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The Six-Legged Canary in the Coal Mine

In the early days of coal mining and up into the early 20th Century, coal miners would bring caged canaries into new underground coal mining areas. Canaries are sensitive to low levels of oxygen, which are caused by high concentrations of methane and carbon dioxide – dangerous to both bird and human – and made them an important asset to miners. So long as the canary would sing, miners knew they were safe from explosions and suffocation.

The mayfly, an aquatic insect, can be used like a canary to detect problems in water quality. Mayflies, like canaries, are sensitive to changes in their environment, but the environment where mayflies live most of their life is in the water. Mayflies will not tolerate poor water quality with low levels of dissolved oxygen.

Mayflies are primarily “detritivores,” eating organic matter that accumulates on the bottom of lakes and rivers. When ready to transform into an adult, mayfly larvae emerge from the water and take to the air, living no longer than a couple of days to mate. Adults cannot even bite. They only have vestigial, or a small remnant, of what once was their mouth.

Why are mayflies so sensitive to polluted water and particularly the type of pollutants, like sewage, that remove oxygen from the water? As part of spending most of their life on the water’s bottom, they inhabit burrows at the sediment-water interface where they grow and develop for at least one year to ultimately transform, like a butterfly from its chrysalis, into a winged mayfly in search of a mate. With low levels of dissolved oxygen, they suffocate.

During the early 1900s, many portions of the Upper Mississippi River teamed with mayflies, a particularly spectacular sight since mayflies often mature and leave the water at the same time by the millions. By mid-century, mayflies essentially disappeared from a lengthy reach of the river immediately downstream of Minneapolis and St. Paul, Minnesota. Culprits included increased sedimentation caused by runoff and chemical pollution. The main factor was poorly treated sewage that dramatically reduced oxygen levels needed by mayflies and many other aquatic creatures to survive.

By the 1990s, mayflies returned to many areas of the river where they had been absent for decades thanks to better sewage treatment efforts and antipollution laws such as the Clean Water Act. Mayflies returned to the river. Big time.

Mayfly emergence events from the Mississippi River have been recorded since explorers searched for the river’s source in the early 19th century and continue to this day.

Spring seems like a long way off, but if you happen to catch an emergence event, record your observations at:

By Scott Covington
Regional Office - External Affairs (On detail from Headquarters - Refuges)


Mayflies cover a road sign near Stoddard, Wisconsin. Photo by USFWS.

Mayflies cover a road sign near Stoddard, Wisconsin. Photo by USFWS.

Last updated: January 6, 2016