Government officials warding off deadly diseases often praise the lowly mosquitofish as a public health messiah. But some environmentalists have dubbed it the “plague minnow” and the “fish destroyer.”
Today, nearly a century after the finger-sized fish was first introduced to California in a Sacramento lily pond, it’s arguably the most ubiquitous freshwater fish in the world. But the mosquitofish also ranks among the world’s worst invasive species.
Balancing the pest-control prowess and ecological destruction of the fish, nearly every mosquito and vector control district in California now deploys the creature with varying strategies.
California has recorded 74 deaths from the mosquito-borne West Nile virus in the last five years, and the voracious fish helps greatly in the fight against infectious diseases. In the wild, however, it also threatens local biodiversity — outcompeting native species and infiltrating seemingly every corner of the Golden State.
“Every crazy-ass place in the state of California seems to have something to do with mosquitofish,” said Eric Palkovacs, a UC Santa Cruz professor of ecology and evolutionary biology who recently published a historical overview of the creature’s complicated relationship with humans.
When first identified in the 1850s, the mosquitofish was native to the southeast U.S. But the fish made its way to California in April 1922, and after being adopted as an alternative to the toxic insecticide DDT by the World Health Organization and various militaries, it spread around the globe.
“People put non-native fish everywhere,” Palkovacs said. “They didn’t give two thoughts to their impact on the food web and native environment.”
The problem, experts say, is that the fish don’t discriminate between snacking on the rice-sized offspring of disease-ridden mosquitoes and critically endangered marine life.
Chad Mitcham, a senior biologist in the Ventura office of the U.S. Fish and Wildlife Service, has seen the impact first-hand. A few years ago, he sampled a pond in north Monterey County with thousands of larval Santa Cruz long-toed salamanders, a federally endangered species on the brink of extinction. Months later, after mosquitofish had been introduced to the pond, only 10 larvae remained.
“It was very disheartening to see,” Mitcham said.
The fish also contribute to algal blooms — sprawling green sheets of microbes that block sunlight and oxygen from other species. When mosquitofish feed on zooplankton, which typically keep algae in check, unbridled algal populations explode and cover ponds.
The fact that mosquitofish can live almost anywhere — from frigid waters to geothermal springs, and in acidity levels ranging from those of coffee to bleach — facilitated the fish’s global domination.
Mosquitofish, however, play an important role for public health agencies around the world. The Russian city of Sochi in 2010 even erected a bronze mosquitofish statue to commemorate the fish’s contribution to fighting malaria.
In California, most vector control districts dispatch mosquitofish to infested bodies of water such as untreated swimming pools.
Tired of maintaining the chlorinated pool at her house in the mountains above Soquel, Deborah Beale decided a few years ago to replace it with a natural pond to nurture local wildlife.
“Sure enough, we got a wonderful explosion in the tree frog population — and dragonflies,” she said. “But we also had more mosquitoes.”
So Beale contacted Santa Cruz County Mosquito and Vector Control, which dumped a small bag of mosquitofish in her new pond. The fish — now numbering in the thousands — protect the Beale residence from pesky bites and dangerous viruses like West Nile and St. Louis Encephalitis.
“They really have improved the quality of life on the property for sure,” she said. “It’s absolutely striking.”
Mosquitofish were especially helpful after last year’s wildfires, when mandatory evacuations left scores of pools abandoned for weeks, said Amanda Poulsen, Santa Cruz County’s vector control manager.
Her agency, however, is cautious with its usage — collecting mosquitofish only from natural waterways and pre-stocked pools, and delivering them only to self-contained bodies of water. It also distributes flyers urging users not to release the fish into the wild.
Yet the strategies of vector control agencies differ vastly among California counties. While three small tubs in Santa Cruz County hold a few hundred mosquitofish year-round, the vector control agency for Sacramento and Yolo counties breeds millions of mosquitofish annually in 22 on-site ponds. It released more than 2 million mosquitofish in 2020 alone.
The operation in Elk Grove is one of the largest in the world. The reason: The Sacramento-Yolo district is responsible for treating mosquito threats in more than 13,000 acres of rice fields, where shallow waters create fertile insect-breeding grounds near dense residential areas.
“If we aren’t treating these rice fields with fish or pesticides, then there’s a huge risk for residents,” said Tony Hedley, the district’s fisheries supervisor. “That’s what weighs more than where these fish are going to end up.”
Prioritizing human health, however, often jeopardizes other species.
“Obviously if there’s any kind of flooding, these mosquitofish are going to go other places,” Hedley said. “I don’t know the full impact of mosquitofish, honestly, but I am more concerned than ever.”
California law permits public health agencies in most counties to plant mosquitofish, even on private property, without permits. While both the Santa Cruz County and Sacramento-Yolo agencies say they typically require the consent of property owners before distributing the fish, they acknowledge that there’s no way to track whether private residents properly use or dispose of them.
That leaves officials continually working to balance the public health promise with the ecological threat of the fish.
“Mosquitofish themselves are neither good nor evil,” Palkovacs said. “We should cast them as neither the hero nor the villain of the story.”
