California scientists talk about the dangers and rewards of mosquito fish


Government officials fighting deadly diseases often praise the humble mosquito fish as the public health messiah. But some environmentalists have called it a “plague minnow” and a “fish destroyer”.

Today, nearly a century after the finger-sized fish was first introduced to a lily pond in Sacramento, it is arguably the most ubiquitous freshwater fish in the world. But the mosquito fish is also one of the worst invasive species in the world.

To balance the pest control abilities and the ecological destruction of the fish, almost every mosquito and vector control district in California, including Marin, is now using the creature with different strategies.

Grant White, a field technician in the Sacramento-Yolo Mosquito and Vector Control District in Elk Grove, is responsible for looking after the mosquito fish. (Anda Chu / Bay Area News Group)

California has recorded 74 deaths from mosquito-borne West Nile virus in the past five years, and the voracious fish is a great help in the fight against infectious diseases. In the wild, however, it also threatens local biodiversity – displacing native species and seemingly infiltrating every corner of the state.

A worker hand sifts a bucket of mosquito fish in the Sacramento-Yolo Mosquito and Vector Control District in Elk Grove. (Photo by Luz Maria Robles)

When the mosquito fish was first identified in the 1850s, it was native to the southeastern United States. However, the fish made its way to California in April 1922, and after it was adopted by the World Health Organization and various military personnel as an alternative to the toxic insecticide DDT, it spread around the world.

“People are putting alien fish everywhere,” said Eric Palkovacs, a UC Santa Cruz professor who recently published a historical review of creatures’ intricate relationships with humans. “They didn’t care about their impact on the food web and the local environment.”

The problem, experts say, is that the fish don’t differentiate between snacking on the rice-sized offspring of disease-causing mosquitoes and endangered marine life.

Chad Mitcham, senior biologist in the US Fish and Wildlife Service’s Ventura office, has seen the effects firsthand. Several years ago, he sampled a pond in northern Monterey County containing thousands of Santa Cruz long-toed salamander larvae, an endangered species that is critically endangered. Months later, after mosquito fish were brought into the pond, only 10 larvae remained.

“It was very disheartening to see,” said Mitcham.

The fish also contribute to algal bloom – extensive green layers of microbes that block sunlight and oxygen from other species. When mosquito fish feed on zooplankton, which normally keeps algae in check, unbridled algae populations explode and cover ponds.

The fact that mosquito fish can live almost anywhere – from icy waters to geothermal springs, and in acid levels from coffee to bleach – facilitated global fish domination.

However, mosquitofish play an important role for public health authorities around the world. The Russian city of Sochi even erected a bronze mosquito fish statue in 2010 to commemorate the fish’s contribution to the fight against malaria.

Workers use a large net to cage mosquito fish in one of 22 ponds in the Sacramento-Yolo Mosquito and Vector Control District in Elk Grove. (Photo by Luz Maria Robles)

In California, most vector control districts send mosquito fish into infested waters such as untreated swimming pools.

The Marin / Sonoma Mosquito and Vector Control District provides residents with mosquito fish for use in man-made pools, fish ponds, water troughs, and unmaintained swimming pools.

“Anything that is not connected to a natural body of water,” says the district spokesman for Nice Sequeira.

About 450 mosquito fish requests were made in 2021, of which about 170 were from Marin residents. Once the fish is delivered, technicians check the fish several times a year. Local residents receiving fish are instructed not to put them in natural waters and the bags come with brightly colored stickers that convey the same message, Sequeira said.

For larger bodies of water such as pools, the district staff can pick up the fish. But mostly the residents themselves are responsible for disposing of them properly, said Sequeira.

Nevertheless, mosquito fish are found in natural freshwater bodies in Marin. At Point Reyes National Seashore, mosquito fish have been found in the estuaries at the lower end of Tomales Bay, where the critically endangered tidal gobies live, according to park wildlife ecologist David Press. While park biologists have no information on the effects of mosquito fish at Point Reyes, Press said the fish are known to affect Pacific tree frogs, “which we have many, of course.”

Eric Ettlinger, an ecologist for the Marin Municipal Water District, said his surveys of Lagunitas Creek and its tributaries did not include mosquito fish.

“We didn’t see any mosquito fish in Lagunitas Creek or two major tributaries, San Geronimo Creek and Devil’s Gulch,” said Ettlinger.

According to Ettlinger, a 2006 study found mosquito fish in the reservoir of Lake Lagunitas.

In Santa Cruz County, Deborah Beale, tired of maintaining the chlorine pool in her home in the mountains above Soquel, decided to replace it with a natural pond a few years ago to care for local wildlife.

“Of course we have a wonderful explosion in the tree frog population – and dragonflies,” she said. “But we also had more mosquitos.”

Beale contacted Santa Cruz County Mosquito and Vector Control, who dumped a small bag of mosquito fish in their new pond. The fish – now thousands – protect the Beale residence from annoying bites and dangerous viruses like West Nile and St. Louis encephalitis.

“They certainly really improved the quality of life on the property,” she said. “It’s absolutely noticeable.”

Mosquitofish were especially helpful after last year’s forest fires when mandatory evacuations left dozens of pools abandoned for weeks, said Amanda Poulsen, vector control manager for the Santa Cruz district.

Your agency is careful with their use, however – they only collect mosquito fish from natural waterways and pre-populated pools, and only deliver them to closed bodies of water. It also distributes flyers urging users not to release the fish.

However, vector control agency strategies vary widely across California counties. While three small vats in Santa Cruz County hold a few hundred mosquito fish year-round, the Sacramento and Yolo counties vector control agency breeds millions of mosquito fish in 22 local ponds each year. In 2020 alone, more than 2 million mosquito fish were released.

The Elk Grove facility 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 water near dense residential areas creates fertile insect breeding grounds.

In an educational basin, mosquito fish feed on mosquito larvae in the Sacramento-Yolo Mosquito and Vector Control District in Elk Grove. (Anda Chu / Bay Area News Group)

“If we don’t treat these rice fields with fish or pesticides, the local residents are at great risk,” said Tony Hedley, the district’s fisheries overseer. “It weighs more than where these fish will land.”

However, prioritizing human health often puts other species at risk.

“Of course these mosquito fish will migrate to other locations in the event of a flood,” said Hedley. “I honestly don’t know the full effects of mosquito fish, but I’m more concerned than ever.”

California law allows health officials in most counties to plant mosquito fish on private property without a permit. While both the Santa Cruz County and Sacramento-Yolo agencies state that they usually need property owner approval before distributing the fish, they acknowledge that there is no way to track whether or not private residents are using them properly to dispose.

That keeps officials constantly working to balance the promise of public health with the ecological threat posed by fish.

“Mosquitofish themselves are neither good nor bad,” said Palkovacs. “We shouldn’t cast them as either the hero or the villain of history.”

Independent journalist Will Houston contributed to this report.

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