Gambusia affinis (Baird and Girard, 1853)

Common Name: Western Mosquitofish

Synonyms and Other Names:

Robert McDowallCopyright Info

Identification: Mosquitofish is a small, live-bearing fish, is dull grey or brown in color with no bars of bands on the sides, and has a rounded tail.  Its body is short, its head flattened, and its mouth pointed upward for surface feeding. Distinguishing characteristics were provided by Rauchenberger (1989) and Page and Burr (1991) (although the latter authors treated the two forms as subspecies). Gambusia affinis and G. holbrooki were long considered subspecies of G. affinis, and were only recently recognized as separate species (Wooten et al. 1988; Rauchenberger 1989; Robins et al. 1991). Complicating matters of identification, most introductions occurred before the recent taxonomic change; furthermore, the origins of introduced stocks were usually unknown or unreported. In addition, both forms were widely available and thought to have been dispersed widely by humans. As a consequence, it often is not possible to determine if many of the earlier records represent introductions of G. affinis or of G. holbrooki.

Gambuisa affinis can be distinguished from G. holbrooki by the following characters (Ross 2001):

  Dorsal fin
Anal fin
Bony claw on
4th gonopodial ray
G. affinis 6-7 ≤ 10 segmented
G. holbrooki 8+ 11+ unsegmented


Size: 6.5 cm

Native Range: Atlantic and Gulf Slope drainages from southern New Jersey to Mexico; Mississippi River basin from central Indiana and Illinois south to Gulf. Gambusia holbrooki is native to Atlantic and Gulf Slope drainages as far west as southern Alabama; G. affinis occurs throughout rest of the range (Rauchenberger 1989; Page and Burr 1991).

Great Lakes Nonindigenous Occurrences: Mosquitofish have been stocked in Alabama (Boschung 1992); Alaska (Krumholz 1948); Arizona (Dees 1961; Miller and Lowe 1967; Minckley 1969,1973; Lee et al. 1980 et seq.; Tilmant 1999; USFWS 2005); California (Dees 1961; La Rivers 1962; Minckley 1973; Moyle 1976a; Lee et al. 1980 et seq.; Moyle and Daniels 1982; Smith 1982; Stockwell et al. 1996; Taylor et al. 1982; Tilmant 1999; Matern et al. 2002; Sommer 2001); Colorado (Woodling 1985; Tyus et al. 1982; Zuckerman and Behnke 1986; Dill and Cordone 1997; Rasmussen 1998; Tilmant 1999; Sommer et al. 2001); Connecticut (Whitworth 1996); Florida (J. Chick, personal communication; J. D. Williams, personal observation); Hawaii (Brock 1960; Maciolek 1984; Tilmant 1999; Mundy 2005); Idaho (Simpson and Wallace 1978; Lee et al. 1980 et seq.; Idaho Fish and Game 1990); Illinois (Dees 1961; Smith 1979; Mills et al. 1993); Indiana (Dees 1961; Simon et al. 1992); Iowa (Harlan et al. 1987; Bernstein and Olson 2001); Kansas (Cross 1954, 1967; Clarke et al. 1958; Dees 1961; Tilmant 1999); Kentucky (Clay 1975; Burr and Warren 1986); Massachusetts (Krumholz 1948; Dees 1961); Michigan (Krumholz 1948; Hubbs and Lagler 1958; Dees 1961; Lee et al. 1980 et seq.); Minnesota (Eddy and Underhill 1974; Phillips et al. 1982); Mississippi (Krumholz 1948); Missouri (Dees 1961; Cross 1967; Pflieger 1997; Young et al. 1997); Montana (Brown and Fox 1966; Brown 1971; Lee et al. 1980 et seq.; Holton 1990); Nebraska (Haynes 1983; Lynch 1988a, 1988b, 1991; Rasmussen 1998; Steven 2004); Nevada (Miller and Alcorn 1946; Dees 1961; La Rivers 1962; Hubbs and Deacon 1964; Bradley and Deacon 1967; Lee et al. 1980 et seq.; Deacon and Williams 1984; Tilmant 1999; Vinyard 2001; USFWS 2001; ); New Jersey (Krumholz 1948; Fowler 1952; Dees 1961); New Mexico (Barber et al. 1929; Koster 1957; Dees 1961; Lee et al. 1980 et seq.; Tyus et al. 1982; Sublette et al. 1990; Plantania 1991); New York (Dees 1961; Lee et al. 1980 et seq.; Smith 1985; Schmidt 1986); North Carolina (Menhinick 1991); various drainage systems in Ohio (Dees 1961; Lee et al. 1980 et seq.; Trautman 1981; Hocutt et al. 1986; Burr and Page 1986); Oregon (Bond 1961, 1973, 1994; Oregon 1995); Pennsylvania (Dees 1961; Cooper 1983); Tennessee (Kuhne 1939; Etnier and Starnes 1993); Texas (Hubbs, personal communication); Utah (Rees 1934, 1945; Dees 1961; Sigler and Miller 1963; Minckley 1973; Lee et al. 1980 et seq.; Tyus et al. 1982; Tilmant 1999; USFWS 2005); Virginia (Jenkins and Burkhead 1994); Washington (Dees 1961; Wydoski and Whitney 1979; Fletcher, personal communication; USFWS 2005); West Virginia (Cincotta, personal communication); Wisconsin (Krumholz 1948; Dees 1961); Wyoming (Hubert 1994; Stone 1995), and probably other states. Also in Puerto Rico (Erdsman 1984; Lee 1983).

