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Western Mosquitofish (Gambusia affinis)

Western Mosquitofish
Gambusia affinis
(Order Cyprinodontoformes; Family Poeciliidae)

Female (above) and male (below) Gambusia affinis.  Image from

Adult Diagnosis: Adults have bodies that are light olive or a dull grey/brown. They have slightly speckled fins without bars or bands and rounded tails. Mosquito fish are characterized by short bodies and flat heads with terminal mouths pointing upward. Females have a rounded, small anal fin and grow to about 6-7centimeters. Males grow to about 4 centimeters and have an elongated intromittent organ (gonopodium).

Juvenile Diagnosis: Juveniles resemble adults in miniature.

Natural History: Mosquito fish were implemented for mosquito control due to their top feeding behavior. Females bear live young; a unique feature of many members of the Poeciliidae family. Sperm transfer occurs when Gambusia males insert their gonopodium in the oviductal opening of the female. Females can store viable sperm for several months. After fertilization young are born in about 24 days. Maturity occurs in 1 month for males and 6 weeks for females. The maximum age of mosquito fish is 4 years. Gambusia affinis prefers temperatures between 12-29 degrees Celsius. However, in deep enough water they are able to reduce activity and go into a hibernation-like state.

Distribution: Gambusia is native to the Atlantic and Gulf slope drainages, but has been stocked (usually for mosquito control) in 38 states around the United States. Some people consider the species invasive because they can flourish in many habitats and outcompete native species or have effects on local populations of amphibians. In Kansas, the Western Mosquitofish is part of the native fauna.

Western Mosquitofish distribution.  Brown color shows native distribution. Map from USGS

Habitat: Gambusia affinis typically require structure for protection in order to maintain a large population. They are found in freshwater or protected brackish water conditions.

Diet: As a topminnow (seeks food at the surface) mosquitofish feed on zooplankton and invertebrates including mosquito larvae. One fish will consume anywhere from 42-167% of its body weight per day. Occasionally they will opportunistically feed on their young.

Species Status: Gambusia affinis is stable in its native range and in many of the states where it has been stocked.



USGS Gambusia affinis fact sheet:
Oklahoma Department of Wildlife Conservation:
Smithsonian Marine Station at Fort Pierce:


Pyke, Graham H. “A review of the biology of Gambusia affinis and G. holbrooki.” Reviews in Fish Biology and Fisheries 15 (2005): 339-365. 
Burner, Allison B, Geoffrey R. Smith, Christopher J. Dibble, Andrew J. Terlecky, Christopher B. Dayer, and Megan E. Ogle. “Effects of Invasive Western Mosquitofish and Ammonium Nitrate on Green Frog Tadpoles.” Copeia 2 (2013): 248-253. 
Krumholz, Louis A. “Reproduction in the Western Mosquitofish, Gambusia affinis affinis (Baird & Girard), and its use in Mosquito control.” Ecological Monographs 18.1 (1948): 1-43. 

Submitted by: Corinne Juju Wellemeyer, July 2013

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