Western Mosquitofish | Mexican Fish.com

Western Mosquitofish, Gambusia affinis

Western Mosquitofish, Gambusia affinis. Fish caught from a freshwater steam of Long Island, New York, July 2021. Length 6.8 cm (2.7 inches). Catch, photograph and identification courtesy of James Lafontaine, Long Island, New York.

Western Mosquitofish, Gambusia affinis. Fish caught from the Gasconade River drainage in Lane Spring, Missouri, June 2017. Length: 6.8 cm (2.7 inches). Catch, photograph and identification courtesy of Ben Cantrell, San Diego, California.

Western Mosquitofish, Gambusia affinis. Fish caught from the Armagosa River, Southeastern, California, September 2019. Length: 4.8 cm (1.9 inches). Catch, photograph and identification courtesy of Ben Cantrell, San Diego, California.

The Western Mosquitofish, Gambusia affinis, is a member of the Livebearer or Poeciliidae Family, that is also known as the Mosquitofish and Gambezi, and in Mexico as guayacón mosquito. Globally, there are forty-two species in the genus Gambusia, of which three are found in Mexican waters, two in the freshwater systems that feed into the Atlantic and this species found in the freshwater systems of both the Atlantic and the Pacific Ocean.

The Western Mosquitofish has a small short body that is moderately robust and moderately compressed at the rear with a large abdomen. The head is large and flattened with a small upturned protrusible mouth that opens in the front and large eyes. They are dull greenish olive to brown dorsally transitioning to blue on the sides and silvery-gray ventrally. The body has a diamond shaped netting pattern and most have small black spots on the body and tail. They have a small dark bar below the eye and the black abdominal cavity can be seen through the belly in living fish. A small percentage of the population is black. The Western Mosquitofish is sexually dimorphic with females being larger than males, a maximum of 7.0 cm (2.8 inches) in length, versus 4.0 cm (1.6 inches) for the males, and have much longer lifespans. Females also have a gravid spot at the posterior of the abdomen and they have rounded anal fins (versus a pointed anal fin in the males). Their anal fin has 8 to 10 rays; their caudal is rounded; their dorsal fin has 7 to 9 rays, is rounded and set behind the anal fin; their pectoral fins have 12 to 14 rays; and their pelvic fins have 6 rays. They have 5 to 12 gill rakers. They are covered with smooth large scales.

The Western Mosquitofish is a non-migratory species that is found in areas with extensive amounts of subsurface vegetation in all types of shallow freshwater systems including backwaters, marshes, ponds and rivers and in brackish sloughs and coastal saltwater habitats. They avoid areas where where larger predatory fishes can frequent. They prefer water temperatures between 24^oC (75^oF) and 30^oC (86^oF). They can survive in water as cold as 0.5^oC (33^oF) and as warm as 42^oC (108^oF) with salinities up to 58 ppt. They can tolerate high levels of pollution and low oxygen levels having the ability to breath oxygen from the air. They are voracious surface feeders that consume detritus, small insects and insect larvae, and zooplankton near the surface with mosquito larvae being only a small component of their diet. They practice cannibalism when ample food is not available. They have a high feeding capacity and are capable of consuming 42% to 167% of their body weight on a daily basis. In turn they are heavily preyed upon by shore birds including Herons. Reproduction is viviparous and involves internal fertilization, a 30-day gestation period, with each female giving birth to approximately 60 live young two to six times a year. At birth there are equal numbers of males and females but the adult populations have significantly more females than males. The females can reach sexual maturity within three weeks and the males within sixty days. Females have the ability to store sperm from multiple males for several months. They have life spans of up to three years.

The Western Mosquitofish is very similar to the the Guppy, Poecilia reticulata, the Sailfin Molly, Pecilia latpinna, and the Southern Platyfish, Xiphophous maculatus but none of these have the net-like pattern on their sides. They are most like confused with the Eastern Mosquitofish, Gambusia holbrooki (11 or more anal fin rays and 8 or more dorsal fin rays).

The Western Mosquitofish is native to freshwater systems of Mexico that drain into the northeast coast of the Gulf of Mexico south to Tamaulipas, Northern Veracruz. They have recently been introduced by humans to the Pacific coastal drainage Baja from Guerrero Negro, Baja California northward along the central and northwest coasts of Baja.

From a conservation perspective the Western Mosquitofish is currently considered to be of Least Concern with expanding global populations and are believed by some to be the single most abundant most widely introduced freshwater fish in the world. Due to their reputation as mosquito-control agents to combat malaria, they were stocked routinely and indiscriminately in temperate and tropical areas around the world. In the United States the first known introductions took place in the early 1900s. They are credited with helping to eradicating malaria in South America, southern Russia and the Ukraine, and for reducing the West Nile virus cases in California. Presently their use in mosquito control or mosquito borne diseases is not viewed to be particularly effective. They are known however to be highly aggressive and predatory on and competitive with native populations of numerous fishes having a large negative impact to local environments altering crustacean, insect and zooplankton communities. In Australia, for example, they are currently considered to be noxious pests posing a threat to native fish and frog populations and no evidence indicates they have controlled mosquito populations. The Global Invasive Species Database currently has the Western Mosquitofish listed as one of the world’s 100 “worst” invasive species.