Gambusia affinis

Invasion History

First Non-native North American Tidal Record: 1941
First Non-native West Coast Tidal Record: 1941
First Non-native East/Gulf Coast Tidal Record: 1986

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General Invasion History:

Western Mosquitofish (Gambusia affinis) is found in the Mississippi drainage from Kansas to southern Illinois, and in Gulf of Mexico drainages from Mobile Bay to San Antonio Bay, and in inland portions of the drainages south to the Rio Grande. Its similar congener G. holbrooki ranges from Alabama and Florida north to Tennessee, Maryland, and Delaware. The two species were formerly (pre-1990s) considered conspecific, as subspecies of G. affinis. Mosquitofish are characteristic of shallow, enclosed fresh and brackish waters, including marshes and swamps, but can occur in lagoons with salinities of 20-40 PSU (Englund et al. 2000; Pyke 2005). Both species of Mosquitofish have been introduced around the world as a biocontrol agent for mosquitos. They are also imported as research animals or aquarium fishes. Most of the established, introduced populations in North America are G. affinis (USGS Nonindigenous Aquatic Species Program 2018). Mosquitofish in Hawaii and New Zealand are G. affinis, originating in Texas. Populations in Europe and Australia have been identified as G. holbrooki (Eastern Mosquitofish, derived from stocks taken from Georgia (Pyke 2005; Pyke 2008; Cardona 2006). However, these Europe and Australia populations were referred to as G. affinis in older (pre-1990s) papers. The species identity of many of the Mosquitofish stocks around the world is unknown (Lever 1996; Pyke 2005; Walton et al. 2012.

North American Invasion History:

Invasion History on the West Coast:

Western Mosquitofish (Gambusia affinis) from Texas were brought to California for mosquito control in 1922 and stocked in a lily pond at Sutter’s Mill. In 1924-1926, they were stocked in 30 California counties. They were well-established in the San Francisco estuary watershed by the 1940s and were found in 27% of the sites sampled during a 1984 survey (Dill and Cordone 1997; Leidy 2007). In 1965, they were collected in Lake Merritt (Cohen and Carlton 1995), and they occur in Suisun Marsh (Matern and Moyle 2002). In lagoons and estuaries along the coast, G. affinis is usually most abundant near inflows of freshwater streams but can persist during periods of high salinity (Nordby and Zedler 1991). They are established in Los Penasquitos Lagoon (Nordby and Zedler 1991), upper Newport Bay (Horn and Allen 1981), Ballona Marsh (1981, USGS Nonindigenous Aquatic Species Program 2018); Malibu Lagoon (Ambrose and Meffert 1999), mouth of Ventura River (1975, USGS Nonindigenous Aquatic Species Program 2018); Goleta Slough (1968, USGS Nonindigenous Aquatic Species Program 2018); Morro Bay (Feirstine et al. 1973), Elkhorn Slough (Kukowski 1973), and Humboldt Bay (2000, Boyd et al. 2002). These localized populations probably result from many separate introductions in nearby fresh waters. Dispersal of Mosquitofish through the ocean seems unlikely.

Elsewhere on the West Coast, G. affinis occurs in tidal freshwater in the Coquille River estuary (2007, Silver et al. 2017); and the Columbia River near Portland, Oregon and Longview, Washington (1995, Systsma et al. 2004; USGS Nonindigenous Aquatic Species Program 2018).

Invasion History on the East Coast:

Some localized populations of Gambusia spp., have been collected north of the native range of G. holbrooki. According to Briggs and Waldman (2002), all vouchered Gambusia from the Hudson River and Long Island Sound have been G. affinis. Western Mosquitofish have been collected from the Bronx River (2000, Rachlin et al. 2007). from Sparkill Marsh, New Jersey on the Hudson River (1991, Mills et al. 1997) and Great South Bay, Long Island (1984-1986, Lent et al. 1990, cited by Briggs and Waldman 2002). In 1999, they were collected in a cranberry bog on the Quashnet River, a tributary of Waquoit Bay, Cape Cod. It is unknown if this population have survived freezing temperatures (Hartel et al. 2002, USGS Nonindigenous Aquatic Species Program 2018).

In the Great Lakes Basin Western Mosquitofish were introduced to the upper Illinois River around Chicago in 1923 and are still established (Mills et al. 1993). They have also been found near Toledo, in the Maumee River near Lake Erie (1980, USGS Nonindigenous Aquatic Species Program 2018).

Invasion History in Hawaii:

In 1905 Western Mosquitofish were introduced to Oahu, and subsequently stocked on all the major islands (Brock 1960; Calton and Eldredge 2009). These fish were found in many Oahu streams and estuaries with salinities below 16 PSU, but were present in Nanakuli Stream, which can have salinities up to 41 ppt (Englund et al 2000).

