies have documented that native fish species often have an equal or greater control effect on mosquitoes than G. affinis. In a laboratory experiment, G. affinis was inferior to Fundulus diaphanus and Pimephales promelas (native to northeastern USA) as a biocontrol agent for mosquito larvae (Bickerton et al. 2018). Similarly, South American native fishes Cnesterodon decemmaculatus and Jenynsia multidentata were equally as effective at controlling mosquito populations as G. affinis (Bonifacio et al. 2019).

Gambusia affinis is also commonly used in medical research. It is a sentinal species used in the study of endocrine disrupting compounds due to G. affinis’s distinguishable hormone-dependent sexual dimorphism (Huang et al. 2016).

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Control
Biological
Recent research has documented several potential biological control methods for G. affinis. The use of male guppies can cause reproductive interference for G. affinis and could be a viable biocontrol method (Tsurui-Sato et al. 2019). Bluegill (Lepomis macrochirus) may moderate the negative impacts of G. affinis by lowering aggressions and foraging behaviors and slow their spread (Clemmer and Rettig 2019).

Physical
There are no known physical control methods for this species.

Chemical
Of the four chemical piscicides registered for use in the United States, antimycin A and rotenone are considered “general” piscicides, but no studies have been found of their effects on Gambusia affinis (GLMRIS 2012).

Increasing CO2 concentrations, either by bubbling pressurized gas directly into water or by the addition of sodium bicarbonate (NaHCO3) has been used to sedate fish with minimal residual toxicity, and is a potential method of harvesting fish for removal, though maintaining adequate CO2 concentrations may be difficult in large/natural water bodies (Clearwater et al. 2008). CO2 is approved only for use as an anesthetic for cold, cool, and warm water fishes the US, not for use as euthanasia, and exposure to NaHCO3 concentration of 142-642 mg/L for 5 min. is sufficient to anaesthetize most fish (Clearwater et al. 2008).

It should be noted that chemical treatment will often lead to non-target kills, and so all options for management of a species should be adequately studied before a decision is made to use piscicides or other chemicals. Potential effects on non-target plants and organisms, including macroinvertebrates and other fishes, should always be deliberately evaluated and analyzed. The effects of combinations of management chemicals and other toxicants, whether intentional or unintentional, should be understood prior to chemical treatment.  Other non-selective alterations of water quality, such as reducing dissolved oxygen levels or altering pH, could also have a deleterious impact on native fish, invertebrates, and other fauna or flora, and their potential harmful effects should therefore be evaluated thoroughly.

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.

Gambusia holbrooki was introduced into New Jersey (Fowler 1952) and into Tennessee near Knoxville and maybe to other locations as well (Starnes, personal communication). Both species have been introduced into Alabama (Boschung 1992). Shapovalov et al. (1981) indicated that both species were introduced into California, but Swift et al. (1993) argued that G. holbrooki never has been taken in the state and probably never was stocked. There was even mention that a hybrid between the two species was released into California waters (Dill and Cordone 1997). In their recent tome on fishes introduced into California, Dill and Cordone (1997) related their strong suspicions that pure Gambusia holbrooki had been introduced into that state. They based their conclusion, in part, on the importance and size of the mosquito control program in California, and the central role mosquitofish played in those attempts. However, Dill and Cordone did admit that there was no real proof that G. holbrooki became established in the state.

In some cases Gambusia stocks native to a particular region of a state were moved within the same state, in Virginia for example (Jenkins and Burkhead 1994). In contrast, Krumholz (1948) reported that mosquitofish from southern Illinois, where the species is native, were introduced into northern Illinois, an area outside its native range. Hubbs and Lagler (1958) reported that intergrades between G. affinis and G. holbrooki have been introduced into southern Michigan, but the stock did not become established. Galat and Robertson (1992) found that the Yaqui topminnow Poeciliopsis occidentalis sonoriensis occurring in some sites increased their fecundity in response to the presence of introduced Gambusia; however, the researchers noted that such habitats must also have certain environmental conditions (e.g., uniform temperatures) for maintenance of vigourous P. o. sonoriensis populations. Galat and Robertson concluded that conservation of some extant populations of P. occidentalis depends primarily on control of Gambusia. When compared to other Gambusia spp., including G. holbrooki, Rehage and Sih (2004) found that G. affinis exhibited the greatest dispersal tendency and as a result was more likely to spread to other habitats after introduction.

Introduction of the Western Mosquitofish into northern California occurred in 1922 when 600 G. affinis were planted in Fort Sutter lily pond.  Members of this population were then introduced into the vicinities of Glenn, Kern, Coachella Valley, and Los Angeles CA during the 1920s and 1930s.  In 1934, G. affinis were also introduced into Fallon, Nevada.  From Fallon, Nevada, G. affinis were introduced into the following areas of Nevada: Wabuska, Garrett, Parker Ranch, and Bonham Ranch in the late 1930s and early 1940s (Stockwell et al. 1996).

Western Mosquitofish have been widely introduced outside of the continential United States for mosquito control purposes (Krumholz 1948; Purcell et al. 2012). Introductions of mosquitofish into New Zealand from the Hawaiian Islands showed a reduction in genetic diversity typical of introduced populations originating from a small number of colonizers (Purcell et al. 2012).