Fish Fry Day: Western Mosquitofish and the Danger of Good Intentions

Despite it’s small size compared to the rainbow smelt (right) and Northern pike (below) the Westernmosquito fish has a big tale to tell.

It’s Fish Fry Day and fish is on the menu! We’re working our way across the “Fishes of Wisconsin” poster finding morsels of info for every species of fishes found in Wisconsin. Today’s special: the Western Mosquitofish.

Take a moment to look at the map below. The orange color shows the native range of the Western Mosquitofish. The red shows all of the places the fish has ended up as a non-native and, often, invasive species. There is little connectivity between the two colors, meaning that the fish didn’t swim upstream and colonize new areas. Nope, this little guy got around via the best kind of public transit an invasive species can find – human beings.

The orange coloring shows the Western Mosquitofish’s native range, the red shows where it’s been introduced via human activity. Courtesy: USGS.

Why, you might ask, would people take the time to move a tiny little fish from its southern home waters to locations all over North America? Well, let me assure you that this isn’t a story of someone dumping a bait bucket or forgetting to clean their boat. No, in the case of the Western Mosquitofish, humans had intent – and they were the best of intentions.

You see, back in the day, our fair country had declared all-out war on mosquitoes. The problem, of course, was the malaria parasite that the annoying little bugs carried. In the late 1800’s the disease was prevalent from the Gulf of Mexico to the Northern Plains and people were doing just about anything to slow its spread.

Courtesy: Malaria Museum

To fight the public health threat of malaria, people went after the carrier of the disease. They drained wetlands, which are prime mosquito habitat and, starting in the early 1900’s, they began to move mosquitofish to new waters.

Western Mosquitofish. Photo: Robert McDowall, USGS

Back then both species of mosquitofish, the Western (Gambusia affinis) and the Eastern (Gambusia holbrooki) had a reputation as excellent mosquito-control agents (Krumholtz, 1948). A voracious surface feeder of plankton and insect larvae, with its flattened head and upward-pointing mouth, the mosquitofish has been documented consuming food equivalent to anywhere from 42 to 167% of its body weight each day (Chips, 2004). Grasping at these attributes as a potential tool in the malaria fighting toolbox, hatcheries started producing mosquitofish, state agencies began stocking their water bodies and even the former U.S. Public Health Service started helping move mosquitofish to new homes.

While the fish doesn’t specifically target mosquito larvae, relatively large, relatively slow-moving mosquito larvae are a huge food source for many small, surface-feeding fishes and the thought was that the mosquito fish would simply be better than other fishes at eating them. And, of course, it was.

Unfortunately, mosquitofish were so successful in their new homes, that they began having big impacts on the resident native species and other ecosystem processes. In other words, in the age-old story of human attempts to use biological controls – there were unintended consequences.

Mosquitofish eating mosquito larvae. Courtesy: Los Angeles County Vector-Borne Disease Control District

Mosquitofish proved to be not only voracious, but aggressive, shredding other fishes’ fins, eating their eggs and young and, in general, displacing them from their spots in the top of the water column. They’ve been particularly troublesome in the American West, contributing to the elimination or decline of federally endangered and threatened species – like the Railroad Valley springfish in Nevada and the Sonoran topminnow in Arizona (Courtenay and Meffe, 1989). What’s more, mosquitofish eat the zooplankton that would otherwise graze on algae, which can lead to nuisance algal blooms (Hurlbet et al. 1972). And, most ironically, mosquitofish can eat so many of a habitat’s natural predators of mosquito larvae that mosquito populations can increase after their introduction! (Hoy et al. 1972, Bence 1988). In fact, mosquitofish have been used so pervasively as biological mosquito-control agents around the world, having similar unintended consequences, that the Global Invasive Species Database has them listed as one of the world’s “worst” invasive species. 

“This is all very interesting,” you may ask, “but aren’t we talking about the fishes of Wisconsin?”

Well, yes, and Wisconsin does have a few established populations of Western Mosquitofish but, for now, they’re confined to four known locations in Grant County – in backwaters of the Mississippi River, the Sugar and Platte Rivers and in Sinnippee Creek. Otherwise, and like many northern states, the mosquitofish’s low tolerance of cold weather seems to be keeping populations in check. Let’s just hope no one finds a cold-water relative and ramps introduction efforts back up anytime soon!

*Big thanks to the USGS’s “Nonindigenous Aquatic Species” Fact Sheet for pointing us toward all sorts of cool info on this post.