Skip to main content

What are Mosquitofish?

Introduced to California in 1922, mosquitofish (Gambusia affinis) has become the most efficient biological control method used in mosquito control today. Mosquitofish are an attractive and effective alternative referred to as biological control, and part of the Districts integrated vector management program. Butte County Mosquito and Vector Control District maintains six fishponds at the Oroville Headquarters.

These ponds produce hundreds of pounds of mosquitofish each year. The mosquitofish are routinely stocked and planted by District Mosquito Control Specialists to control mosquito populations in sources such as irrigation ditches, industrial, ornamental and artificial ponds, un-maintained swimming pools, semi-permanent and permanent urban sources, and at times in rice fields and wetlands. California law allows the District to plant fish in water sources other than those located on private property. To avoid competition with sensitive native fish and other native aquatic organisms, the District does not stock mosquitofish in habitats where such species are known to be present.


Mosquitofish are small live-bearing fish closely related to the common guppy. They are extremely prolific, breed throughout the summer months and may have 3 to 5 broods annually. Each brood can contain 40-100 young fry. For optimal growth water temperatures are best between 77-86 degrees Fahrenheit. Mosquitofish size varies from ¾ of an inch to 3 inches in length. Females continue to grow after sexual maturation but the male fish stop once they become sexually mature. The small size of these fish allow them to swim and hunt in shallow waters and penetrate dense vegetation where larvae and pupae hide.

Feeding Behavior

Mosquitofish are omnivorous and have a voracious appetite for mosquito larvae. The flattened head and protruding mouth enable the fish to readily prey on surface feeding mosquito larvae and pupae. A large female can consume up to 500 larvae per day! All ages, sexes, and sizes of these fish eat mosquito larvae, other small aquatic invertebrate insects, and algae. The fish are visual predators and feed during daylight hours.


Mosquitofish can tolerate a wide temperature spectrum in waters, from 33 to 104 degrees Fahrenheit. They can tolerate temperature changes, organic pollution, salinity, poor food supply, and overcrowding. During the cooler months, the fish hibernate in the lower water depths, and reappear in early to late spring when water temperatures rise. The fish prefer sunlit areas of the body of water they live in and do not thrive in a heavily shaded environment. Their hardiness, adaptability, ease of handling during transportation, and relative lack of susceptibility to disease contribute to their effectiveness.

Mosquitofish Care

Mosquitofish are low maintenance and seldom need supplementary food. During the cooler months of the year when mosquito larvae and other aquatic organisms are scarce it is acceptable to feed the fish a diet of tropical fish food flakes and / or dry dog or cat food. Overfed fish may not feed on mosquito larvae and excess food may cause the water source to become polluted creating a negative effect to the fish. If predators are present in (other fish) or around (raccoons, opossums, birds) the water source a rock, log, or vegetation should be provided for shelter to the mosquitofish. Organic matter can create a negative impact on mosquitofish as well. Certain leaves like pine, oak, and eucalyptus contain chemicals that are harmful to the fish and if a large accumulation of these leaves impact their environment it could result in sick fish that will not feed on larvae. Small amounts of algae are beneficial to mosquitofish so their fry can seek shelter and it also provides a substantial food source, but too much algae will limit the fish’s effectiveness to hunt and prey upon mosquito larvae.

Mosquitofish Pick-Up or Delivery

Butte County Mosquito and Vector Control District provides FREE mosquitofish to residents for placement on their property. If you have a small pond, animal trough, or un-used swimming pool fish can be picked up at the District office in Oroville and at the Chico Substation. The District also maintains free fish pick up stations at several nurseries and feed stores in the county during the summer months. When you get your fish home, acclimate them to their new homes. Place the container with the fish directly into the water they will be residing in for 30-60 minutes or until the water and the container's water are nearly the same temperature. Then release the fish. If you have a large source of an acre or more, call the District to schedule a fish delivery or fill out the service request form.

To obtain information on mosquitofish please call the District headquarters at (530) 533-6038 or (530) 342-7350.

Join our mailing list