Mosquito Fish

While there are many kinds of fish that are called mosquito fish, the main species is Gambusia affinis, which was introduced to this country from South American in approximately 1958. In parts of the Southern and Eastern United States, Gambusia affinis and Gambusia holbrooki are considered native. Their usefulness as a natural mosquito predator for home users is limited to water gardens, ponds, container water features, animal water troughs and unused swimming pools. Obviously, they do nothing to address mosquito-breeding places like tree hollows, in bromeliads, used cans and bottles, or as is common in parts of Asia, coconut hulls.

Both the males and females are a silvery brown/gray color, but the females are much larger. The males average between .05-2 inches, the females between 1-3 inches.

They are moderately hardy, and can survive water temperatures from 33-100 degrees, Fahrenheit. Their preferred range of water temperature is from 60-80 degrees, Fahrenheit. They generally live from 1-3 years, unless you live in a climate with very cold temperatures. Many will not over winter, and will have to be replaced the next year.

As mosquito eaters, they are superb. One of the larger females can consume 100-200 mosquito larvae per day. Their preferred diet appears to be other insect larvae and small fish, or fry. For this reason, they are not recommended neighbors for ponds with other fish, as they will eat most of the fry of other species. They are very aggressive feeders and have a very assertive behavior pattern. Mosquito fish will readily consume small amphibian larvae, like newts, salamanders, toads and frogs.

Many people who have introduced them into backyard water features have been so displeased with Gambusia’s aggressive bullying, they have begun to refer to them as “dambusia”.

In an environment where other, larger fish are present, they will inhabit the shallows to avoid predation. Since mosquito larvae are more likely to live in shallow water, this helps keep the larvae exposed to the highest concentration of mosquito fish.

Mosquito fish are live breeders, giving live birth rather than laying eggs. And, after mating, the female can store sperm and produce several batches of offspring from one mating.

In a little less than a month after mating, the female will give birth to 10-100 small fry. If she is not removed from the fry, she will eat them. The fry are approximately 3/8 inch at birth, which means they are quite large enough to begin feeding on mosquito larvae immediately. Aside from their mother’s cannibalistic behavior, the fry are tasty morsels for other fish as well. The fry become mature enough to reproduce in about 6-8 weeks, so several generations can breed in one summer.

Mosquito fish prefer a protected environment, with plenty of aquatic plants to provide hiding places. It is also a good idea to provide a few large rocks for hiding places also; they need protection from herons, egrets, raccoons, opossums, and yes, . . . . even the family cat. Most algae are not harmful to them, but don’t allow a heavy growth of duckweed, a small floating plant that can cover the water’s surface. It will prevent them from feeding, and lower the oxygen content of the water.

Large fish, like Koi, do not feed on mosquito larvae. And, unlike larger species, they are small enough to thrive in smaller water features.

It is open to debate whether Gambusia does more harm than good. They have been transplanted into non-native habitats, world wide, premised on their ability to eat mosquito larvae.

Some experts are concerned that mosquito fish may actually eat some of the mosquito larvae’s other natural predators, in preference to mosquito larvae.

There have been scientific studies worldwide suggesting Gambusia has been responsible for the decline in population of numerous kinds of minnows, frogs and newts. Of course, these studies focused on their release into natural habitats.

As a consequence, if you choose to add mosquito fish to your mosquito-killing arsenal, it should only be under very controlled circumstances, such as being certain your water feature is completely isolated from natural habitats. And, mosquito fish should NEVER be released into a natural environment.

If you do decide to use them, most governmental vector control organizations will give them away for free. And, they can also be obtained commercially. They do exist in the wild, but before harvesting that way, be sure to check with your local authorities to see if there are any restrictions on capture or transport.

If you are undecided about whether to use mosquito fish, keep in mind that there are other species that will eat the larvae. Small goldfish and fathead minnows will also feed on mosquito larvae and they are both less aggressive than mosquito fish. And, don’t forget, besides mosquito fish, swallows, bats, purple martins and dragonflies also eat mosquitoes.

If you do have a pond or water garden, you may decide to try mosquito fish for one season and see if they work for you. If not, you can try another kind of fish. It is all a matter of preference. For additional research on gambusia, check out the Gambusia Control Association.