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Montana Field Guide

Montana Field Guides

Western Mosquitofish - Gambusia affinis

Non-native Species

Global Rank: G5
State Rank: SNA

Agency Status


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General Description
The only livebearer species widely distributed, the mosquitofish, is established in a thermal spring east of the Divide and can be found in other non-thermal waters in the eastern drainages during the summer. The wider summer distribution is the result of the State Board of Health planting them in mosquito-infested areas. Mosquitofish are planted because they eat crustaceans and insects on or near the water's surface, including a high number of mosquito larvae when available. Mosquitofish thus serve as a biological control for mosquitoes. Mosquitofish do not overwinter in Montana in non-thermal waters. This species shows marked sexual differences with females becoming larger than males. Females can reach about 3 inches in length.

Diagnostic Characteristics
Color is light olive to dull silver. Lateral line is absent.

Species Range
Resident Year Round

Western Hemisphere Range


Observations in Montana Natural Heritage Program Database
Number of Observations: 3

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density



(Observations spanning multiple months or years are excluded from time charts)

Year round survival in Montana is restricted to waters with thermal influences (Elser et al. 1980).

Food Habits
Includes all kinds of tiny aquatic plants and animals. Usually feeds at surface in very shallow water where mosquito larvae are typically found (Brown 1971).

Females larger than males.

Reproductive Characteristics
Sexually mature in 4-6 wks. Internal fertilization. The eggs hatch internally and young emerge as free-swimming individuals. Fry produce March - Oct. in one Montana pond studied (Brown 1971).

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Citation for data on this website:
Western Mosquitofish — Gambusia affinis.  Montana Field Guide.  .  Retrieved on , from