Identification: Distinguishing characteristics were provided by Rauchenberger (1989a) and Page and Burr (1991) (although the latter authors treated the two forms as subspecies). These two species 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.
Gambusia holbrooki can be distinguished from G. affinis 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 |
| Anal fin |
| 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 |
| Anal fin |
| G. affinis || 7 || 10 |
| G. holbrooki || 8 || 11 |
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). 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 1976; 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). 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).
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).
References: (click for full references)
Arthington, A.H., and L.N. Lloyd. 1989. Introduced poeciliids in Australia and New Zealand. Pages 333-348 in G.K. Meffe and F.F. Snelson, Jr., editors. Ecology and evolution of livebearing fishes (Poeciliidae). Prentice Hall, Englewood Cliffs, NJ.
Boschung, H.T., Jr., and R.L. Mayden. 2004. Fishes of Alabama. Smithsonian Books, Washington, DC.
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.
Danielsen, T.L. 1968. Differential predation on Culex pipiens and Anopheles albimanus mosquito larvae by two species of fish (Gambusia affinis and Cyprinodon nevadensis) and the effects of simulated reeds on predation. PhD dissertation, University of California, Riverside.
Deacon, J.E., C. Hubbs, and B.J. Zahuranec. 1964. Some effects of introduced fishes on the native fish fauna of southern Nevada. Copeia 1964:384-388.
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.
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.
Meffe, G.K. 1983. Attempted chemical renovation of an Arizona spring brook for management of the endangered Sonoran topminnow. North American Journal of Fisheries Management 3:315-21.
Meffe, G.K. 1985. Predation and species replacement in American southeastern fishes: a case study. Southwestern Naturalist 30:173-87.
Meffe, G.K., D.A. Hendrickson, and W.L. Minckley. 1983. Factors resulting in decline of the endangered Sonoran topminnow Poeciliopsis occidentalis (Atheriniformes: Poeciliidae) in the United States. Biological Conservation 25(1983):135-159.
Moyle, P.B. 1976. Inland fishes of California. University of California Press Berkeley, CA. http://books.google.com/books?id=8ZCStnV581kC&printsec=frontcover&dq=fishes+of+california&hl=en&sa=X&ei=t0dOT-P-Nsna0QH88rS7Ag&ved=0CDUQ6AEwAA#v=onepage&q=fishes%20of%20california&f=false.
Myers, G.S. 1967. Gambusia, the fish destroyer. Australian Zoology 13(2):102.
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.
Stephen, B.J. 2004. The mosquitofish problem. Nebraska Land. 82(4): 36-37.
Walters, D.M., and B.J. Freeman. 2000. Distribution of Gambusia (Poeciliidae) in a southeastern river system and the use of fin ray counts for species determination. Copeia 2000(2):555-559. http://www.jstor.org/stable/1448206
Ward, F. 1931. Notes on the food and parasites of the mosquito fish (Gambusia holbrooki) in Florida. Trans. Am. Fish. Soc. 61: 208-214.
Whitmore, S. 1997. Aquatic nuisance species in Region 6 of the Fish and Wildlife Service. US Fish and Wildlife Service, Great Plains Fish and Wildlife Mangement Assistance Office, Pierre, SD.
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.
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.