Hawaii just wanted to control mosquitoes. So, in 1905 it thought of a simple solution. It introduced a small fish from Texas to Oahu, the western mosquitofish, to eat mosquito larvae. What happened next is described in a 1999 paper by researcher Ronald Englund in the Journal of Insect Conservation. The fish was supposed to solve one problem; it may have caused another one.
A small fish with a big appetite
The western mosquitofish is tiny. It only reaches a size of about 2 inches. However, it consumes a variety of small water organisms, not only the mosquito larvae. That’s a pretty big appetite for such a small fish. It could survive in both freshwater and saltwater and eventually found its way to the other Hawaiian islands. It was introduced to many other regions of the world for the same reason. The Hawaiian damselflies had a lot to lose. There are 23 species of damselflies belonging to the genus Megalagrion that are endemic to Hawaii. Their young, nymphs, live in ponds and streams. They evolved in places with few fish, and they swim and feed in the open, which makes them vulnerable.
What the 1999 study found
Englund sampled streams, wetlands, and estuaries on Oahu. He noted where native damselflies were still found and where introduced fish existed. The two did not coincide. Native damselflies were absent from sites where the fish were present. Englund concluded that predation by the fish was the likely explanation, though the paper does not prove it. The study also pointed to seven species of damselflies and dragonflies introduced to Oahu since 1936 as another possible threat, which makes the mosquitofish the leading suspect rather than the only one. Because the research focuses on damselflies, wider effects on other native wildlife are beyond the scope of the study.
{{/usCountry}}Englund sampled streams, wetlands, and estuaries on Oahu. He noted where native damselflies were still found and where introduced fish existed. The two did not coincide. Native damselflies were absent from sites where the fish were present. Englund concluded that predation by the fish was the likely explanation, though the paper does not prove it. The study also pointed to seven species of damselflies and dragonflies introduced to Oahu since 1936 as another possible threat, which makes the mosquitofish the leading suspect rather than the only one. Because the research focuses on damselflies, wider effects on other native wildlife are beyond the scope of the study.
{{/usCountry}}Deep Dive
What impact did the introduction of the western mosquitofish have on Hawaii's native damselflies?
Why is the introduction of non-native species like the western mosquitofish problematic for ecosystems?
How did the researchers conclude that mosquitofish were a significant threat to Hawaii's damselflies?
One species down to a thin strip of stream
Englund's later work shows how serious this can get. From 1997 to 2000, Englund tracked the orange-black Hawaiian damselfly for a study published in Odonatologica in 2001. Over that period, the Oahu population was confined to just over 100 meters of stream habitat, or about 330 feet. Threats included alien fish, drying streams, and altered habitat. The paper warned that the Oahu population could soon go extinct if its stream dried up, which nearly happened in June 1997, early in the monitoring period. It recommended establishing another wild population in a stream without alien fish.
A fix with lasting effects
Fish introductions are not the only threat. Drying streams and habitat alteration also endanger the damselflies, so the damage cannot be blamed on the fish alone. Still, Englund's work points to introduced fish as one of the most serious threats to Hawaii's native damselflies. The story isn't just about Hawaii. It's also about how easy it is to fix one problem and create another. The intent was to control mosquitoes, and the plan seemed reasonable at the time.
When a new species is introduced into an ecosystem, the effects can take years to show up. That is a good reason to think twice about moving animals from one place to another. Nearly a century after the introduction, these studies were still documenting the damselflies' struggle. Next time a quick fix sounds perfect, it's worth asking what else it might change.