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NFC: Mosquito FISH kILL frogs

Mosquitofish Imperil Frogs 

Mosquitoes are a nuisance. They also transmit a variety of diseases, so
getting rid of them is generally a good idea. Governments in many
Southern states released mosquitofish in large numbers into ponds and
streams over the past four decades to control mosquito populations. The
fish have a great appetite for the water-dwelling larvae—unfortunately,
they find endangered amphibian tadpoles just as appetizing.

"The common idea was that they can't do much harm," says Lee Kats of
Pepperdine University in California, one of the authors of a new study on
mosquitofish. Considering its modest size, two inches, this relative of
the popular aquarium guppy can be surprisingly voracious. In lab and
field experiments, Kats and his colleagues found that the fish devoured
as many Pacific treefrog tadpoles as mosquito larvae, even when offered a
relatively larger amount of mosquito larvae. Gut samples from wild fish
showed that the fish frequently dined on frog or newt offspring also
under natural circumstances. 

"There’s this control practice of the government that is dangerous to the
amphibians," Kats says. "The county does little to regulate the usage." 

The agency in charge of mosquito control, the LA County Vector Control
District, does not deliberately release mosquitofish into natural water
bodies, says Lu Ann Munn, a spokesperson for the district. Others put
them there. "We do give them to the public and they may be misused," she

"Most districts are starting to take a critical look where they put the
fish," says David Brown of the Sacramento district. He disagrees,
however, with Kats' view that mosquitofish are not necessary anymore
today. Several cases of the mosquito-borne Western equine encephalitis
have been detected in recent years, and other diseases could rebound
without the fish.

The vector control districts are studying and applying alternative means
of mosquito control. Examples are a fungus that very specifically only
kills mosquitoes and black flies, and highly specific biocides, such as
insect growth suppressants. But these control agents have their own
problems. The fungus depends on good water quality and insects can become
resistant to pesticides. 

"If anything, the policy should be re-evaluated," Kats says. It may be
too late for the amphibians, now that mosquitofish are resident in many
water bodies. "I'm pessimistic for at least two species," he says. The
California newt and the California tree frog may already be beyond the
point of no return

—Beate Kittl
Posted 8/9/1999

Related web sites: 
MosquitoNet from the University of California-Davis 
Alameda County Mosquito Abatement District 
Mosquitofish Page from the USGS 
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