THE MOSQUITOFISH affinis

Adult male (above) and adult female (below) Gambusia affinis affinis. Courtesy of Dr. Louis A. Krumholz.

UNITED STATES DEPARTMENT OF THE INTERIOR FISH AND WILDLIFE SERVICE BUREAU OF COMMERCIAL FISHERIES Fishery Leaflet 525 THE MOSQUITOFISH, Gambusia affinis

By Lola T. Dees Branch of Reports Division of Resource Development

CONTENTS

Page Introduction ...... 1 Range ...... 1 Foreign introductions ...... 2 Habitat ...... 2 Life history ...... 2 Description ...... 2 Food ...... 3 Reproduction ...... 3 The young ...... 3 Enemies ...... 3 Culture ...... 3 Dealers ...... 4 References ...... 4

INTRODUCTION Several of Gambusia, of the Cyprinodontes and the Poe- An insignificant-looking little fish, Gam- ciliidae, occur in the warm or tropical busia affinis, is largely responsible for mak- areas of the United States, Mexico, and ing many parts of the world livable for man. Central America. The species used to con- In 1918 the great value of Gambusia as an trol mosquitoes is Gambusia affinis (Baird and eradicator of the aquatic stages of the Girard). This species has two subspe- was first clearly shown when cies: Gambusia affinis affinis and Gambusia affinis these fish were used at Camp Hancock in holbrooki. They are known also as mosquito- Augusta, Georgia. fish, because they feed on mosquito larvae, Gambusia eat aquatic larvae of mos- and as topminnows, since they are shaped quitoes, including the species that cause like a and feed at the surface of the malaria and yellow fever. One female, water. 2 1/3 inches long, ate 225 large mosquito larvae within 1 hour, and a male, 1 1/4 RANGE inches long, ate 28 larvae during the same period. At birth Gambusia commence feed- The natural range of G. a. affinis extends ing on newly hatched mosquito larvae, which, along the Gulf of Mexico coast from the when first hatched, are threadlike mouth of the Rio Grande as far west as almost microscopic in size. Bay St. Louis, Mississippi, and through the If there are no aquatic plants or debris Mississippi River and its tributary waters (which furnish hiding places for mosquito as far north as central . Introduc- larvae) or predators, Gambusia will con- tions have extended the range of G. a. affinis trol a mosquito population. Experiments northward, even into Canada. They have been extending over two summers in Michigan successfully introduced into Arizona, Cali- revealed that Gambusia reduced mosquito fornia, northern Illinois, , Kansas, production 80 to 95 percent in ponds where Massachusetts, Michigan, Missouri, Ne- natural conditions were simulated (Krum- vada, New Jersey, New Mexico, New York, holz, 1948b). When introduced into a fish- Ohio, Pennsylvania, Utah, Washington, Wis- free pond at the height of the mosquito consin, and probably other States. b reeding season, these mosquitofishbrought The other subspecies, G. a. holbrooki, about an 80 percent reduction in mosquito ranges from southern New Jersey along the reproduction within a week. Atlantic coast to the Florida Keys and along

2 the Gulf of Mexico coast as far as Apa- sewage. In this environment they stayed at lachicola, Florida. It is not hardy enough the surface at all times and appeared to be to survive northern winters (Wascko, 1950). respiring at the airwater interface.

