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| NAS - Nonindigenous Aquatic Species |
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Common Name: western mosquitofish
Taxonomy: available through 
Identification: Mosquitofish are small, live-bearing fish, are dull grey or brown in color with no bars of bands on the sides, and have a rounded tail. Their bodies are short, their head flattened, and their mouths pointed upward for surface feeding. Distinguishing characteristics were provided by Rauchenberger (1989a) 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 1989a; Robins et al. 1991a). 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.
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 1989a; Page and Burr 1991).
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Interactive maps: Continental US, Alaska, Hawaii, Caribbean
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; 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 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 1966, 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; 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); 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 (IL) 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.
Ecology: Western mosquitofish are small live-bearing fish that feed 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 refuge 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 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). 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.
Impact of Introduction:
According to Courtenay and Meffe (1989), mosquitofish have 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 Gambusia are particularly effective in reducing mosquito populations or in reducing the incidence of mosquito-borne diseases (Courtenay and Meffe 1989; Arthington and Lloyd 1989). Because of their aggressive and predatory behavior, mosquitofish may negatively affect populations of small fish through predation and competition (Myers 1967; Courtenay and Meffe 1989), and benefit mosquitos by decreasing competitive pressure from zoooplantion and predation pressure from predatory invertebrates (Blaustein and Karban 1990). In some habitats, introduced mosquitofish reportedly displaced select native fish species regarded as better or more efficient mosquito control agents (Danielsen 1968; Courtenay and Meffe 1989).
Introduced mosquitofish have 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 1976a; Meffe et al. 1983, Meffe 1985). Western mosquitofish use the same habitat as the plains topminnow Fundulus sciadicus and have displaced these topminnows 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
Introducing mosquitofish also can precipitate algal blooms when the fish eat the zooplankton grazers (Hurlbert et al. 1972), or in an increase in the number of mosquitoes if the fish eat the invertebrate predators (Hoy et al. 1972, Bence 1988). 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).
Mosquitofish, and other introduced poeciliids, have 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 fish on the insects (Englund 1999).
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 P. 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 (2004) found that G. affinis exhibited the greatest dispersal tendency and as a result was more likely to spread to other habitats after introduction.
Bence, J. R. 1988. Indirect effects and biological control of mosquitoes by mosquitofish. The Journal of Applied Ecology. 25(2):505-521.
Benoît, H. P., Post, J. R., and Barbet, A. D. 2000. Recruitment Dynamics and Size Structure in Experimental Populations of the Mosquitofish, Gambusia affinis. COPEIA 2000(1): 216-221.
Blaustein, L and R. Karban. 1990. Indirect effects of the mosquitofish Gambusia affinis on the mosquito Culex tarsalis. Limnology and Oceanography 35(3):767-771.
Courtenay, W. R., Jr., and G. K. Meffe. 1989. Small fishes in strange places: a review of introduced poeciliids. Pages 319-331 In: G. K. Meffe, and F. F. Snelson, Jr., editors. Ecology and evolution of livebearing fishes (Poeciliidae). Prentice Hall, Englewood Cliffs, NJ.
Chipps, S. R., and D. H.Wahl. 2004. Development and evaluation of a western mosquitofish bioenergetics model. Transactions of the American Fisheries Society 133 (5): 1150-1162.
Erdsman, D.S. 1984. Exotic fishes in Puerto Rico, p. 162-176, In: W.R.Jr. Courtenay and J.R.Jr. Stauffer, eds. Distribution, Biology, and Management of Exotic Fishes. John Hopkins. Baltimore> and
Englund, R.A. 1999. The impacts of introduced poeciliid fish and Odonata on the endemic Megalagrion (Odonata) damselflies of Oahu Island, Hawaii. Journal of Insect Conservation 3:225-243.
Gramradt, S.C. and L.B. Kats. 1996. Effect of introduced crayfish and mosquitofish on California newts. Conservation Biology 10(4): 1155-1162.
