Hungry Minnow Run Amok?

A hungry minnow let loose to eat mosquitoes appears to be munching on some dwindling amphibians, too. Introduced mosquito fish are threatening California treefrogs, researchers report in this month's Conservation Biology. The finding highlights the potential risks of biological control, in which natural predators are used to fight pest populations.

Native to ponds and slow-moving streams in the southeastern United States and northeastern Mexico, mosquito fish feast on the insect's waterborne larvae. As a result, mosquito control officials have handed out the minnow by the bucket load around the world in a bid to fight mosquito-borne diseases, such as malaria. They say the fish has helped reduce both mosquito populations and the use of expensive and potentially dangerous pesticides. Some biologists, however, worry that the fish threatens more than just mosquitoes, blaming it for declines in native fish and amphibian populations in New Zealand and elsewhere. But mosquito fish advocates have argued that there are few studies showing that the released fish has a taste for anything other than insects.

Now, however, ecologists Lee Kats and Jeff Goodsell of Pepperdine University in Malibu, California, have shown that the minnows will devour treefrog tadpoles even when other prey is plentiful. Their suspicions were raised when they examined the stomachs of 36 mosquito fish netted from a stream near Los Angeles and found that 65% contained tadpoles, while just 56% had mosquito larvae. They then tested the fish's prey preference in the wild and in the laboratory by setting up in-stream tanks and tabletop tubs stocked with minnows, Pacific treefrog tadpoles, and mosquito larvae. Even when the tanks were swarming with larvae, the minnows went for the tadpoles. The pair concluded that the presence of the minnows "is an important factor in the [frogs'] reproductive success." Indeed, Kats says he has watched frog populations drop in streams where the fish were only recently introduced. "I'm not saying that they are the only cause, but they are a cause," he says.

The study confirms that biocontrol advocates need to consider both the benefits and potential ecological costs of their control agents, says ecologist Daniel Simberloff of the University of Tennessee, Knoxville. "We've ignored the downsides for too long," he says. And Kats hopes the results will convince California officials to reconsider their policy of handing out the fish to anyone who asks. "Twenty years from now," he says, "we are going to look at this practice and say: 'What were we thinking?' "