Invasion History Elsewhere in the World:

Gambusia affinis and G. holbrooki are widely introduced, mostly for biocontrol of mosquitos. Western Mosquitofish were introduced to Taiwan (from Texas, via Hawaii) to Taiwan) in 1911 (Lever 1996), and to China in 1924. A second introduction (from Texas, via the Philippines) was made to Shanghai in 1927 (Gao et al. 2017). Genetic and morphological studies of Gambusia spp. from 10 sites in eastern China, from Hebei Province to Hainan, found only G. affinis (Gao et al. 2017). In 1916, G. affinis was introduced from Taiwan to Japan. It is now established from Okinawa to central Honshu (National Institute for Environmental Studies 2018). In 1913, an American biologist transferred 'some two dozen' Mosquitofish from Hawaii to Manila, which multiplied to 'many thousands' (Lever 1996). Gambusia spp. were introduced to Thailand, Singapore, and India, but the identity of the species is not clear.

Gambusia affinis from Hawaii were brought to New Zealand in 1930 and released in lakes and swamps on the North Island, beginning in 1933, and is now widespread in still and slow-flowing waters (Purcell et al. 2012). Western Mosquitofish were also introduced in 1930 to Papua New Guinea, where they are now widespread (Lever 1996). By 1948, they had been introduced to Tahiti, Guadalcanal, the Caroline Islands, the Mariana Islands, the Marshall Islands, and Fiji (Krumholz 1948; Maciolek 1984). In many cases, this was probably for mosquito control for Japanese and US troops during World War II.

Gambusia affinis was introduced to South Africa in 1936 and distributed in various parts of the country. By 1970, it was well established in estuarine lakes of the Cape Province (Olds et al. 2015; Sloterdijk et al. 2015).

We have probably overlooked many estuarine introductions of G. affinis. In Europe, Australia, South America, the predominant mosquitofish was known, or believed to be G. affinis. In other locations, the identity of the introduced Gambusia (Lever 1996; Walton et al. 2012).

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Description

Western Mosquitofish (Gambusia affinis) is a small livebearing fish which inhabits, fresh, brackish, and occasionally marine waters. Livebearing fishes, of the family Poecilidae, have dramatic sexual dimorphism, with the male's anal fin being elongated into an intromittent organ, the gonopodium. Poecilid fishes have the mouth upturned and the head flattened, adaptations for feeding at the surface. They have a single dorsal fin and lack dorsal and anal fin spines. They also lack a lateral line. The scales of Gambusia affinis are outlined in black, giving a cross-hatched appearance. Females are larger than males, reaching a maximum 70 mm, compared to males (51 mm), but both sexes are usually smaller. Females are often pregnant, and look pot-bellied, with a dark spot near the urogenital opening. This gravid spot grows large as embryos develop. The males have the upper 4-6 pectoral rays curved upward, for a bowed appearance. In both sexes, there is a dusky-black teardrop-shaped spot across the eyes, and one to three rows of small black spots arching across the dorsal and caudal fins (Robins et al. 1983; Page and Burr 1991; Moyle 2002; Froese and Pauly 2018).  

Western Mosquitofish occupy a range covering the Gulf coast and lower western Mississippi drainage from western Alabama to the Rio Grande and north to southern Kansas and Illinois. A similar species, G. holbrooki (Eastern Mosquitofish) ranges from Alabama to Florida, and north to Tennessee, Maryland, and Delaware. The two species were once considered conspecific, as subspecies of G. affinis. Gambusia affinis usually has six dorsal fin rays and the male lacks teeth on the third ray of the gonopodium, while G. holbrooki usually has 7-8 rays, and males have a toothed gonopodium (Page and Burr 1991). Hybrids between the two species occur in the vicinity of Mobile Bay, Alabama (Wilk and Horth 2017). In the United States, Western Mosquitofish were widely introduced outside their native range, while introductions of Eastern Mosquito fish are less common (USGS Nonidingenous Species Database). However, both species have been introduced to other continents (Lever 1996; Pyke 2005; Walton et al. 2012). Though the two species are genetically distinct, they seem very similar ecologically, with regard to temperature and salinity tolerances, habitat preferences, life history, and feeding (Pyke 2005; Pyke 2008).

