Gambusia affins (mosquito fish) have been stocked throughout the world for mosquito control, particularly to combat malaria.  Its appetite for mosquito larvae is well known. A single mosquito fish can devour numerous mosquito larvae each day.

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Mosquito fish were successfully introduced into Utah from Selby County, Tennessee, in 1931.  They are found abundantly in warm springs and are restricted in their spread by the severity of winter.  The species is now established and can be found throughout the state.  Many of the mosquito districts in Utah have a mosquito fish stocking program.  mosquito fish are primarily used to control mosquito larvae in small ornamental pools and fish ponds.

The mosquito fish is a comparatively small species; the full growth females are usually under 2.5 inches in total length, while males are typically under 1.5 inches.  The muted silver and light olive green body color is common to both  sexes.  In addition, they are able to lighten or darken to the immediate environment.

Mosquito fish exhibit a tremendous tolerance for a wide range of water temperatures.  Previously acclimated fish may tolerate minimum and maximum temperatures of 33º and 104º F; although sudden drastic changes of temperature are often lethal.  The preferred water temperature appears to be between 77º and 86º F.  When surface water temperatures approach higher lethal limits, mosquito fish will move into shallow waters which are warmed more easily by the sun.  During winter, mosquito fish often congregate about a water supply inlet when this water is warmer than that of their overall habit.

Mosquito fish are ovoviviparous: that is, a female is fertilized internally, the eggs hatch within her body and are delivered as free-swimming independent juveniles.  This is the only fish in Utah that brings forth live young.  Their gestation period is quite variable (18-35 + days) as are their broods (10-300 + young).  Several broods of young may be produced from a single fertilization and are delivered during the warmer seasons in temperate climate.  In tropical climates almost continual reproduction may occur.  One can easily see that water temperature is very important to the growth and reproduction of this fish.  Cannibalism by the adults on the young can be very substantial, especially in areas where the newborn have nowhere to hide.  Juvenile fish may reach maturity in four to six weeks during the summer and may commence breeding activities shortly thereafter.

 The Mosquito Abatement District-Davis County (MAD-D) provides mosquito fish to any Davis County resident for placement in ornamental pools at their residence.  The MAD-D maintains a file of all known ornamental pools.  A trained inspector will deliver the mosquito fish to all pools on file, beginning in June.  Each ornamental pool will be inspected at least twice during the summer months.  In general, mosquito fish are stocked in very small numbers because they quickly reproduce to the maximum population levels that a particular habitat may sustain.

If supplement feed is needed, especially in temporary situations where very few natural food organisms exist, almost any lice or dried feed (even crushed dog food or chicken feed) may be sparingly applied.  Due to their opportunistic feeding habits supplemental feedings are not usually needed.

 In small, confined mosquito sources, such as ornamental pools, small fish ponds, bird baths and cattle water troughs, 10-15 adult fish will provide rapid mosquito control and soon reproduce to increase their numbers.  Ornamental pools and other sources need to be cleaned before fish are introduced.  Caution must be taken with the use of pesticides in or around pools in which fish reside.  Consideration of other mosquito predators and animal life must be taken into account before stocking of fish takes place.

If the aquatic environment receiving the fish begins drying up later in the year or is drained, the fish may be reduced by netting them.  Mosquito abatement personnel can assist in retrieving of the fish and transposing them for relocation.

• Keep the water level at one foot or more.

• Provide the mosquito fish with some shade or shelter, such as an overhead sun screen or even aquatic plants.  (Aquatic plants may be purchased at any large nursery.)

• Do not apply chlorine to pond

• Read labels carefully when using Algaecides to control algae growth in your ponds.  Make sure it is compatible with your fish.

• Fish will over winter in pond if the pond doesn't completely freeze over.