After establishing a new small garden pond, by the end of the first summer, there are dragonfly larvae and other mosquito predators in the pond. These larval dragonflies and other predators will happily eat all the mosquitoes. Before then -- before the dragonfly populations build up, a few fish in the pond will eat the mosquitoes. Once established, these ponds do not need fish. (But we check for mosquito larvae occasionally – and if we see any, we add a few goldfish.)
Most of our mesocosms (these ponds) have goldfish. Some of our goldfish are colored and easy to see. Others have the ‘wild-type’ bronze or brown color. Native fish would be a wiser choice, but are harder to find and keep established. Goldfish are a non-native invasive species originally from Eurasia, where they live in shallow waters such as streams and small ponds. Normally, we would never hold an invasive species. But populations of goldfish in freshwaters in the eastern USA have long since stabilized. They are now a part of the food web in much of the USA. Here on campus, where students have been releasing their pet goldfish into Crim Dell and Lake Matoaka for probably 100 years, the ecological damage caused by goldfish is already complete. And wild predators (such as barred owls) will visit small garden ponds to snack on the goldfish – a little hard on the goldfish, but good for other wildlife.