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.