What Ontario is doing

To prevent these unwanted invaders from coming into the province, Ontario has regulated Eastern and Western mosquitofish (Gambusia holbrooki and Gambusia affinis) as prohibited invasive species under the Invasive Species Act.

Learn about the Invasive Species Act  and regulations.

Background

The Eastern and Western Mosquitofish are small and slender fish that do not lay eggs but instead give birth to live fish. They are considered one of the world’s worst invasive species by the Invasive Species Specialist Group of the IUCN Species Survival Commission.

Outside of Canada, Eastern and Western Mosquitofish have been intentionally introduced for mosquito control in many parts of the world. While this is no longer a common practice due to its low effectiveness for controlling mosquitoes, the intentional stocking of Eastern and Western mosquitofish was the primary pathway for their initial introduction.

Once established, further spread can occur by natural means such as through connected waterbodies or flooding events. Illegal use of mosquitofish as bait, in aquaria and water gardens are also potential pathways of spread into natural systems.

Native range

Western Mosquitofish are native to Central America and parts of North America. They can be found in the Mississippi River basin from central Indiana and Illinois in the United States south to the Gulf of Mexico.

Eastern mosquitofish are native to Atlantic and Gulf Slope drainages as far west as southern Alabama.  

Introduced range  

Both Western and Eastern mosquitofish are not yet known to occur in Ontario. However, there have been 3 detections in the United States parts of Lake Erie since the 1980s and as recently as 2009. In the United States, they are established widely in the west, Midwest and northeast, including several states bordering the Great Lakes. They are established on all continents except Antarctica. 

View an up-to-date distribution map of Eastern mosquitofish and Western mosquitofish.

Impacts

Western and Eastern mosquitofish impact local biodiversity by:

  • outcompeting native fish species for food and habitat through aggressive feeding on zooplankton, eggs and larvae of other aquatic animals
  • reducing biodiversity, causing native species decline and even potential extirpation of native species

How to identify

Western and Eastern mosquitofish share similar identifying features, such as:

  • elongated and somewhat compressed body
  • flattened head with upward pointed mouth
  • grows up to 6.5 centimetres, females grow larger than males and have a visible distended (bloated) abdomen when pregnant
  • dull grey or brown in colour with no bars or bands on the sides
  • both species have a rounded tail
  • the dorsal fin (top fin) of the Eastern mosquitofish is located closer to the head compared to the Western mosquitofish

What you need to know

Learn how to identify Eastern and Western mosquitofish and how to prevent the introduction or spread of these fish to local waterways.

It is illegal to import, deposit, release, possess, transport, propagate, buy, sell or trade Eastern or Western mosquitofish in Ontario.

Never buy or use Eastern or Western mosquitofish as bait or release any live fish into Ontario waterbodies.

Report illegal activity

If you have any information about the illegal importation, distribution, or sale of Eastern or Western mosquitofish, report it immediately to either:

If you’ve seen Eastern or Western mosquitofish or another invasive species in the wild, please: