Western mosquitofish, small freshwater fish typically about two and a half inches long, have been distributed far and wide from their native habitat in the southern U.S. and northern Mexico. Because they feed on insect larvae, including, depending on the circumstances, substantial numbers of immature mosquitoes (hence their name), humans have introduced them to waterways throughout the world.
In fact, these introductions are still occurring today – if you live in California’s Alameda County, you can request a delivery of mosquitofish right to your own pond.
Of course, biological controls rarely function exactly as intended. Mosquitofish are aggressive toward other fish, and have even been known to replace native fish species – which, in some cases, are more “efficient mosquito control agents” than the introduced mosquitofish that displaced them.
Mosquitofish also eat lots of other types of insects, as well as zooplankton and aquatic vegetation – meaning the effects of their eating habits can ripple outward from multiple parts of their food webs. Far from solely reducing mosquito populations, mosquitofish can cause big changes in their new homes (which is why some states regulate the expansion of their range).
Water bodies are not isolated from the environments that surround them – scientists call the flow of nutrients and resources between aquatic and terrestrial areas a ‘boomerang flux’ or ‘reciprocal subsidy.’ For example, many aquatic insects (including mosquitoes) spend their larval stages underwater, and later emerge as winged adults – the energy they incorporate into their bodies from the aquatic environment is transferred to terrestrial environments when those adults are eaten by predators like spiders or birds. New research recently published in the journal Freshwater Science suggests that introduced mosquitofish can alter that transfer of energy.
A team of scientists working at Utah’s Fish Springs National Wildlife Refuge, a series of spring-fed wetlands in the middle of the desert 125 miles southwest of Salt Lake City, measured how well aquatic insects survive to adulthood in the presence of western mosquitofish, as well as two native fish species.
The researchers set up experimental tubs of spring-water, complete with sediment and aquatic vegetation, to which they added either one species of fish or a combination (they also kept fish out of some of the tubs, so they could compare the results from the fish-filled tubs to the fish-less ones). Then they captured the adult aquatic insects that emerged from the tubs’ water surface – presumably the insects that did not survive their underwater larval stages became fish food. (None of the adult insects they collected were mosquitoes.)
The scientists found that the biomass of aquatic insects that emerged unscathed from the tubs that didn’t contain any mosquitofish was much larger than the biomass captured above the tubs where they were present: about 70 percent of the insect biomass made it out of the tubs containing native fish species, while only about 40 percent survived the mosquitofish tubs.
As the researchers note in their paper, “[w]estern mosquitofish have the potential to negatively affect the flow of energy from springs to the terrestrial environment,” in the form of reduced adult insect biomass. Though we humans may cheer when those adult insects are mosquitoes, we should remember that mosquitoes are not the only insects western mosquitofish consume, and the effects of introducing the fish to new locations are rarely as straightforward as we might like.