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Taxonomy

Ecology

General:

Food:

insects, copepods, cladocerans, fish eggs, algae

Consumers:

fishes, birds

Competitors:

small surface-feeding fishes

Trophic Status:

Non-feeding

NonFed

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Habitats

General Habitat	Nontidal Freshwater	None
General Habitat	Fresh (nontidal) Marsh	None
General Habitat	Tidal Fresh Marsh	None
General Habitat	Grass Bed	None
General Habitat	Unstructured Bottom	None
General Habitat	Salt-brackish marsh	None
General Habitat	Canals	None
Salinity Range	Limnetic	0-0.5 PSU
Salinity Range	Oligohaline	0.5-5 PSU
Salinity Range	Mesohaline	5-18 PSU
Salinity Range	Polyhaline	18-30 PSU
Salinity Range	Euhaline	30-40 PSU
Tidal Range	Subtidal	None
Vertical Habitat	Nektonic	None

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Tolerances and Life History Parameters

Minimum Temperature (ºC)	5	Experimental, Otto (1973), cited by Pyke (2005)
Maximum Temperature (ºC)	43	Experimental, Otto (1973), cited by Pyke (2005)
Minimum Salinity (‰)	0	This is a freshwater species
Maximum Salinity (‰)	40	Experimental, Chervinski (1983) cited by Pyke (2005). Salinity tolerance varies among populations and with acclimation. This species shows a strong preference for fresh water and low-salinity brackish water (Pyke 2005).
Minimum Dissolved Oxygen (mg/l)	2.8	Pyke 2005
Minimum pH	4.7	Walton et al. 2012
Maximum pH	10.2	Walton et al. 2012
Minimum Length (mm)	12.6	Males, South Africa; Females, South Africa; Females, 14.7(Sloterdyjke et al. 2015)
Maximum Length (mm)	65	Females, more usually 39 mm; Males, 51 mm, more usually 20-25 mm (Sloterdyjke et al. 2015; Froese and Paul 2018)
Broad Temperature Range	None	Warm temperate-Tropical
Broad Salinity Range	None	Nontidal Limnetic-Euhaline

General Impacts

Western Mosquitofish (Gambusia affinis) and Eastern Mosquitofish (G. holbrooki) were widely introduced around the world as part of campaigns against mosquitos as vectors of malaria, yellow fever, and other mosquito-borne diseases, as well as mosquitos as simple annoyances at lakes, beaches, and waterfront communities. Widespread introductions of mosquito fish began in the early 20th century (Seal 1910; Krummholz 1948; Lever 1996). For many decades, their perceived benefit in mosquito control was generally accepted. ''Gambusia are so effective for this purpose that it is doubtful that a more valuable fish swims in North American waters' (Hildebrand and Schroeder 1928). However, many of the major disease-carrying mosquitos (Aedes spp.) breed in containers such as cans, bottles, and flowerpots, not accessible to fish. Against malarial mosquitos, Anopheles spp., and general pest mosquitos, which do breed in natural waters, fish are less effective than in laboratory trials, because they are generalist feeders, which have many other potential foods besides mosquito larvae. Early experimental trials were poorly controlled, and mosquitofish releases were concurrent with many other mosquito control and public health methods. Increasingly, the use of introduced fishes for mosquito control has been recognized as a threat to native fish biodiversity, while the benefits from mosquitofish stocking are unproven (Pyke 2008; Azevedo-Santos et al. 2017). Gambusia 'affinis' (including G. holbrooki) has been listed by the Invasive Species Specialist Group of the World Conservation Union (IUCN) as one of the '100 worst invasive species' and now are among the most widespread fishes in the world (Pyke 2008).

Economic Impacts

Western Mosquitofish (Gambusia affinis) were introduced around the world, largely for the control of disease-carrying mosquitos. Other uses include as an aquarium fish, a forage fish, or for bait (Froese and Pauly 2018). However, because of negative ecological effects, some review articles on Gambusia discuss methods of eradication of Gambusia (Pyke 2008; Walton et al. 2012). Health- Western Mosquitofish were introduced to control of mosquitos. One major area of releases was in the Central Valley of California, in areas of rice culture, where the use of larvicide or oil for mosquito control was undesirable, and where malarial mosquitos were breeding. Widespread releases for mosquito control were made in 1923-1924 (Dill and Cordone 1997). Early experiments with Gambusia used small sample sizes and limited replicates, but generally found reductions in mosquito larvae. Mosquitofish predation was less effective in vegetated ponds. Some experiments found few or no effect on mosquito abundance, but these results tended to be overlooked. However, increasing concern about negative impacts of non-native fishes on local species and ecosystems and more thorough examination of the efficacy of Gambusia and other exotic fishes for mosquito control has led many scientists to advise against non-native fishes for biocontrol , and to advise other mosquito control methods (Pyke 2008; Walton et al. 2012; Azevedo-Santos et al. 2017). However, releases by individuals and local governments continue to the present day.

Ecological Impacts

Competition - Gambusia spp. are hardy, fast-reproducing fish, and tend to be aggressive towards fish of other species that are similar in size and habitat. Negative impacts have been reported from small, confined bodies of water, and relatively few from estuaries. Negative impacts have been reported for fishes in desert springs, pools, and springs, including species in North America and Australia. Amphibian larvae, tadpoles and juveniles are also especially vulnerable because they often rely on similar habits (Lever 1996). Examples include the California Red-Legged Frog (Rana aurora) and the Chiricahua Leopard Frog (R. chiricahuaensis). Among affected species in North America are desert fishes, including the threatened/endagered pupfishes (Cyprinodon spp.), Railroad Valley Springfish (Crenichthyes baileyi), Sonoran topminnow (Poeciliopsis gracilis) Global Invasive Species Database 2018). In brackish lava-rock pools in Hawaii, G. affinis competed with a native shrimp, Halocaridina rubra for algal food (Capps et al. 2009).

