The native range of the mosquitofish is not well known because human introductions started in the early 1900s. The two species were likely restricted to Gulf and Atlantic coastal plains and the lower Mississippi River Valley. Due to the broad spectrum of their diet and water quality tolerance, mosquitofish are habitat “generalists” that are able to flourish in a wide range of conditions. These traits, combined with their high reproductive rate and frequent dispersal by humans, have resulted in two species that are very successful invaders.
Western Mosquitofish (Gambusia affinis). From http://www.issg.org/database/image.asp?ii=1636&ic=e
Despite their small size, invasive mosquitofish can have a big impact on their environment. Introductions of mosquitofish can lead to algal blooms if they have consumed the zooplankton that keeps algae under control. They can cause populations of native fishes to go extinct through competition for food and direct aggression. They also carry parasites that infect native fish. Worst of all, research has shown that mosquitofish are no more effective at controlling mosquito populations than native mosquito predators. They can even increase the size of mosquito populations by feeding on or outcompeting the mosquito’s native predators.
Mosquitofish can be a problem here in Tennessee because they threaten the Barrens topminnow (Fundulus julisia). This fish occurs in only in three counties in Tennessee and is part of our natural heritage. Unfortunately, the larvae of the Barrens topminnow are eaten by the mosquitofish, and larger juveniles have to compete with the invasive species for food. The Tennessee Aquarium Conservation Institute is currently reintroducing Barrens topminnows to springs with where mosquitofish have been excluded in order to prevent negative impacts from this invasive species.