Gambusia holbrooki was recorded in the Great Lakes basin (OH) in 1947, but extirpated in that location in 1948. We are uncertain whether this may have been G. affinis. See USGS fact sheet on G. holbrooki for more information.

Table 1. Great Lakes region nonindigenous occurrences, the earliest and latest observations in each state/province, and the tally and names of HUCs with observations†. Names and dates are hyperlinked to their relevant specimen records. The list of references for all nonindigenous occurrences of Gambusia affinis are found here.

Full list of USGS occurrences

State/ProvinceYear of earliest observationYear of last observationTotal HUCs with observations†HUCs with observations†
Michigan196120133Detroit; Great Lakes Region; Lake Erie
Ohio194720092Black-Rocky; Lower Maumee

Table last updated 1/22/2020

† Populations may not be currently present.

Ecology: Western Mosquitofish is a small live-bearing fish that feeds primarily on zooplankton and invertebrate prey at the top of the water column. Adults are known to feed on their young opportunistically (Benoit et al. 2000). This species is also well known for its high feeding capacity. Chips (2004) observed maximum consumption rates of 42–167% of their body weight per day. These organisms also require a high density of refuges to maintain populations at or near their asymptotic density (Benoit et at. 2000). Interestingly, equal numbers of male and female mosquitofish occur in the ovary and at birth while adult populations contain a disproportionately large number of females and exhibit increased male mortality after recruitment (Haynes and Cashner 1995). This is probably due to the females' ability to store sperm, a trait that renders males largely unnecessary after insemination and whose presence becomes merely increased competition for developing young.

Means of Introduction: Because of their reputation as mosquito-control agents, both G. holbrooki and G. affinis have been stocked routinely and indiscriminately in temperate and tropical areas around the world. In the United States the first known introductions of mosquitofish took place in the early 1900s (Krumholz 1948). In 1905 about 150 G. affinis were introduced into Hawaii from Texas to test their effectiveness in preying on mosquito larvae (Seale 1905), and by 1910 their descendants had been released into parts of Oahu, Hawaii, Maui, Kauai, and Molokai (Van Dine 1907; Stearns 1983). Also, in 1905 Gambusia, reportedly from North Carolina, were released into New Jersey waters for the purpose of controlling mosquitoes (Seal 1910; Krumholz 1948). In 1922 mosquitofish from Texas (900 from Austin and 300 from Hearne) were introduced into a lily pond a Sutter's Fort. That lily pond served as a hatchery used to spread G. affinis across California and Nevada during the 1920s and 1930s (Stockwell et al. 1996). Mosquitofish were commonly and widely introduced during the following decades by such organizations as the former U.S. Public Health Service, in large part because they were thought of as an effective and inexpensive means of combating malaria (Krumholz 1948). In more recent years, employees of many state and local health departments apparently view the use of mosquitofish to control mosquito larvae as an attractive alternative to the use of insecticides. In some areas range extensions have occurred through natural dispersal far from sites where originally introduced (e.g., Pflieger 1997).

Status: Established in most states where stocked outside its native range. Its establishment and spread in northern states is greatly restricted because the species are not, in general, cold tolerant. In most cases, overwintering in colder regions requires surfacing groundwater springs (e.g., Woodling 1985; but see Lynch 1988b). Established in Nebraska, although the populations suffer heavy (up to 99%) winter mortality (Haynes 1983). Pflieger (1997) noted that Gambusia affinis is more widespread and abundant in Missouri now than it was half a century ago. For instance, Pflieger indicated that, by the early 1980s, it had become established northward along the Mississippi River to Clark County, Missouri, and westward near the Missouri River to Andrew County, a range expansion attributed to a combination of natural dispersal and undocumented introductions.