Goodsell, J.A. and L.B. Kats. 1999. Effect of introduced mosquitofish on Pacific treefrogs and the role of alternate prey. Conservation Biology 13(4): 921-924.
Haynes J. L. and R. C. Cashner. 1995. Life history and population dynamics of the western mosquitofish: a comparison of natural and introduced populations. Journal of Fish Biology 46(6): 1026-1041.
Hubbs, C. and J.E. Deacon. 1964. Additional introductions of tropical fishes into southern Nevada. The Southwestern Naturalist. 9(4): 249-251.
Maciolek, J. A. 1984. Exotic fishes in Hawaii and other islands of Oceania. Pages 131-161 in W. R. Courtenay, Jr., and J. R. Stauffer, Jr., editors. Distribution, biology, and management of exotic fishes. The Johns Hopkins University Press, Baltimore, MD.
Matern, S.A., P.B. Moyle, and L.C. Pierce. 2002. Native and alien fishes in a California estuarine marsh: twenty-one years of changing assemblages. Transactions of the American Fisheries Society. 131: 797-816.
Mills, E.L., J.H. Leach, J.T. Carlton, and C.L. Secor. 1993. Exotic species in the : a history of biotic crises and anthropogenic introductions. J. Great Lakes Research. 19(1): 1-54.
Page, L. M., and B. M. Burr. 1991. A field guide to freshwater fishes of North America north of Mexico. The Peterson Field Guide Series, volume 42. Houghton Mifflin Company, Boston, MA.
Rasmussen, J.L. 1998. Aquatic nuisance species of the Mississippi River basin. 60th Midwest Fish and Wildlife Conference, Aquatic Nuisance Species Symposium, Dec. 7, 1998, Cincinnati, OH
Rehage, J. S. and A. Sih. Dispersal behavior, boldness, and the link to invasiveness: a comparison of four Gambusia species. Biological Invasions 6(3): 379-391.
Sommer, T, B. Harrell, M. Nobriga, R. Brown, P. Moyle, W. Kimmerer, and L. Schemel. 2001. California's Yolo Bypass: Evidence that flood control can be compatible with fisheries, wetlands, wildlife, and agriculture. Fisheries. American Fisheries Society. 26 (8): 6-16.
Tilmant, J.T. 1999. Management of nonindigenous aquatic fish in the U.S. National Park System. National Park Service. 50 pp.
Young, B. A., T. L. Welker, M. L. Wildhaber, C. R. Berry, and D. Scarnecchia, editors. 1997. Population structure and habitat use of benthic fishes along the Missouri and lower Yellowstone rivers. Annual Report of Missouri River Benthic Fish Study PD-95-5832. U.S. Army Corps of Engineers and the U.S. Bureau of Reclamation. 207 pp.
Zuckerman, L. D., and R. J. Behnke. 1986. Introduced fishes in the San Luis Valley, Colorado. Pages 435-452 In: R. H. Stroud, editor. Fish culture in fisheries management. Proceedings of a symposium on the role of fish culture in fisheries management at Lake Ozark, MO, March 31-April 3, 1985. American Fisheries Society, Bethesda, MD.
Other Resources:
Eastern and western mosquitofishes (from USGS Status and Trends book)
Species Summary for Gambusia affinis mosquitofish (FishBase)
Distribution in Illinois - ILNHS
Gambusia affinis (Global Invasive Species Database)
Great Lakes Water Life Photo Gallery
Author: Leo Nico, Pam Fuller and Greg Jacobs
Contributing Agencies: 
NOAA - GLERL
Revision Date: 4/13/2006 Citation for this information:
Leo Nico, Pam Fuller and Greg Jacobs. 2009. Gambusia affinis. USGS Nonindigenous Aquatic Species Database, Gainesville, FL.
<http://nas.er.usgs.gov/queries/FactSheet.asp?speciesID=846> Revision Date: 4/13/2006
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