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The Newsletter of the Colorado Herpetological Society

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Natural History

Why Frogs Fall Victim to Mosquito-eating Fish

By D'Vora ben Shaul
The Jerusalem Post, 9-5-99

A number of environmental factors have been investigated as causes for their disappearance, including pollution and pesticides, which were found to be contributing factors, but not the prime culprit.

Lee Katz, at the Pepperdine University in California, claimed that the amphibia were being destroyed by another life form. Seen as unlikely at first, Katz's theory is now pretty well accepted by limnologists and wildlife biologists, mainly because Katz found out who was eating the tadpoles.

Amphibia, by their very nature, are inevitably linked to pools and streams, where they breed. It is at this stage that the newly hatched amphibia disappear. Katz's culprit is the most ubiquitous of water denizens, the mosquito fish (Gambusia affinis). This fish has been widely introduced into lakes, ponds, rivers, streams, and even water reservoirs all over the world as a natural means of controlling mosquito-carried disease. A native of South America and some areas of the U.S., the little mosquito fish is a voracious eater of mosquito larvae.

Over the past 80 years, millions of Gambusia have been introduced to waters outside their original habitat. As a result, many Gambusia were introduced to waterways where the native amphibian population had the mosquito larvae well under control.

Katz designed a series of experiments that showed conclusively that the little mosquito fish also has a real taste for the tadpoles and eggs of spawning amphibia. In one experiment, he found twice as many Gambusia with undigested tadpoles in their stomachs as those that had fed on larvae, but in most cases the fish had fed on both. Since the Gambusia breed almost all year, producing dozens of live young on a monthly basis, and the amphibia breed only once a year, there isn't really any doubt of the outcome, Katz says.