Economic/Ecological Importance: Gambusia affinis is among the most widely introduced fish species worldwide, and a number of countries have reported negative environmental consequences in the wake of mosquitofish introduction (Fishbase 2004).Although effective as a mosquito control agent and widely introduced for this purpose, mosquitofish have been found to compete with or displace indigenous fish and to otherwise disrupt habitats. For example, McDowall (1990) reports that selective predation by mosquitofish can alter zooplankton, insect and crustacean communities.G. affinis has been nominated as among the 100 "World's Worst" invaders by the Invasive Species Specialist Group (ISSG 2006).