Western Mosquitofish

Western Mosquitofish, Gambusia affinis

The Western Mosquitofish, Gambusia affinis, whose common Spanish name is guayacón mosquito, is a species in the Livebearer or Poeciliidae Family, known collectively as topotes and espadas in Mexico. This fish is also commonly referred to as the Gambusia or simply the Mosquitofish. Globally, there are forty-two species in the genus Gambusia, four of which are found in Mexican waters, three in the Atlantic and one in the majority of freshwater systems within Mexico.

The Western Mosquitofish are a small fish with a moderately robust short body with a large abdomen, a large flattened head, an upturned protrusible mouth, and large eyes.  They are greenish-olive to brown dorsally, blue-gray on the sides with a net-like pattern, and silvery-white ventrally with small black dots on the body and caudal fin.  Some fish are very dark and almost black in color. The caudal and dorsal fins are rounded. They have a small black bar under the eye and the gut cavity is dark. They do not have bars or bands on their sides. They have 8 to 10 anal fin rays and 7 to 9 dorsal fin rays. The dorsal fin originates opposite the seventh anal ray. The anal fin base is less than half of its distance from the caudal fin; the dorsal fin originates opposite the seventh anal fin ray; the pectoral fins end before the anal fin; and the pelvic fins reach the pectoral fins. The Western Mosquitofish have a pronounced sexual dimorphism with females reaching 7.0 cm (2.8 inches) in length and males 4.0 cm (1.6 inches); the anal fins in the females are rounded and in the males are pointed.  Mature females have a gavid spot on the posterior of their abdomens just above the anal fin. Equal numbers of males and females occur at birth however the males have much shorter life spans and thus females outnumber males.

The Western Mosquitofish are non-migratory being found in all types of freshwater systems with water temperatures between 12oC (54oC) and 29oC (84oF) that have thick vegetation. Being able to tolerate high saline levels they can also be found in brackish environments including salt marshes and other low-salinity coastal habitats with standing or slow-flowing waters. They are also toleration of pollution, low dissolved oxygen levels but are not cold tolerant limiting their northern range. The Western Mosquitofish has an enormous apatite consuming more than their body weight of zooplankton, invertebrates, eggs, larvae, and juveniles of various fishes and detritus on a daily basis. In turn they are preyed upon by various pelagic and surface predatory fishes as well as numerous sea birds including herons. They are also known to practice cannibalism on their juveniles. Reproduction is viviparous with internal fertilization with females capable of storing sperm from multiple males for long periods of time and have gestation periods of 24 to 30 days giving birth to 30 to 60 live pups. Life spans in the wild are about one year, however, can survive for up to three years in captivity.

In Mexican waters the range of the Western Mosquitofish are not well documented, however, it is logical to assume they are found in all fresh water systems that have year round water temperatures between 12oC (54oC) and 29oC (84oF).

The Western Mosquitofish is virtually identical and indistinguishable from the Eastern Mosquitofish, Gambusia holbooki. They are also very similar to the Mangrove Gambusia, Gambusia rhizophorae (rows of small black spots on the sides), and the Sailfin Molly, Poecilia latipinna (large sail-like dorsal fin).

The Western Mosquitofish have been introduced indiscriminately in all global temperate and tropical locations as effective and inexpensive mosquito-control agents and are alternatives to the use of insecticides. They have been credited with the control of malaria and the West Nile Virus. Of late, however, a debate rages within the scientific community that they are not particularly effective in controlling mosquito populations and in reducing the incidence of mosquito-borne diseases. They are known to be highly invasive however, having a significant negative impact on native populations of federally endangered and threatened fishes, frogs and newts. In some countries they are considered to be noxious pests and a listed as one of the top 100 most invasive global fishes. The Western Mosquitofish are of minor commercial importance being sold on a limited basis by the aquarium trade, for use in home ponds for mosquito eradication and as a live food for carnivorous aquarium fishes.  From a conservation perspective they are considered to be of Least Concern being very abundant with a broad global distribution and stable populations. Some believe it to be the most abundant freshwater fish in the world.

Western Mosquitofish, Male or Immature Female, Gambusia affinis. Fish collected with a net out of the Rio de Santiago river basin in the Las Cuevas section of the East Cape of Baja California Sur, December 2017. Length: 4.8 cm (1.9 inches). Catch, photo and identification courtesy of Brad Murakami, Surrey, British Columbia, Canada.

Western Mosquitofish, Mature Female, Gambusia affinis. Fish collected with a net out of the Rio de Santiago river basin in the Las Cuevas section of the East Cape of Baja California Sur, December 2017. Length: 6.6 cm (2.6 inches). Catch, photo and identification courtesy of Brad Murakami, Surrey, British Columbia, Canada.  Productive discussions with H.J. Walker, Jr., Scripps Institution of Oceanography,  La Jolla, CA noted. I believe there is a high likelyhood that this is a Dusky Millions Fish (aka Speckled Mosquitofish), Phalloceros caudimaculatus, a native of South America but agree with H.J. that it is a long swim to Baja California Sur via freshwater systems.