Adult Gambusia affinis are extremely aggressive and attack other fish, shredding fins and sometimes killing them. Controversy has followed the introduction of mosquito fish, as they have been accused of being little better at destroying mosquitoes than native fish species, as well as being responsible for eliminating many of these same species (Myers, 1965; Haas et al., 2003). Selective predation by mosquito fish has also been shown to alter zooplankton, insect and crustacean communities (McDowall, 1990). Mosquito fish are potential hosts of helminth parasites, which have been transmitted to native fishes (FishBase, 2003).
Location Specific Impacts:
United States (USA)
Other: Gambusia affinis have been found to often attack, shred fins, and sometimes kill other species. Mosquito fish are known to prey on eggs, larvae, and juveniles of various fishes, including those of largemouth bass and common carp; they are also known to prey on adults of smaller species. Introduced fishe, including mosquito fish, are likely at least partially responsible for the decline of the 'Vulnerable (VU)' Chiricahua leopard frog (see Rana chiricahuensis in IUCN Red List of Threatened Species) in southeastern Arizona.
Threat to endangered species: Courtenay and Meffe (1989) state that mosquito fish have contributed to the elimination or decline of populations of federally endangered and threatened species; mosquito fish are known to prey on eggs, larvae, and juveniles of various fishe, including those of largemouth bass and common carp; they are also known to prey on adults of smaller species. Several studies have shown that introduced mosquito fish impact negativly on native species, Nico and Fuller (2005) catalogue these impacts. Specific examples of their negative effects include a habitat shift and a reduction in numbers of the 'Vulnerable (VU)' Railroad Valley springfish (see Crenichthys baileyi in IUCN Red List of Threatened Species) in springs in Nevada and the local elimination of the 'Near Threatened (NT)' Sonoran topminnow (see Poeciliopsis occidentalis in IUCN Red List of Threatened Species) in Arizona. Western mosquito fish use the same habitat as the plains topminnow Fundulus sciadicus and have displaced these topminnows and other species with their aggressive behavior. The mosquito fish is also responsible for the elimination of the 'Vulnerable (VU)' least chub (see Iotichthys phlegethontis in IUCN Red List of Threatened Species) in several areas of Utah. Mosquito fish have been founf to be very aggressive, even toward larger fish.
Hawaii (United States (USA))
Predation: Mosquito fish, and other introduced poeciliids, have been implicated in the decline of native damselflies Megalagrion spp. including the ' Vulnerable (VU)' (see Megalagrion oahuense in IUCN Red List of Threatened Species) on Oahu, Hawaii. Often the distributions of the damselflies and introduced fishes were found to be mutually exclusive, probably resulting from predation of the fish on the insects (Englund, 1999).