FOREIGN INTRODUCTIONS LIFE HISTORY Both subspecies have been introduced into many foreign lands (Hildebrand, 1931; Description Krurnholz, 1948b). In about 1920 the Inter- national Red Cross asked the United States The two subspecies are almost identical Bureau of Fisheries for brood stocks of in color, form, and size. While the color is Gambusia for Italy and Spain. The ship- usually olive or dull silvery, darkest on ment of G. a. holbrooki from Augusta, Georgia, the head and lightest on the belly, it varies to Italy failed. However, the fish of the according to the character of the habitat. same species sent to Spain arrived in Individuals living in ditches and drains are fairly good condition and multiplied rapidly. usually pale, and those in dark-colored Within a year or so Italy obtained a brood water of swamps are dark green, with a stock from Spain but the fish were unable distinct purple bar below the eye. There to withstand the winters in northern Italy. is frequently a fine dark line along the In 1927, G a. affinis, hardier than G. a. holbrooki, sides, sometimes a dark blotch below the were shipped from Carbondale, Illinois, eye; a dark purplish blotch is usually pres- to Trieste, Italy. These fish survived and ent on the side above the vent inthe female. multiplied. The has 2 or 3 transverse rows From the original introductions into of fine black spots. The anal fin in the Spain and Italy, Gambusia have been dis- female is dark-edged; the caudal fin has 3 tributed to nearly all south European coun- or 4 irregular transverse rows of dark tries and to Germany and Austria. They spots; the other fins are dusky. The fins were introduced also into Palestine, the are small, and the scales large in propor- Philippine Islands (from thence to China tion to the size of the fish. Dark pigment and Japan), , Thailand, Union of outlines each scale. South Africa, Hawaiian Islands, the West There are a few differences between the Indies, and Argentina. From these places two subspecies. G. a. holbrooki, has 8 rays in of introduction, Gambusia spread far and the dorsal fin, and the third ray of the wide until they have become almost cos- gonopodium (sexual organ) shows a deep mopolitan in the warmer sections of the split when examined under the micro- world. scope (Innes, 1956). G. a. affinis has only 7 rays in the dorsal fin, and the split in the third ray of the gonopodium is lacking. HABITAT The sexes of the same species differ in The two subspecies are found in many form and structure. The female has a types of water whose temperature may deeper body than the male. Females, whose 0 range from 40° to 100 F. They flourish maximum size depends on the fertility of in sluggish and standing water, fresh, the water in which they live, grow through- brackish or marine, clear or muddy, deep out life and may reach a length of 2 1/2 or shallow, open or overgrown, but prefer inches. Males grow little after the gono- clear shallow water. They probably seek podium is formed (Turner, 1941) and do shallows to escape large predacious fish, not exceed 1 1/2 inches. The female has a such as largemouth black bass, and to feed small, rounded anal fin with the rays con- on mosquito larvae, which are more nected by a membrane, and the male has numerous there than elsewhere. the third, fourth, and fifth anal fin rays Mosquitofish occur in ditches and pools united and lengthened to form the gono- containing either hard or brackish water. podium (picture on cover). Common characteristics of these environ- Generally mosquitofish, which are rela- ments are a relatively high mineral content tively free from disease, die during the in the form of dissolved solids and an summer in which they mature. Some females abundant supply of plant and food. mature in their first summer of life; others They thrive too in artesian well discharges, do not mature until their second summer cisterns, water tanks, potholes, and rain and die when about 15 months old. Less barrels. They have been known to live also hardy than females, males die at an earlier in an open municipal septic tank containing age.

3 Food

While adult Gambusia eat principally larvae and nymphs of aquatic insects, they also eat plant food such as diatoms, de smids , and filamentous algae. The young feed mostly on plankton (the passively floating or weakly swimming animal and plant life).

Reproduction Gambusia are ovoviviparous; that is, a female produces eggs that hatch within her body. The male fertilizes the eggs in the female by using the gonopodium to deposit sperm in her genital tract. A female is able to produce several broods of young after being fertilized once. Studies of 30,093 specimens of G. a. affinis collected during 1938, 1939, and 1940 from eight ponds in northeastern and central Figure 1.—Pregnant female Gambusia affinis affinis. The brood Illinois reveal much about their breeding of 79 were removed later. The yolk sac has been habits (Krumholz, 1948b). Males matured completely absorbed, and the embryos are nearly fully at smaller sizes and at slightly earlier ages developed and about ready for birth. Courtesy of Dr. Louis A. Krumholz. than females. Some females in two ponds became sexually mature for the first time early in their second summer of life when 8 to 10 months old and 1 to 2 inches long. They reproduced for 10 to 15 weeks, usually bearing 3 to 4 broods. The offspring of their first and possibly their second brood became sexually mature during their first summer of life when 4 to 6 weeks old and about 1 inch long. They reproduced for 4 to 10 weeks, generally bearing only 1 or 2 broods. In the first brood, males and females were in almost equal numbers, but in suc- ceeding broods the females gradually out- numbered the males. The average number of young in a brood decreased as the length of the mother increased. Some metabolic factor, coincident with the approaching end of the reproductive season, may control the number of young.

The young Figure 2.- -Adult male (above) and adult female (below) Gambusia Fertilized eggs hatch in 21 to 28 days. affinis affinis and young. The young are about 4 hours old. Within several hours a female expels from Courtesy of Dr. Louis A. Krumholz. a few to several hundred live young (figs. and 2). About three-eighths of an inch long ENEMIES at birth, they are well developed and com- Birds, frogs, large fish, andwater snakes pletely formed. They have prominent black devour Gambusia. eyes and are often uniformly yellowish. The fins are dusky, and the caudal fin CULTURE usually has a cross series of dots. At birth they are able to swim. Gambusia can be easily bred in aquariums0 Growth of mosquitofish is rapid, depend- at water temperatures of about 75 F. ing on the food supply and to a lesser extent While they prefer live foods, they will eat on the water temperature. prepared foods.

