Forest Preserve District of Cook County, Illinois
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The Mosquitofish
Nature Bulletin No. 182-A   March 6, 1965
Forest Preserve District of Cook County
Seymour Simon, President
Roland F. Eisenbeis, Supt. of Conservation

In our forest preserves we have restored many marshes, both large and small, that had been drained or partially drained by former owners of the land. By blocking tile drains and building low dams at the natural outlets from valleys or low wet areas, we not only have restored old marshes but have created many new lakes, lagoons, ponds and sloughs.

Such areas soon become populated with aquatic plants and animals. Then they attract many kinds of wildlife that come there to drink, bathe, prey and feed, or build their homes. Such areas have life, beauty and interest the whole year round. We call them "Wildlife Oases". But they present one problem, important in a county of 4,500,000 people: mosquitoes.

Fortunately, the Chicago region has a fish immigrant from the southern state with which we control the mosquitoes that breed in such waters. It is the Mosquitofish, or Gambusia, one of the little topminnows or killifishes, and a near relative of the guppies, swordtails and moons -- popular aquarium fishes also from warmer climates. Like them, and unlike the other native killifishes of the northern states, the young of the Gambusia are born alive. It is called the mosquitofish because, more than any other kind, it regularly feeds among trash and vegetation in shallow water and along shores where mosquitoes breed.

It was first successfully introduced into northern Illinois in 1923, when some of these fish were brought from a pond on the campus of Southern Illinois Normal University, at Carbondale, and placed in a garden pool in Winnetka, a north shore suburb of Chicago. Carbondale, over 300 miles south, is near the northern limit of the Gambusia's normal range.

In 1928 and 1929, more of these little fish from the Carbondale pond were placed in ponds on golf courses near Chicago by the DesPlaines Valley Mosquito Abatement District; but none survived in a winter. So, in 1933, this organization obtained mosquitofish from the Winnetka pool and placed them in ponds in our forest preserves and elsewhere. Enough of these survived and multiplied so that these ponds have served as hatcheries for further distribution of this hardy "naturalized" northern strain, obtained by unique good fortune from one or more rare individual fish adapted to survive long winters beneath the ice. Since 1941, some of this same strain have been successfully planted in a variety of Michigan waters as far north as the Straits of Mackinac.

The two sexes of the mosquitofish are more strikingly different in size than any other native fish. The mature female is usually less than two inches long but she is twice as long and about ten times as heavy as the mature male. Females usually produce 3 or 4 broods in a season and an average of about 50 young per brood, but exceptionally large females may give birth to broods of 300 young. Apparently, mosquitofish do not often live longer than two years.

Mosquitoes lay little raft-like masses of eggs on the surface of water. These hatch out larvae which must come frequently to the surface to breathe. After they become pupae, the pupa dangles from the surface film of the water by a tube thru which it breathes. Mosquitofish have upturned mouths, and they work along the surface, gobbling down mosquito eggs, larvae and pupae. In some waters these fish multiply until they not only effectively control mosquitoes but also serve as an important item of food for hook-and-line fish. It may develop that the introduction of Gambusia into waters of the "vacation regions" of the northern states may accomplish more, in two ways, to increase the pleasure of vacationists than many more expensive programs of fish management and of mosquitofish control.

Water plus Gambusia equals recreation minus mosquitoes.

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