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Gambusia (western mosquito fish)
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Gambusia affinis affinis (Baird and Girard)
Common Names: Western mosquitofish, potguts, western gambusia


This fish has been stocked throughout the world for mosquito control, particularly to combat malaria, and its appetite for mosquito larvae is well known. In Utah, however, it is restricted in its spread by the severity of the winters, for it will not tolerate prolonged cold (below 40 degrees F). Nevertheless, it has become established in the state, and is used extensively in organized mosquito abatement districts, especially in ornamental pools and small ponds. It is not effective, however, in extensive freshwater marshes. The species also serves to some extent as a bait and forage fish.


This mosquitofish is native to the central United States, from southern Illinois and southern Indiana to Alabama and the mouth of the Rio Grande, Texas.

In Utah it has been reported (Rees, 1945; personal communication, 1959) from Salt Lake City and vicinity, Saratoga Springs and Payson, Utah County, and in many localities north of Salt Lake City, in Davis, Weber, Morgan and Box Elder Counties, and also in southern Utah, Washington County. On June 6, 1942, it was abundant in a bubbling spring just off Lake Point at the north end of the Oquirrh Mountains along the southern margin of Great Salt Lake, Tooele County. On August 21, 1960, it was abundant at Bluff, San Juan County, in the overflow of a piped spring. The species was successfully introduced from Selby County, Tennessee, in 1931 (Rees, 1934). It is most abundant in springs.


The rounded caudal fin, large scales on the head as well as the body, superior position of the mouth, posteriorly placed dorsal fin originating behind the origin of the anal, and the modification of the anal fin of the male into a spike-like reproductive organ, all serve to identify the species. The dorsal fin usually has six rays, sometimes seven. There are about 30 scales along the side, which lacks a true lateral line.

In life, mosquitofish are olive or dull silvery, darkest on the head and back and lightest on the belly. The scales are outlined by dark pigment and there is usually one or several rows of black spots across the caudal fin, more prominant in females. Frequently there is a wedge-shaped bar below the eye, and a very narrow dark line running along the midside from the head to the base of the caudal fin. Pregnant females are pot-bellied and have a black spot just above the anus.


This is the only species of fish in Utah that brings forth its young alive, like the familiar guppy. Fertilization is internal, the male transferring the sperm to the genital tract of the female by means of its specialized anal fin. The breeding season occurs during the summer of Utah, or, in warm springs, it may continue throughout the year, although not as many young are born during the winter as in other seasons.

The number of offspring produced in a single brood varies with the size of the female, larger ones generally producing more young; from one to more than 300 may be born per birth. The number of broods liberated by a female during one season depends chiefly on the age at which maturity is attained and varies from one to five. Increasing age brings about a decrease in fecundity because mosquitofish and other members of the family Poeciliidae, unlike most fishes, go through a period of sterile senility following the reproductive period. Newborn young are usually eight to nine mm. long, a little more than one-third of an inch, and are able to fend for themselves from birth. The interval between births varies from about three weeks to one month.

The female is larger than the male because she keeps growing throughout life, whereas the male ceases to grow when maturity is reached, usually in four to six weeks. The size attained by males is not more than one and one-fourth inches, whereas large females may be two and one-half inches long.

The food of mosquitofish consists chiefly of small animals such as crustaceans and insects, but diatoms and algae are also eaten. The species feeds at the surface and invades water only a few inches deep, thereby permitting the fish to prey effectively on mosquito wrigglers. The life span is from about six to fifteen months. (Krumholz, 1948).


This species inhabits lakes, rivers, creeks, ponds, springs, and ditches, seeking the quieter, shallower waters where the vegetation often is dense. It survives the winter in Utah in warm springs and also in cold-water pools that do not freeze completely or do so for only a few days at a time. Habitats that are maintained by running water originating in springs or artesian wells meet these conditions and allow the survival of a brood stock for restocking in the spring. Mosquitofish tolerate temperatures from about 40 degrees F to over 100 degrees F.

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