Fish flies exist to reproduce, feed other animals, and recycle nutrients from water to land. Despite their brief and seemingly pointless adult lives, they play an outsized role in freshwater ecosystems as a food source for fish, birds, and bats, and they serve as one of nature’s most reliable indicators of clean water. The name “fish fly” is a regional term, used heavily around the Great Lakes, for burrowing mayflies in the genus Hexagenia, one of the most widespread mayfly groups in North America.
What Fish Flies Actually Are
Fish flies are mayflies, insects belonging to an ancient lineage that originated more than 300 million years ago. The species most commonly called a fish fly is Hexagenia limbata, a large burrowing mayfly found across much of North America. As nymphs, they live in the soft sediment at the bottom of lakes and rivers for roughly a year, feeding on organic material. Then they emerge, mate, and die within about 24 hours.
That extreme lifespan imbalance is the defining feature of mayflies. The adult stage isn’t really for “living” in any conventional sense. Adults don’t eat. Many species don’t even have functional mouthparts. The entire purpose of the winged adult form is to find a mate, reproduce, and deposit eggs back into the water.
A Critical Link in the Food Web
The biggest ecological purpose of fish flies is feeding other animals, both underwater and in the air. As nymphs burrowed into lake and river bottoms, they’re consumed by stoneflies, caddisflies, dragonfly larvae, water beetles, leeches, and crayfish. Fish eat enormous quantities of them. In one study of a Norwegian mountain lake, trout consumed an estimated 30 to 40 percent of the total annual mayfly production.
Once adults emerge and take flight, they become prey for birds, bats, dragonflies, and spiders. Even amphibians and small mammals like shrews feed on them. The sheer volume of biomass they represent makes them a cornerstone species. Without fish flies and other mayflies, freshwater food webs would lose one of their most important energy transfers: converting decomposing organic matter on lake bottoms into protein that feeds animals up the food chain.
Why They Emerge All at Once
If you’ve ever seen fish flies blanketing a gas station or piling up under streetlights, you’ve witnessed one of their key survival strategies: synchronized mass emergence. Millions of adults emerge within the same narrow window, typically over just a couple of weeks in late spring or early summer.
This timing isn’t accidental. It serves two purposes. First, emerging together overwhelms predators. Birds, bats, and dragonflies can only eat so many in a night. Research on synchronized mayfly emergence found that the percentage of adults killed by predators on any given day drops as the total number of emerging adults increases. By flooding the environment all at once, each individual fish fly improves its own odds of surviving long enough to mate. Biologists call this predator satiation.
Second, when your adult life lasts a single day, finding a mate quickly is everything. Emerging at the same time as millions of others solves that problem. The massive swarms that annoy people near lakefronts are actually mating aggregations, ensuring that males and females find each other during their vanishingly short window.
Living Water Quality Monitors
Fish flies are among the most sensitive biological indicators of water health. Hexagenia nymphs cannot survive in polluted or oxygen-depleted water. This sensitivity made their disappearance from Lake Erie in the 1950s one of the most dramatic ecological collapses ever documented in the Great Lakes. Two periods of severe oxygen depletion in the summers of 1953 and 1955, caused by heavy nutrient pollution, wiped out burrowing mayflies across the western basin of the lake.
Nymphs were absent from most of western Lake Erie’s sediments for decades afterward. Small populations persisted near the shoreline, but the open basin remained empty. By 1993, surveys found several small populations returning near the western and southern shores. By 1995, nymphs had spread throughout the western half and eastern end of the basin, a sign that water quality was improving. Scientists with the U.S. Geological Survey have used Hexagenia populations as a direct measure of ecosystem health across the Great Lakes for this reason. When fish flies return to a body of water, it generally means dissolved oxygen levels and sediment quality have improved enough to support sensitive aquatic life.
So if you live near a lake and fish flies are swarming in huge numbers, that’s actually good news about your local water.
Nutrient Recycling From Water to Land
Fish fly nymphs spend a year feeding on decaying organic matter in lake-bottom sediments. When millions of adults emerge, fly over land, mate, and die, their bodies deposit nutrients like nitrogen and phosphorus onto terrestrial surfaces. This transfer of nutrients from aquatic to land-based ecosystems is a meaningful ecological process, fertilizing shoreline soils and feeding decomposers on land. It’s the same principle behind salmon runs enriching forest soils, just on a smaller, more widespread scale.
Why They Swarm Around Lights
The reason fish flies pile up around streetlights, gas stations, and lit storefronts has nothing to do with attraction to light in the way most people assume. Flying insects don’t steer toward light sources. Instead, they use the brightest part of the sky to keep themselves oriented, a reflex called the dorsal light response that keeps their top side pointed upward during flight.
When an artificial light is nearby, it corrupts this orientation system. The insect tilts its back toward the light source instead of toward the sky, which sends it into disoriented loops. Research published in Nature Communications confirmed this by tracking insects with motion-capture technology. In every species tested, insects flew perpendicular to the light rather than directly toward it, creating the orbiting and erratic flight patterns people observe. They aren’t attracted to the light so much as trapped by it.
Reducing unnecessary upward-facing and unshielded outdoor lighting can help. Downward-directed fixtures and shielded bulbs produce less skyward light scatter, giving fish flies fewer false orientation cues and reducing the pileups on buildings and sidewalks.