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The Nonindigenous Occurrences section of the NAS species profiles has a new structure. The section is now dynamically updated from the NAS database to ensure that it contains the most current and accurate information. Occurrences are summarized in Table 1, alphabetically by state, with years of earliest and most recent observations, and the tally and names of drainages where the species was observed. The table contains hyperlinks to collections tables of specimens based on the states, years, and drainages selected. References to specimens that were not obtained through sighting reports and personal communications are found through the hyperlink in the Table 1 caption or through the individual specimens linked in the collections tables.

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Common name: Western Mosquitofish

Taxonomy: available through www.itis.gov

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 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:

	Dorsal fin rays	Anal fin elements	Teeth on 1st gonopodial ray (3rd anal fin ray)
G. affinis	6	9	absent
G. holbrooki	7	10	present

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

	Dorsal fin elements	Anal fin elements
G. affinis	7	10
G. holbrooki	8	11

 

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).

Alaska	Hawaii	Puerto Rico & Virgin Islands	Guam Saipan

Native range data for this species provided in part by NatureServe

Hydrologic Unit Codes (HUCs) Explained
Interactive maps: Point Distribution Maps

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.

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 Utah (Whitmore 1997). Meffe (1983, 1985) found that mosquitofish are very aggressive, even toward larger fish. They often attack, shred fins, and sometimes kill other species. Mosquitofish are known to prey on eggs, larvae, and juveniles of various fishes, including those of largemouth bass and common carp; they are also known to prey on adults of smaller species (Meffe 1985; Courtenay and Meffe 1989). Courtenay and Meffe (1989) listed impacts on a variety of native fishes.

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).

Introductions of Western Mosquitofish have been implicated in the current restricted distribution of plains topminnow in Nebraska and may be affecting populations in Wyoming (Rahel and Thel 2004; Wyoming Fish and Game Department 2010). Schumann et al. (2015) examined the impacts of mosquitofish on populations of plains topminnow and plains killifish (Fundulus kansae) in Nebraska using mesocosm trials, finding increased fundulid mortailty through direct predation on larval fishes and aggression towards juveniles, as well as alteration in activity patterns and microhabitat use by the native species in the presence of mosquitofish.

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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Other Resources:
Gambusia Control Homepage

Distribution in Illinois - ILNHS

Gambusia affinis (Global Invasive Species Database)

Great Lakes Waterlife

US Fish and Wildlife Service Ecological Risk Screening Summary for Gambusia affinis

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

Peer Review Date: 1/25/2016

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.