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

****:THE MOSQUITOFISH

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.