A small fish introduced to Hawaii in 1905 to control disease-carrying mosquitoes has become a major threat to the islands’ native freshwater species. The eastern mosquitofish, or Gambusia affinis , was brought from the southeastern United States and released into streams, drainage ditches and taro fields. Instead of feeding only on mosquito larvae, the invasive fish began preying on a wide range of native aquatic animals.
According to conservation assessments by the U.S. Fish and Wildlife Service (USFWS), mosquitofish have contributed to declines in several native Hawaiian damselflies, including threatened Megalagrion species. The fish’s aggressive feeding habits, rapid reproduction and ability to survive in different environmental conditions have made it particularly difficult to control.
Hawaii’s native damselflies are especially vulnerable because many evolved in freshwater habitats where predatory fish were largely absent. Their young, known as naiads, live in calm pools and along stream edges. When mosquitofish spread into lower- and middle-elevation streams, they began feeding on these young damselflies.
Some species have suffered major losses. The crimson Hawaiian damselfly ( Megalagrion xanthe ) and flying-earlobe Hawaiian damselfly ( Megalagrion nigrohamatum ) have disappeared from large parts of their former ranges on Oahu, Molokai, Maui and Hawaii Island in areas where invasive fish are present.
The problem goes beyond damselflies. Mosquitofish also compete with Hawaii’s endemic freshwater gobies, known as o'opu, for food. They prey on native freshwater crustaceans, including endemic shrimp called o'pae kala'ole, while their feeding can also alter stream ecosystems and affect organisms that help regulate algae growth.
What makes the invasive fish particularly difficult to manage is its ability to change its diet. When mosquito larvae become scarce, mosquitofish can switch to other small aquatic animals. Females also give birth to live young rather than laying eggs, allowing populations to establish rapidly when fish enter new habitats.
Removing established populations is difficult because Hawaii’s waterways are interconnected with springs, agricultural areas and coastal ecosystems. Trapping and chemical treatments can also put native species at risk.
Conservation efforts are therefore focused on protecting vulnerable habitats, particularly high-elevation streams where waterfalls can act as natural barriers. Wildlife managers are also using barriers to prevent invasive fish from entering protected areas. In some locations, captive-bred Hawaiian damselflies are being released into safer headwater streams to help rebuild populations.
More than a century after the mosquitofish was introduced for mosquito control, Hawaii’s experience has become a warning about invasive species. A fish brought in to solve one problem ended up creating another, threatening native animals that had evolved without defenses against such predators. Protecting Hawaii’s remaining freshwater species now depends on keeping invasive fish out of vulnerable habitats and preventing further spread.