Predation- Gambusia spp. are omnivorous, and capable of feeding on a wide variety of invertebrates, fish larvae and juveniles and tadpoles. As with competition, impacts are greatest in small, confined bodies of water (Lever 1996). In Hawaiian lava rock pools, they preyed on the shrimp Halocaridina rubra, although predation was minimized by the diurnal feeding of the fish and the nocturnal habitats of the shrimp (Capps et al. 2009). Because Gambusia spp. are omnivorous, and can feed on such a wide range of food, including algae, zooplankton, benthic, invertebrates and fishes, its effects can range over several trophic levels (Pyke 2008 Hinchliffe et al. 2017)

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Regional Impacts

SP-XXI	None		Ecological Impact	Predation
Gambusia affinis is a predator on the native Hawaiian shrimp Halocaridina rubra. However, in the presence of G. affinis, the shrimp is active only at night, minimizing predation (Capps et al. 2009).
SP-XXI	None		Ecological Impact	Competition
When the two species are present together in brackish ponds in Hawaii, Gambusia affinis and the native Hawaiian shrimp Halocaridina rubra both feed largely on periphyton algae, since the major potential animal prey for G. affinis, the shrimp, are only active at night. Consequently, both species are grazers on algae on the rocks, G. affinis by day, and H. rubra by night (Capps et al. 2009).
SP-XXI	None		Ecological Impact	Herbivory
In brackish ponds in Hawaii, Gambusia affinis feeds largely on periphyton algae, and has a trophic position of 2.2 (Capps et al. 2009).
SP-XXI	None		Economic Impact	Fisheries
Inroduced mosquitofish threaten traditional fisheries for the native Hawaiian shrimp Halocaridina rubra (opae’ula). Eradication attempts, using rotenone, have been unsuccessful (Nico and Walsh 2011)
P090	San Francisco Bay		Ecological Impact	Competition
In experiments in ponds in San Joaquin County, the presence of Western Mosquitofish (Gambusia affinis) and introduced Bullfrogs (Lithobates catebianus both adversely affected tadpoles of the native Red-Legged Frog (Rana aurora draytonii). Gambusia affinis did not reduce numbers of tadpoles, but did cause injuries and reduced growth of the tadpole, probably reducing recruitment. Bullfrogs were much more effective as predators (Lawler et al. 1999).
P090	San Francisco Bay		Economic Impact	Health
Western Mosquitofish (Gambusia affinis) were introduced to San Francisco Bay for perceived benefits of mosquito control (Cohenn and Carlton 1995; Dill and Cordone 1997)
CA	California		Ecological Impact	Competition
In experiments in ponds in San Joaquin County, the presence of Western Mosquitofish (Gambusia affinis) and introduced Bullfrogs (Lithobates catebianus both adversely affected tadpoles of the native Red-Legged Frog (Rana aurora draytonii). Gambusia affinis did not reduce numbers of tadpoles, but did cause injuries and reduced growth of the tadpole, probably reducing recruitment. Bullfrogs were much more effective as predators (Lawler et al. 1999).
CA	California		Economic Impact	Health
Western Mosquitofish (Gambusia affinis) were introduced to San Francisco Bay for perceived benefits of mosquito control (Cohenn and Carlton 1995; Dill and Cordone 1997)
HI	Hawaii		Ecological Impact	Competition
When the two species are present together in brackish ponds in Hawaii, Gambusia affinis and the native Hawaiian shrimp Halocaridina rubra both feed largely on periphyton algae, since the major potential animal prey for G. affinis, the shrimp, are only active at night. Consequently, both species are grazers on algae on the rocks, G. affinis by day, and H. rubra by night (Capps et al. 2009).
HI	Hawaii		Ecological Impact	Herbivory
In brackish ponds in Hawaii, Gambusia affinis feeds largely on periphyton algae, and has a trophic position of 2.2 (Capps et al. 2009).
HI	Hawaii		Ecological Impact	Predation
Gambusia affinis is a predator on the native Hawaiian shrimp Halocaridina rubra. However, in the presence of G. affinis, the shrimp is active only at night, minimizing predation (Capps et al. 2009).
HI	Hawaii		Economic Impact	Fisheries
Inroduced mosquitofish threaten traditional fisheries for the native Hawaiian shrimp Halocaridina rubra (opae’ula). Eradication attempts, using rotenone, have been unsuccessful (Nico and Walsh 2011)

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