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From the 'It Seemed Like a Good Idea at the Time' Dept.
INVASIVES WEEK: We took the mosquito fish to the river and dropped them in the water. Now, these little fish are making a big mess of freshwater ecosystems.

This story is a part of OnEarth's Invasive Species Week.

Patrick L. Rakes recalls scouring the waterways of south-central Tennessee, looking for Barrens topminnows, a species found nowhere else on Earth. This was back in the 1980s, when he was a graduate student in zoology at the University of Tennessee. He did find the species, whose breeding males turn a striking, iridescent blue-green color, in 14 locations, alleviating fears that they were on the verge of extinction.

Then a foreigner arrived. Over the next decade, Rakes watched these Barrens topminnow populations wink out one by one, replaced by a much less dazzling specimen: a small, dull-gray creature known as the mosquito fish. Though benign in appearance, mosquito fish are aggressive and deadly to larger, less combative fish. And so, by 2000, Barrens topminnows were all but gone; today they naturally occur in only three known places in the world. “Everywhere the mosquito fish are present, the topminnows have disappeared,” says Rakes, now the codirector of Conservation Fisheries, a nonprofit that helps protect and restore populations of rare fish.

It’s a pattern emerging across the globe. In temperate and tropical habitats on every continent except Antarctica, mosquito fish have made life difficult for small fish, amphibians, and insects, outcompeting them for food, eating their eggs and young, and, at times, relentlessly harassing adults. This destructive species continues to spread, abetted by humans, who ignore the consequences of what seemed like an easy solution to another problem: mosquitoes and the malaria and other diseases they carry.

Mosquitoes lay their eggs in water. Fish, in general, like to eat mosquito larvae before it hatches, but mosquito fish are especially voracious feeders. For that reason, health officials and others have introduced the fish as a way to curb mosquitoes. They were released widely in post-Katrina New Orleans, for example, to head off outbreaks of West Nile disease.

Less than 2.5 inches long, mosquito fish would appear to be ready food for larger fish and other aquatic species. Instead, they dominate ecosystems with the gusto of a much larger predator, in part because they’re prolific breeders that give birth to live young. They also punch above their weight. “They’ll go after a fish ten times their size and nip their fins,” says Peter Unmack, a research fellow at the University of Canberra who has watched the predators eliminate red-finned blue-eye, which he discovered in the Australian outback, from all but two springs.

Two mosquito fish

Mosquito fish can also survive in relatively polluted waters. One Santa Cruz, California, public health worker observed more than a dozen mosquito fish placidly swim in the murky shallows of a freshwater lagoon that sat adjacent to a sewage treatment plant.  So getting rid of the hardy creatures is nearly impossible: Like cockroaches, they can survive being flushed down a toilet. Even poison has had only mixed success.

Native to much of the eastern and central United States, mosquito fish began their global invasion in 1905, soon after it was discovered that mosquitoes transmit diseases. That year, Hawaii funded David Starr Jordan, a fish biologist and the founding president of Stanford University, in a bid to control the islands’ biting insects. Jordan dispatched his student Alvin Seale to the Houston-Galveston area, where he collected three predatory fish species and brought them on the long journey to Hawaii. Of those species, western mosquito fish, then known only by its scientific name, Gambusia affinis, proved to be by far the most aggressive at going after mosquito larvae, attacking them like “a pack of wolves ravaging a flock of helpless sheep,” Seale wrote in a paper published in The Philippine Journal of Science. Having declared the experiment a success, Hawaii’s government transported mosquito fish around the islands so quickly that, within a few years, their population purportedly totaled in the millions.

Seale and other mosquito fish proponents, including the American Red Cross and the Rockefeller Foundation, aided the global spread, bringing them to Asia in the 1910s and to the western United States in the ’20s. Meanwhile, the closely related eastern mosquito fish, Gambusia holbrooki, was being transported to Europe and Australia. All told, western and eastern fish were introduced to at least 37 U.S. states and 50 countries, becoming one of the most widespread freshwater fish in the world. Even New Zealand, which didn’t have a mosquito problem, got them. As early as 1936, scientists began recording the deleterious effects that mosquito fish were having on native species.

What was easy to introduce has been almost impossible to eradicate. It’s been more than 75 years since scientists first learned of the consequences of their quick-fix pest solution, but those sounding the alarm have proven ineffective at halting the mosquito fish’s incursions. Although now closely regulated in certain states (California, for instance), mosquito fish remain a key component of government-run mosquito-control programs. And, although its use for pest control has generally decreased, the fish can be acquired on the Internet by anyone with a mosquito problem, purchased from breeders either unaware or unconcerned about their harmful impact.

Though some scientists continue to tout them as a singularly effective biocontrol agent, others believe native species could just as easily get the job done. “If there are fish already present in moderate numbers, then the number of mosquito larvae that will hatch out of that pond is negligible,” Unmack tells me. “The species doesn’t really matter.” Unfortunately, that’s a distinction biologists somehow missed when they first touted the species. So, lesson learned—right, humankind?

(Well, maybe. Now read Ginger Strand’s account of another adventure in invasives killing invasives—a real-life noir crime drama set in her own backyard.)

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Jesse Greenspan is a Brooklyn-based freelance journalist who writes about history and the environment. His work has appeared in such publications as the New York Times, Scientific American and Audubon. MORE STORIES ➔