Gambusia affinis (Baird & Girard, 1853): western mosquitofish

Biological Description

Body enlarged anteriorly, compressed, deep in females; lateral line complete; scales 29 to 32 in a lateral series. Head moderate, broad and flattened above; mouth small, lower jaw projecting beyond the upper; teeth pointed and in broad villiform bands on both jaws; eye moderate. Dorsal fin small, 7 to 9 rays, origin behind that of anal fin. Caudal fin rounded, Anal fin of female large, 8 to 10 rays, in the male developed into a narrow long copulatory process. Color is light olive with a dark predorsal stripe on back; scales outlined with dark pigment; blackish spot or bar below the eye.

Life History

The mosquitofish belong to the group of fishes known as live-bearers. The males and females differ in external physical appearance and fertilization is internal. The anal fin of the male is elongated for transfer of sperm to the female. A female may produce several broods from a single fertilization. Eggs ripen in the ovaries and are ejected in a series into the ovarian cavity where they are fertilized and develop. Length of pregnancy depends on temperature, nourishment, and age of the female. However, as a rule the normal period is 21 to 28 days. The young are about 0.375 inches at birth; the average number of young produced by a female is about 50, but some females have produced 100 to 300. Since the gestation period is short, more than one generation per year is possible if the growing season is long enough. In nature there probably is a winter pause in reproduction.

Growth is rapid and fish reach mature size of 1 to 1.5 inches for males and 2.5 inches for females in a month or 6 weeks. Life span is short, seldom extending beyond 15 months.

Geographic Distribution

The natural range of the mosquitofish was New Jersey south to Florida and the Gulf of Mexico, and central United States from southern Illinois to Alabama and Mexico.

Habits and Habitats

Mosquitofish inhabit the quiet, shallow, densely vegetated areas of lakes, ponds, rivers, creeks, ditches, and springs. They survive, grow, and reproduce best in warm water, and tolerate temperatures between 40° and 100° F. They tolerate brackish water well and have lived in unusual places, such as rain barrels and sewer outfalls. This species usually does best in waters with a high mineral content and an abundant supply of plants and food organisms.


The food of the mosquitofish includes a variety of animals, such as zooplankton and small insect larvae, but also includes tiny aquatic plants such as diatoms and algae. Feeding is at the water surface and extends to the shoreline in water only an inch deep. The fish feeds heavily on the aquatic larvae of mosquitos. In one test, a single 2.3-inch female ate 225 large mosquito larvae in an hour, and a 1.25-inch male ate 28. Even at birth the newly hatched mosquitofish feed on the young, threadlike mosquito larvae.

 Idaho Conservation Status

Idaho Native or Import

Introduced. In 1974 the Idaho Department of Health and Welfare stocked 4 ponds in Elmore County with mosquitofish. In April 1975, they reported the fish reproduced well during the summer and fall of 1974 and appeared to have wintered well. Recently this species was stocked in the levee ponds at Lewiston and apper to have overwintered successfully.


It is world renowned because of its use in controlling mosquitoes. Field experiments in Michigan have revealed that mosquitofish were effective in reducing mosquito populations 80 to 95 percent in experimental ponds where natural conditions were simulated. It is doubtful it has had or will have much effect in Idaho in controlling mosquitoes because many of the mosquito-producing waters are temporary ponds or cool below 40° F and would not support the mosquitofish.


Simpson and Wallace 1982.Wydoski and Whitney 1979.Brown 1971Image Copyright Joseph Tomelleri. Used by permission.