Gambusia affinis (western mosquitofish) has a maximum length of 56 mm, and is always found in high densities in the lower stream reaches, usually in the presence of another poeciliid fish, Poecilia mexicana. The highest elevation at which Gambusia affinis has been found was 185 m in Poliwai Gulch, near the Waiahole Ditch.  Gambusia affinis exhibits a wide salinity tolerance of 0 to 40 ppt, but is usually found in areas having salinities of 16 ppt or less. Gambusia affinis stomachs from Pearl Harbor streams were examined and found to contain various aquatic fauna such as chironomids, shrimp, and ants. Native damselflies were not found in these stomachs, but this was not surprising because of the complete lack of native damselflies in all areas where Gambusia affinis was found.

Predator-prey relationships between fish and Odonata have been extensively studied in temperate regions, and fish have been shown to affect the distribution of many species of Odonata. In Sweden, experiments found that dragonfly larvae inhabiting only fishless lakes and acid bog ponds attempted to actively escape from fish predators, while species from lakes containing fish feigned death and remained still when attacked. This naïve larval escape response to the threat of predation was similar to that observed for Hawaiian Megalagrion damselfly larvae both in the field and in aquarium observations. Similar naïve behaviors have been seen in Enallagma damselflies from fishless lakes, which attempted to swim away from predators. While this behavior was successful against attacks by immature dragonflies, it failed to deter fish predation.

Poeciliid predation was also correlated with the elimination of native Odonata species in Australian lakes. In one study, lakes containing Gambusia affinis had only 3-4 species of Odonata, while nearby lakes without introduced fish contained 11 species. In the presence of mosquitofish, declines in Odonata populations were also found in California rice fields. Although most research on the negative effects of poeciliids on native species has involved Gambusia affinis, some authors believe that the carnivorous Poecilia reticulata might also have impacts similar to that of Gambusia affinis on native aquatic biota.