Overview:

The western mosquitofish is a ray-finned, live-bearing freshwater fish of the same family which includes the well-known live-bearing aquarium fishes guppy, molly, platy, and swordtail.⁴ G. affinis is native to North and Central America, from the Mississippi River basin south to the Gulf of Mexico.² Part of their diet includes insect larvae and the mosquitofish has been introduced widely around the world throughout the 1900s for the purpose of mosquito control and malaria prevention.³ Their role in mosquito control is now disputed and countered by the documented native impacts these fish have on aquatic communities.⁵
The western mosquitofish tolerate a wide range of environmental conditions and have high reproductive capacity. Gestation is short and females may bear five or more clutches in a season.⁵ However G. affinis populations are regulated by food availability and competition with other species.⁵
Western mosquitofish feed primarily on zooplankton³ and also native aquatic invertebrates such as odonate larvae (damselflies & dragonflies), rotifers, and crustaceans.⁵ This fish has been associated with declines in amphibian populations due to predation on eggs and tadpoles.⁵ Additionally, similar-sized native fish species have declined due to predation or competition where G. affinis have been introduced. Some of these native fish species are considered to be more efficient at mosquito larvae control.¹ In turn mosquitofish are consumed by aquatic birds, larger fish, larger aquatic invertebrates, etc.⁵
G. affinis can be confused with G. holbrooki, which were once considered a subspecies.¹ At time of writing, there are no known wild populations of western mosquitofish in Alberta. To date, the only known population of western mosquitofish in Alberta is in the Cave and Basin pools near Banff.
As of January 1, 2016, the possession, sale, or transport of this species in Alberta is unlawful under the Fisheries Act.

Habitat:

Western mosquitofish prefer standing to low-flowing waters which are well vegetated and occupy the shallower edges.² They occur in water temperatures from 0 to 45 °C, salinities 0 to 41 ppt, pH 4.5 to 9, dissolved oxygen 1 to 11 mg/L, and turbidity.⁵ They are tolerant of pollutants and have been observed to survive under ice in some locations, although cold temperatures do limit distribution. Mainly found at lower elevations.⁵

Identification:

The western mosquitofish is small, commonly 3.9 cm total length², dull grey to brown in colour, with a terminal and upward pointing mouth for feeding at the surface of the water.¹ The body is short and the head flattened.³ The dorsal fin is small, rounded and originates behind the anal fin. The dorsal fin has 7-9 soft rays, anal soft rays 9-10. There are no spines. There are 8 horizontal scale rows between the back and abdomen.²
In males the first few rays of the anal fin are elongated¹ - the anal fin of males is modified into a copulatory organ.² Mature females are larger than males.¹ The maximum reported length is 7.0 cm.²

Ecology:

Female mosquitofish are fertilized internally by males. Females carry an average of 30 (range of 5 to 1045) alevins (fry with a yolk sac)² and gestation lasts 22 to 25 days.5 Males can mate anytime during the breeding season and females can store viable sperm and may adjust when fertilization occurs. Clutch size increase with food availability and female body length or weight.⁵
All males and most females live less than 12 months, but some females have lived 2 to 3 years.⁵

Economic Impacts:

Western mosquitofish compete directly with similar-sized native fish for food and also prey on the eggs and larvae of native fishes.³ Mosquitofish can very aggressive of larger fish by shredding fins.³ The decline of native sportfish populations could have impacts on recreation and tourism.

Environmental Impacts:

Western mosquitofish can negatively impact populations of similar-sized fish via predation and competition for food resources. Mosquitofish prey on the eggs and juveniles of native fishes. Mosquitofish can also negatively affect trophic relationships of aquatic communities by the consumption of zooplankton and invertebrates.³

Sociological Impacts:

The transformation of native aquatic communities results in the intrinsic loss of natural capital and enjoyment of natural areas.

Prevention:

Learn how to identify western mosquitofish and how to prevent spread. This fish is currently promoted by pest control companies and is widely available on the internet. Do not purchase or grow western mosquitofish. Never, ever empty your aquarium into natural water bodies.

Control:

Currently, there are no widely established control options for the western mosquitofish other than preventing introduction. Drawdown and drying of water bodies where possible will kill mosquitofish. If caught, western mosquitofish should be killed and not released. Report any sightings.

References:

¹Datasheet report for Gambusia affinis (western mosquitofish). Invasive Species Compendium. www.cabi.org/isc/datasheetreport?dsid=82079Accessed: March 2, 2017.
²Gambusia affinis (Baird & Girard, 1853) Mosquitofish. www.fishbase.ca/summary/3215.html Accessed: March 2, 2017.
³Zander. Nico, L., P. Fuller, G. Jacobs, M. Cannister, J. Larson, A. Fusaro, T.H. Makled and M. Neilson. 2017. Gambusia affinis. USGS Nonindigenous Aquatic Species Database, Gainesville, FL. nas.er.usgs.gov/queries/factsheet.aspx?SpeciesID=846 Revision Date: 1/25/2016. Accessed: March 2, 2017.
⁴Poeciliidae. Wikipedia. en.wikipedia.org/wiki/Poeciliidae. Accessed March 6, 2017.
⁵Pyke, Graham H. Plague Minnow or Mosquito Fish? A Review of the Biology and Impacts of Introduced Gambusia Species. The Annual Review of Ecology, Evolution, and Systematics. 2008. 39:171-91. www.jstor.org/stable/30245159. Accessed: 06-03-2017 18:12 UTC.

Resources

• Global Invasive Species Database - Invasive Species Specialist Group

Selected Images

Maps

Invasive Listing Sources

• Alberta Fisheries Act
• Invasive Species of Concern in Georgia
• Maryland Invasive Species Council - Invasive Species of Concern in Maryland
• Minnesota DNR invasive species list
• New York Regulated and Prohibited Invasive Species - Prohibited
• Ontario’s Invading Species Awareness Program Tracked Species List
• West Virginia Invasive Species Strategic Plan and Volunteer Guidelines 2014
• Wisconsin's Invasive species rule – NR 40