Great Lakes Impacts: Current research on the environmental impact of Gambusia affinis in the Great Lakes is inadequate to support proper assessment.

According to Courtenay and Meffe (1989), mosquitofish 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).

Because of its aggressive and predatory behavior, G. affinis may negatively affect populations of small fishes through predation and competition (Courtenay and Meffe 1989, Myers 1967), and could even benefit mosquitoes by decreasing competitive pressure from zooplankton and predation pressure from predatory invertebrates (Bence 1988, Blaustein and Karban 1990, Hoy et al. 1972). Research has shown that, in some waterbodies, abundance of mosquitofish is negatively related to invertebrates such as rotifers, water beetles, and crustaceans; however, this has not been the case in all waterbodies (Pyke 2008). Introduction of mosquitofish can also precipitate algal blooms via heightened consumption of zooplankton grazers (Hurlbert et al. 1972).

Gambusia affinis often attacks, shreds fins of, and sometimes kills other fish species. Mosquitofish 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 (Courtenay and Meffe 1989, Meffe 1985). In some habitats, introduced mosquitofish reportedly displaced select native fish species regarded as better or more efficient mosquito control agents (Courtenay and Meffe 1989, Danielsen 1968). Courtenay and Meffe (1989) and Pyke (2008) have reviewed impacts on a variety of native fishes and organisms.

Introduced mosquitofish 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 (Meffe 1985, Meffe et al. 1983, Moyle 1976a). Introduced fishes, including mosquitofish, 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). Western mosquitofish use the same habitat as the plains topminnow (Fundulus sciadicus) and have displaced the topminnow and other species with their aggressive behavior (Whitmore 1997). The mosquitofish is also responsible for the elimination of the least chub (Iotichthys phlegethontis) in several areas of Utah (Whitmore 1997). Meffe (1983, 1985) found that mosquitofish are very aggressive, even towards larger fishes.

Mosquitofish, 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).

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

Current research on the beneficial effect of Gambusia affinis in the Great Lakes is inadequate to support proper assessment.

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 mosquitofish 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).

Management: Regulations (pertaining to the Great Lakes)
The Western Mosquitofish is prohibited in Wisconsin under Wis. Admin. Code § NR 40.04, meaning that no person may transport, possess, transfer, or introduce the species without authorization.

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

There are no known biological control methods for this species.

There are no known physical control methods for this species.

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.

Remarks: Summaries and reviews of mosquitofish introductions were provided by Krumholz (1948), Hardy (1978), Courtenay and Meffe (1989), and Dill and Cordone (1997). The identity of some mosquitofish populations introduced into selected areas is correctly known. In most cases this is because the source of the stock was reported. The Western Mosquitofish, G. affinis, has been documented as introduced into Arizona (Dees 1961; Miller and Lowe 1967); California (Dees 1961; Moyle 1976a; Swift et al. 1993; Dill and Cordone 1997); Florida (Chick, personal communication; Williams, personal observation); Hawaii (Stearns 1983); Illinois (Krumholz 1948; Dees 1961); Indiana (Dees 1961); Kansas (Dees 1961; Cross 1967); Massachusetts (Dees 1961); Michigan (Dees 1961); Missouri (Dees 1961; Pflieger 1997); Nebraska (Lynch 1988a, 1988b); Nevada (Miller and Acorn 1946; Dees 1961); New Jersey (Dees 1961); New Mexico (Dees 1961); New York (Dees 1961); Ohio (Dees 1961); Pennsylvania (Dees 1961); Utah (Rees 1945; Dees 1961); Washington (Dees 1961); and Wisconsin (Dees 1961).

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).

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Author: Nico, L., Fuller, P., Jacobs, G., Cannister, M., Larson, J., Fusaro, A., Makled, T.H., and Neilson, M.E.

Contributing Agencies:

Revision Date: 9/26/2019

Peer Review Date: 1/25/2016

Citation for this information:
Nico, L., Fuller, P., Jacobs, G., Cannister, M., Larson, J., Fusaro, A., Makled, T.H., and Neilson, M.E., 2020, Gambusia affinis (Baird and Girard, 1853): U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, and NOAA Great Lakes Aquatic Nonindigenous Species Information System, Ann Arbor, MI,, Revision Date: 9/26/2019, Peer Review Date: 1/25/2016, Access Date: 2/27/2020

This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.