Earlier this month, visitors at the Minnesota Valley National Wildlife Refuge noticed a concerning trend: dead birds strewn about near Blue Lake in Shakopee.

This week, the refuge reported that the eight birds — including swans, mallards and Canada geese — tested positive for lead poisoning. The necropsy showed what appeared to be lead shot fragments, said refuge manager Sarena Selbo. 

Before the refuge was founded, waterfowl hunting had been popular along the Minnesota River. While lead shot has been banned for waterfowl hunting since the late '80s, fragments can linger at the bottom of lakes, Selbo said. While it's unclear if the birds ingested the lead in Blue Lake, the lake's water levels were recently drawn, which could have shifted some lead shot buried in the bottom of the lake, she said. 

"Lead fragments as small as a couple grains of rice can make a bird ill, causing birds to become very weak, stop feeding and then starve," Selbo said in an email. 

About 100 panfish also died from a lack of oxygen in shallow pockets of the lake — a common occurrence in winter, as ice and snow cover the lake and limit sunlight for the oxygen-producing plants underwater, according to the Minnesota Department of Natural Resources. 

In response, the refuge will increase the water levels "to help plants grow and create quality feeding and resting habitat for waterfowl," Selbo said in an email.  

The Minnesota Pollution Control Agency said it doesn't track the number of lakes with incidences of lead-poisoned waterfowl. However, the issue is common enough for the agency to launch a "Get the Lead Out" campaign, which encourages fishers to switch to non-lead tackle. According to the MPCA, national research indicates lead poisoning accounts for about 14 percent of loon deaths in Minnesota.

In recent years, Vadnais Lake has also reported lead-poisoned waterfowl. 

In January, Minnesota state Sen. Charles Wiger and Rep. Peter Fischer, both Maplewood Democrats, introduced a bill to ban the sale and use of lead jigs and sinkers. 

Wiger told the Star Tribune earlier this month he's optimistic about gaining bipartisan support, as last year the Legislature passed a ban on most industrial use of trichloroethylene. 

"We hear a lot from residents worried about the swans. They are a charismatic species and people see them," Dawn Tanner, a conservation biologist and program development coordinator with the Vadnais Lake Area Water Management Organization, told the Star Tribune. "A legislative change is probably the push that is needed."

Correction: A previous version of this story incorrectly spelled Selbo's last name.