Gambusia: Mosquitofish ID, Pond Use and Guppy Differences

Gambusia is a genus of small livebearing fish in the family Poeciliidae, and two of its 45 species, Gambusia affinis and Gambusia holbrooki, are the fish sold and stocked as mosquitofish (Wikipedia). The rest of the genus are usually called topminnows. Two species are already extinct, two are critically endangered and nine are listed as vulnerable, so most Gambusia are not the hardy pond fish people picture.

The genus is most diverse in Mexico, Texas and the Greater Antilles. The name is thought to come from the Cuban word gambusino, a free-lance miner, or possibly from the Spanish gamusino, an imaginary animal used in pranks like a snipe hunt (Wikipedia).

Because it eats mosquito larvae and was stocked around the world for that reason. Wikipedia says a female can eat up to hundreds of larvae in one day and that the fish was credited with helping to eradicate malaria in parts of South America, southern Russia and Ukraine between the 1920s and 1950s; there is a monument to it in Sochi.

The name oversells it. Mosquito larvae are only a small part of the diet: the fish also eats zooplankton, beetles, mayflies, caddisflies and mites (Wikipedia). The US Fish and Wildlife Service risk screening quotes the Global Invasive Species Database: gambusia was formerly used to control mosquitoes but has been shown to be generally ineffective, and since 1982 the World Health Organization has not recommended it for malaria control.

Mosquitofish are small, dull grey fish with a large belly, rounded dorsal and tail fins and an upturned mouth (Wikipedia). The risk screening adds a bluish sheen on the sides, a silvery belly and colourless fins with rows of small black spots. Check these marks:

• Female: up to about 6 to 7 cm, deep-bodied, with a dark gravid spot just above the vent when pregnant
• Male: about 3.5 to 4 cm, slimmer, with the anal fin turned into a long rod called a gonopodium
• Species: in G. affinis the third ray of the gonopodium is smooth; in G. holbrooki it carries small teeth (Wikipedia), which needs a magnifier

Wikipedia notes guppies, sailfin mollies and platies are commonly misidentified as mosquitofish. A side-on photo in the FishLens app gives a quick first check.

No. They are relatives in the same family, Poeciliidae, but different genera: the guppy is Poecilia reticulata, from northeastern South America and the southern Caribbean, including Trinidad and Tobago, Venezuela and Guyana (Wikipedia). Both give birth to live young and both were spread around the world for mosquito control. How to tell them apart:

• Colour: male guppies carry spots, splashes or stripes in almost any colour; mosquitofish males are plain grey with at most black blotches
• Size: male guppies are 1.5 to 4 cm and females 3 to 7 cm (Wikipedia), so size alone does not separate them
• Temperament: the US Fish and Wildlife Service screening describes mosquitofish as very aggressive, even towards larger fish, attacking and shredding fins

Only after checking your local rules, and never where the water can reach a creek, lake or wetland. The US Fish and Wildlife Service screening rates the eastern mosquitofish as high risk for the contiguous United States, with a high climate match and thoroughly documented harm where it has been introduced. The documented harm includes:

• the local elimination of the endangered Sonoran topminnow in Arizona and declines of other threatened desert fish
• eating the eggs and tadpoles of frogs, including endangered Australian species
• algal blooms, or even more mosquitoes, when the fish eat the animals that normally eat algae or mosquito larvae

The European Union lists mosquitofish as an invasive alien species and bans importing and moving them (Wikipedia). They are still used: the US FWS screening quotes CABI that they are likely farmed in the US for mosquito control in rice fields and that mosquito control authorities have moved them between sites, and Wikipedia notes California and Nevada bred them for West Nile virus control in 2008. If you want them, ask your local mosquito or vector control agency whether they are allowed where you live. They also attack and kill fry, so keep them away from ponds where you want other fish to breed.

None of the sources on this page describe Gambusia as a food fish; the uses they list are mosquito control and feeder fish for aquarium predators (US Fish and Wildlife Service screening). At 4 to 7 cm there is little to eat. More useful: a fish that small, taken from a ditch or pond, may be a protected topminnow or a young fish of another species, so identify it first.

• Gestation: 16 to 28 days (Wikipedia); 3 to 4 weeks (FishBase, quoted by US FWS)
• Brood: generally 40 to 60 young, up to 354 (FishBase, quoted by US FWS)
• Maturity: 4 to 6 weeks, allowing 3 generations a year (FishBase, quoted by US FWS)
• Lifespan: usually less than a year in the wild, up to 1.5 years; over 3 years in captivity (Wikipedia)
• Tolerance: low oxygen, salt up to twice sea water, and 42 C (108 F) for short periods (Wikipedia)

This is why a few fish released into a pond can become thousands within a season.