4 They should not be mixed with other 1925. A study of the top minnow Gambusia species in aquariums, because Gambusia holbrooki in its relation to mosquito will often attack other fish by nipping control. U. S. Public Health Service, pieces from the fins and tails. Bulletin No. 153, 36 p. In nature, female Gambusia seldom eat 1931. Gambusia in foreign lands. Sci- their young because a good supply of natural ence, vol. 74, no. 1930, p. 655-656. food is usually available. In aquariums, however, the young must hide among the Howard, H. H. plants to escape the mother. For protec- 1920. Use of top minnow (Gambusia affinis) tion, the young should be separated from as an agent in . the adults. There are two methods to Rockefeller Foundation International accomplish this. One is to provide the young Health Board, Report No. 7486, 59p. with hiding places, such as dense growths of plants, and the other is to use a parti- Innes, William T. tion to separate the mother from her off- 1956. Exotic aquarium fishes. 19th ed. spring (Innes, 1956). Innes Publishing Company, Phila- delphia, Pennsylvania, 541 p. DEALERS Krumholz, Louis A. Most tropical fish dealers either have 1944. Northward acclimatization of the Gambusia in stock or can order them. western mosquitofish, Gambusia affinis affinis, Copeia, 1944, no. 2, p. 82-85. REFERENCES 1948a. The mosquitofish, Garn.busia, es- tablished in the Great Lakes region. Barnickol, Paul G. Copeia, 1948, no. 2, p. 144. 1941. Food habits of from Gambusia affinis 1948b. Reproduction in the western Reelfoot Lake, Tennessee, with spe- mosquitofish, tial reference to malaria control. Gambusia af finis affinis (Baird & Girard), and its use in Report of the Reelfoot Lake Bio- mosquito control. Ecological Mono- logical Station, vol. 5, p. 5-13. graphs, vol. 18, no. 1, p. 1-43.

Gerberich, John B. Mail, G. Allen. 1946. An annotated bibliography of [298] 1954. The mosquito fish Gambusia affinis papers relating to the control of (Baird and Girard) in Alberta. Mos- mosquitoes by the use of fish. Ameri- quito News, vol. 14, no. 3, p. 120. can Midland Naturalist, vol. 36, no. 1, p. 87-131. Moore, J. Percy. 1922. Use of fishes for control of mos- quitoes in northern fresh waters of Hess, A. D., and Clarence M. Tarzwell. the United States. Report of the U.S. The feeding habits of Gambusia affinis 1942. Commissioner of Fisheries for 1922, affinis, with special reference to the app. 4, 60 p. malaria mosquito, Ano ph ales auadri- maculatus. American Journal of Mosquito News. Hygiene, vol. 35, no. 1, p. 142-151. 1960. The ability of the top minnow, Gambusia affinis (Baird & Girard) to reproduce and overwinter in an out- Hildebrand, Samuel F. door pond at Winnipeg, Manitoba, 1921. Top in relation to malaria Canada. Its vol. 20, no. 1, p. 55-56. control, with notes on their habits and distribution. U. S. Public Health Phillips, W. J. Service, Bulletin No. 114, 34 p. 1930. Use of fishes for control of mos- quitoes. New Zealand Journal of 1922. Fishes as guardians of health. Science and Technology, vol. 12, Outlook, vol. 130, no. 12, p.465-467. no. 1, p. 19-20.

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Rees, Don M. Rice, Lucile A. 1934. Notes on mosquito fish in Utah, 1941. Gambusia affinis in relation to food Gambusia affinis (Baird and Girard). habits from Reelfoot Lake, 1940, Copeia, 1934, no. 4, p. 157-159. with special emphasis on malaria control. Report of the Reelfoot Lake 1945a. Supplemental notes on mosquito Biological Station, vol. 5, p. 77-87. fish in Utah, Gambusia affinis (Baird and Girard). Copeia, 1945, no. 4, Turner, C. L. p. 236. 1941. Morphogenesis of the gonopodium in Gambusia affinis affinis . Journal of 1945b. The utilization of fish by a mos- Morphology, vol. 69, no. 1, p. 161- quito abatement district; their effec- 185. tiveness and limitations. Proceed- ings of the Thirty-Second Annual Wascko, Harold. Meeting of the New Jersey Mosquito 1950. Gambusia in northwestern Ohio. Extermination Association, p. 211- Ohio Department of Natural Re- 216. sources, Bulletin No. 240, 4 p.

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