A new report concludes that mayfly populations are in a precipitous decline in Great Lakes regions, likely due to pollution and algal blooms. This should be a concern not just for anglers, but for anyone who cares about the health of the ecosystem.

These insect explosions provide food for a wide variety of animals, from perch and other commercially important freshwater fish to birds and bats. But new research shows that mayflies are in decline.

“We were really surprised to see that there was a decline year after year,” says lead author Phillip Stepanian, a bio-meteorologist at the University of Notre Dame. “That was really unexpected.”

These swarms are so big and thick that they appear on weather radar used to track rain and snow, and for a long time, meteorologists ignored these signals as a type of “noise,” he says. But Stepanian, who was trained as a meteorologist, realized these signals could provide useful information about populations and movements of animals like birds and insects, including mayflies.

An article in Smithsonian Magazine suggests that a “possible culprit is fertilizer runoff from farms, which have been triggering algal blooms in Lake Erie. Algal blooms release toxins into the environment, to which mayflies are “highly sensitive,” according to the researchers.

Pesticides are also flowing into the Great Lakes tributaries. One 2018 study, for instance, found that concentrations of the neonicotinoid class of insecticides was up to 40 times higher than acceptable limits set by U.S. Environmental Protection Agency Aquatic Life Benchmark. The mayfly species Hexagenia, on which the new study focused, “are among the most sensitive aquatic insects to a suite of these commonly applied pesticides,” the researchers write.

The reduction in mayfly numbers is also disconcerting because these insects play a crucial role in the food chain. As underwater nymphs, they act as a crucial food source for fish and wading birds; once they emerge onto land, they are eaten by other insects, birds and bats.