Mosquitofish Threaten Amphibians

"Handing out exotic mosquitofish as a control of mosquitoes is an outdated government policy that needs to be changed," says Lee Kats of Pepperdine University in Malibu, California, who presents this research with Jeff Goodsell in the August issue of Conservation Biology. This is the first study to demonstrate unequivocally that mosquitofish show no preference for mosquito larvae.

Distributed by the Los Angeles County Mosquito Abatement and Vector Control Center, mosquitofish have become established in many streams in the Santa Monica Mountains within the last 15 years. Because these streams have no native fish, the amphibians that live there lack defenses against fish predation. Amphibians native to these streams include the California newt, the Pacific treefrog and the California treefrog, which is a candidate for protected status.

Previous studies have shown that mosquitofish eat amphibians such as California newt larvae in Santa Monica Mountain streams. But these studies have been criticized because the mosquitofish were not offered alternate prey, leaving them no choice but to eat newt larvae.

Kats and Goodsell addressed this criticism by offering mosquitofish both Pacific treefrog tadpoles and mosquitoes. The researchers put bottomless two-by-two-foot tubs in fishless Santa Monica Mountain streams and added two mosquitofish, 20 tadpoles and 40 (low-density) or 80 (high density) mosquito larvae. These predator and prey densities approximated those naturally found in local streams.

After six hours, the mosquitofish in the "low-density" tubs had eaten all the mosquito larvae and more than 85% of tadpoles. Similarly, the mosquitofish in the "high-density" tubs had eaten more than half of both prey types.

The fact that mosquitofish eat tadpoles even when there are plenty of mosquito larvae shows that the fish have no preference for mosquito larvae. "If mosquitofish are to be an effective means of biological control for mosquitoes, they should consume primarily mosquito larvae rather than other native organisms," say Kats and Goodsell.

The researchers urge local governments to reconsider their policies of using mosquitofish to control mosquitoes. "Mosquitoes are not a major problem in Southern California--this is a desert with little freshwater--and there are other controls including a pathogen that kills larvae," says Kats.