News Release

Mosquito fish may be wiping out amphibians

Reports and Proceedings

New Scientist

ONE of the oldest methods of biological control-introducing mosquito fish to eat disease-carrying mosquito larvae-is spawning an environmental disaster. It turns out that the fish prefer eating tadpoles, and this may be a leading cause of the crash in amphibian populations round the world.

"Exotic predators are one of the top three causes of amphibian decline, and mosquito fish are one of these major predators," says Lee Kats of Pepperdine University in Malibu, California. He calls use of the fish "archaic" and says it needs to be re-examined.

Mosquito fish (Gambusia affinis) are native to the US and Mexico. Since early this century governments in the US, Hawaii, Canada, the former Soviet Union, New Zealand and Australia have introduced the fish to exploit their appetite for mosquito larvae.

But Kats suspected that the fish also have a taste for other delicacies. In 1996, he showed that they eat the larvae of the California newt (Taricha torosa), but critics said the studies revealed little, as the newts were the only food on offer.

So Kats and his colleague Jeff Goodsell tested whether the fish eat amphibians when mosquito larvae are also available. They placed two mosquito fish in tanks containing either mosquito larvae, tadpoles of the Pacific treefrog (Hyla regilla), or a mixture of the two with twice as many larvae as tadpoles, to replicate natural conditions in Santa Monica's rivers. The fish had completely undiscerning tastes. They munched their way through all mosquito larvae and tadpoles in the single-prey tanks and also finished off the mixed tank, gobbling up tadpoles and mosquito larvae in no particular order (Conservation Biology, vol 13, p 921).

Recreating the experiments in rivers showed that wild mosquito fish actually have a preference for tadpoles. Around 65 per cent of stream-caught fish had tadpoles in their stomachs, but only 56 per cent had mosquito larvae.

In the Santa Monica area, numbers of Pacific and California treefrogs, and California newts, may be declining because they haven't had time to evolve defences to mosquito fish. Kats believes that the same is probably true for amphibians elsewhere. California treefrogs are already being considered for protected status.

Kats adds that in many places the fish are given out freely to the public to distribute, and they often end up in areas that don't have a mosquito problem. "These fish are still being used. It's not a one-time mistake, it's an 80-year mistake," he adds.

However, mosquito control expert Chris Curtis of the London School of Hygiene and Tropical Medicine cautions that alternative methods of mosquito controls, such as insecticide spraying, may be just as environmentally damaging. "We have to be careful about downgrading what is a pretty useful control," he says.

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Author: Matt Walker

New Scientist issue 25th August 99

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