The western mosquitofish, Gambusia affinis, is native to much of the south-central United States and Mexico. It is not native to Utah, although it has been introduced to the state and it is now established in warm springs and ponds in many parts of Utah. The species is a member of the livebearer family, meaning that it gives birth to live young (all other fish species in Utah produce eggs that are fertilized outside of the female's body). Other members of the livebearer family include the popular aquarium fishes guppies, platies, and mollies. The western mosquitofish breeds throughout the spring and summer. Gestation takes less than one month, so several groups of offspring can be produced each year.
The western mosquitofish feeds predominantly at the surface, where it eats insect larvae and small invertebrates. It also preys upon the eggs and young of fish and amphibians, often negatively impacting native species in areas where the mosquitofish becomes established.