Mosquitofish (Gambusia affinis) are small fish in the guppy family, which are used extensively throughout the world by mosquito control agencies for control of mosquito larvae.

A single adult mosquitofish can eat up to 500 mosquito larvae a day. The adaptability and hardiness of the mosquitofish, coupled with its ability to produce large numbers of fry (newly born fish) during their lifetime, has made them a valuable biological control agent.

Female mosquitofish are 2 to 2 ½ inches in length, while the males seldom exceed 1 ½ inches. These fish are live bearers, producing 60 to 100 fry in a single brood. Under favorable conditions, the fry reach sexual maturity in six to eight weeks. Females may bear three to four broods in a season, the first of which may number only a few fry. Reproductive efficiency declines with age and later broods become smaller.

The District uses mosquitofish in situations where the fish cannot escape into natural bodies of water, such as contained backyard ponds, ornamental pools, fountains, and neglected swimming pools. This service is done at no additional cost to residents. If you have a pond and would like to have fish or request a pond inspection, please call the District Office at 650-344-8592. Mosquitofish are not allowed to be used in natural sources where they can reach creeks or other natural waterways.

TAP WATER IS TOXIC TO MOSQUITOFISH AND MUST BE TREATED FOR CHLORAMINES BEFORE MOSQUITOFISH CAN BE ADDED

Tap water in San Mateo County is treated with chloramines, not chlorine. Chloramines are toxic to fish, shellfish, reptiles and amphibians, but do not affect humans or mosquitoes.

Chloramines are type of chemicals containing chlorine and ammonia. Chloramines are considered safer for drinking water than chlorine because they reduce the formation of hazardous by-products and last much longer than simple chlorine. Unlike chlorine, chloramines do not evaporate from water if it is left standing overnight.

Water used for fish must be treated to remove both the chlorine and ammonia components of chloramines. This can be done by running the water through a properly sized activated charcoal filter, biological filter, or by treating it with chemicals. Chemicals designed to remove chloramines are also available at pet stores and fish supply stores. Always pre-treat your water before adding it to your pond, no matter how little you add. Replacing as little 1% of the water in a pond with fresh tap water will add enough chloramines to kill the fish. Chloramine residuals in treated water should be below 0.1 mg per liter. Kits to test the amount of chloramines in the water can be purchased at most pet and fish supply stores.

Mosquitofish eat a lot!  And they are very good at increasing their populations. Only 6 to 10 fish are enough to start the population in most situations. If you have a water source that already has a lot of mosquito larvae, it may be necessary to clean the pond or to have the District treat it with larvacides prior to adding mosquitofish.

Mosquitofish are hardy, but they need to be transported carefully. Large plastic containers (like buckets from 1-5 gallons) are most practical; glass and metal containers can overheat the fish and some metals may be toxic to the fish. Keep the bucket out of direct sunlight while transporting the fish. Do not seal the container completely, so that some air can enter/exit the container.

It's best to NOT dump the fish directly into the water. Set the container upright in the water for about 30-60 minutes before releasing the fish into the water. This will allow them to get used to the water temperature and have a better chance of surviving.

Mosquitofish in an outdoor pond do not need be fed during the warmer part of the year.  Don't worry - they are very resourceful at finding food in the water! Along with eating mosquito larvae, they will also eat some other kinds of aquatic insects and algae in the water. While mosquitofish will happily eat fish food flakes, sometimes this results in them eating fewer mosquito larvae, so we don't recommend it.

In the colder months, the fish slow down and eat less. They typically do not need much food when the water is cold.

 Overfeeding can also cause the water to become very dirty, which can be bad for the fishes' health.

Preparation:

• Treat all tap water with a chloramines removal kit
• To provide a safe space from raccoons and other predators, ponds should be at least two feet deep, have steep sides, and some cover like rocks and aquatic plants in the water.
• Provide a circulation pump to enhance water quality
• Pond lining: If it is a new cement pond, allow several weeks to leach out the lime. Rinse the pond several times and fill with fresh water. After 48 hours (and pretreatment for chloramines), request mosquito fish for pond.

Maintenance:

• Thin or remove excess aquatic plants and keep landscape plants trimmed away from the pond edge
• Fallen leaves can be toxic to fish; remove dead vegetation frequently
• Avoid shallow, sloping edges

Shade vs. Sun

Although mosquitofish prefer the shelter of rocks, overhanging plants, and banks, they do not thrive in heavily shaded ponds. Normally, the fish seek out a sunny section of a pond and orient themselves to shallow areas with submerged vegetation, locations often frequented by mosquito larvae and pupae. It is recommended that some aquatic vegetation be provided in ponds as shelter since mosquito fish are cannibalistic and will feed on smaller fish. In large ponds, the mosquitofish will sometimes avoid the shaded areas even though numerous mosquito larvae are present. This may occur if the fish are fed artificially or if there is an abundance of naturally occurring food in other areas of the pond.

Algae

Limited amounts of algae can be beneficial to mosquitofish. An overabundance of algae should be removed. Removal can be achieved by physically cleaning the pond or application of an appropriate algaecide. If an algaecide is selected, care must be practiced when following the directions. An overdose of the chemical could be fatal to the fish.

Bird baths or plastic wading pools are NOT suitable as fish ponds. Shallow water does not provide adequate protection from elevated temperatures or predators. In addition, high temperatures enhance algal growth and deplete oxygen in the water.

Mosquitofish are fairly hardy and can live under a variety of conditions. In California, mosquitofish can live for two to three years. They can often live through the winter in outdoor ponds, but at very low temperatures (below freezing), they may not survive.