The scrappy little mosquitofish, the pit bull of ichthyology and the region's leading defense against West Nile virus, is a savior to insect-infested waters.
Unless you're a frog.
Yes, the fish dines on larval mosquitoes, as intended. But scientists have learned it also has an appetite for the tadpoles of frogs, toads and other amphibians — including the threatened red-legged frog and the endangered Santa Cruz long-toed salamander.
"Frog populations have disappeared from a lot of the places where fish were introduced," said UC Davis biologist Sharon P. Lawler, who has studied the problem.
"Unintended consequences are the hallmark of introduced species," she said. "They often have good and bad effects."
For example, thistle makes delicious honey, but destroys pastures, she said. The tamerisk tree blooms with gorgeous pink flowers, but drains streams.
However, Santa Clara County Vector Control relies on the voracious guppy-like fish. It is so effective against mosquitoes that the agency buys it by the hundreds of pounds each year, then distributes it to residents for free to use in their backyard pools and ponds.
First brought from the East Coast in the 1920s, for decades, they were handed out like candy at Halloween.
The mosquitofish has established a firm foothold in natural waters all over the Bay Area, including Los Gatos and Guadalupe Creeks, as well as the storm water canals near San Francisco International Airport and innumerable ponds.
Two years ago, following the lead of California Department of Fish and Game, Santa Clara and most of other counties banned releasing mosquitofish into natural sources, such as ponds, creeks and marshes. It is a violation of state Fish and Game regulation for private citizens to release mosquitofish in state waters without a permit. But authorities acknowledge that there is no oversight once the mosquito fish are distributed.
Scientists stop short of urging a ban on the fish — when not allowed to escape, the fish are safe, cheap and effective, they concede. But they favor more innovative and targeted controls, such as new mosquito-killing bacteria, which do not harm amphibians.
With begrudging admiration, vector control director Tim Mulligan calls the fish "a terrific tool, in the right circumstances. It's biological control. It means we use fewer pesticides."
Indeed, some scientists were initially skeptical that such a small-mouthed fish, a surface feeder, could even harm a frog. University of California biologist Jeff Wilcox, steward of the San Jose-based Blue Oak Ranch Reserve, tested the premise: he picked up some tadpoles and tossed them into a stream.
"The mosquitofish swarmed them like a school of piranhas and tore them into bits in a matter of a few seconds," he said.
The omnivores are handy for vector control purposes — they forage through the winter after mosquitoes are long gone, so do not have to be resupplied every year. But it also means that they'll eat anything that doesn't eat them first.
Fish are just one of many assaults against the amphibians, which have also declined worldwide because of disease, habitat destruction, pesticide use, and pollution.
But the non-native fish have caused new problems
What they can't eat, they harass. Studies show that tadpoles in ponds with mosquitofish suffer greater injuries and weigh 34 percent less than their fish-free counterparts.
Though officials no longer place the fish in natural streams, they find their way there.
They are remarkably hardy creatures, surviving places that few other fishes would tolerate. And females can delay birth until the time and place are right. So she might mate in a lowly horse trough — but not spawn until out of the trough, into a nearby ditch, then liberated into a wild pond.
"They can disperse," said Jeff Miller of the Center for Biological Diversity, an environmental advocacy group based in San Francisco. "That's the concern."
Mulligan agreed. "There are few places that you can go, if you look long and hard enough, that you won't find a mosquitofish in residence. It's in virtually all the creeks in the county."
But it needs to learn its place, agree both critics and supporters.
"People see a stranded fish and want to save it," Lawler said. "It might make sense to the individual fish — 'Please move me!' — but it could end up in sensitive frog habitat." Lawler said.
To kill it, Mulligan recommends super-chlorinating the water. Or letting the fish dry out. Just don't flush it down the toilet or bathtub. For the sturdy mosquitofish, this is an invitation to paradise.
"Some have survived sewage treatment," said Mulligan.
Contact Lisa Krieger at email@example.com or 408-920-5565.