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Eastern and Western Mosquitofish

Gambusia holbrooki & Gambusia affinis
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What is Ontario Doing?

To prevent these unwanted invaders from coming into the province, Ontario has regulated the Eastern Mosquitofish and the Western Mosquitofish as prohibited under the Invasive Species Act, 2015.


Mosquitofishes are small, live-bearing (give birth to live young) fish that are dull grey or brown in colour. Native to the Atlantic and Gulf Slope drainages from southern New Jersey to Mexico, Mosquitofish were introduced worldwide as early as the 1900s for biological control of mosquito larvae. Since their introduction, there is little evidence that mosquitofish have been effective at controlling mosquitoes. Mosquitofishes displace and compete with native fishes, can impact populations of invertebrates, and can facilitate algal blooms.


Western Mosquitofish were brought from Texas to California in 1922 and have since been found in lagoons and estuaries along the west coast of the US where they are more abundant near inflows of freshwater streams. However, they can also persist during periods of high salinity. Western Mosquitofish are also found in the Columbia River near Portland, Oregon and Longview, Washington, and in most states outside its native range in the US. 

Eastern and Western Mosquitofish have been introduced to New Jersey, Ohio, Tennessee, and have been moving northward along the eastern coast as far north as Boston. Both species prefer slower moving freshwater streams, ponds, and lakes.

Impacts of Mosquitofishes

  • Mosquitofishes are aggressive and can negatively impact native fishes and invertebrate populations through predation and direct competition. 
  • By displacing native insectivores, Mosquitofishes can facilitate mosquito population growth. 
  • Mosquitofishes prey on eggs, larvae, and juveniles of native fishes and have even been shown to impact endangered fish species. 
  • Mosquitofishes are carriers of the necrosis hematopoietic necrosis virus (EHNV) that can be fatal to Rainbow Trout. 

How to Identify

  • Eastern and Western Mosquitofish are small, non-descript fish that grow to be 1 (0.39 in) – 6.5 cm (2.56 in) long. 
  • They have large eyes that take up most of their face. 
  • They have a dull grey colouration with a large abdomen, rounded dorsal and caudal fins, and an upturned mouth. 
  • They have a distinct smoky black teardrop below the eye. 
  • Reproducing females have a large black blotch above the anus. 
  • They have 1-3 rows of black spots found on the dorsal and caudal fins of each species. 

    What You Can Do

    • Learn how to identify the Eastern and Western Mosquitofish and prevent the spread of this unwanted species. 
    • Do not release any live fish into Ontario’s lakes, rivers, or streams. Return or donate unwanted aquarium fish to a pet store or local school.  
    • Never buy or keep live Mosquitofishes. 
    • If you’ve seen an Eastern or Western Mosquitofish or another invasive species in the wild, please contact the toll-free Invading Species Hotline at 1-800-563-7711, visit EDDMapS, or search for the ‘Invasive Species in Ontario’ project on to report a sighting. 


    OFAH/OMNRF Invading Species Awareness Program. (2023). Eastern and Western Mosquitofish. Retrieved from:
    This factsheet may be reproduced for non-commercial purposes.

    Header photo, Western Mosquitofish by dloarie | iNaturalist 

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