Hundreds of dead fish wash up on Bloomington's Lower Penn Lake

On a recent visit to Lower Penn Lake with his kids, Bloomington resident Mark Gabe Jensen noticed some dead fish near the boat landing. At first he thought there were just a few.

"Dad, there's like 50 of them," his 10-year-old daughter said to him.

Turns out it was more like several hundred dead fish, mostly invasive carp, according to a report this spring from the Minnesota Department of Natural Resources. The reason for the die-off, according to the report: anoxia, or lack of oxygen, likely due to winter ice.

Bloomington officials were first alerted March 24 about dead fish along Lower Penn Lake's northern shore. While 98% were carp, there were some sunfish, perch and bass, along with softshell turtles and mussels.

This kind of die-off is called winterkill, said Tom Burri, a freshwater scientist with the DNR. "Sometimes [in] our most extreme, harshest winters we have more complete kills, more fish die [and] more species," he said.

"What's happening now is people go out to put their dock in or they go out to walk the dogs … and then they see a whole bunch of dead fish. But the fish have probably been dead for quite a while. They died earlier in the winter. But the ice is gone and the wind blew them all onto shore."

Lower Penn Lake has a history of winterkill because it's a shallow lake with poor water quality due to high levels of phosphorous. Plants in lakes covered with ice and snow decompose during winter months from a lack of sun, Burri said. Instead of creating oxygen via photosynthesis, dying plants eat up the oxygen — so little is left for the fish.

The DNR so far this spring has received winterkill reports from 15 other lakes around the metro area, including Twin Lakes in St. Louis Park, Ryan Lake in Robbinsdale and Thole Lake in Shakopee.

Rice Lake in Maple Grove earlier this month reported hundreds of dead fish, mostly bluegills. The lake was drawn down 5 feet to control curly leaf pond weed, said Daryl Ellison, DNR fisheries supervisor in the west metro. Like Lower Penn, Rice Lake is shallow and has a history of winterkill.

As northern Minnesota lakes thaw out, Burri said more winterkill will be reported. The DNR says that as many as 500 fish die-offs happen every year in Minnesota.

Jack Distel, Bloomington's water resource specialist, said this many fish don't often wash up at Lower Penn Lake. There is a bright side, he said: Most of the fish killed were invasive species. And he said eagles and other wildlife benefit from the abundance of food on the shoreline.

Distel said staffers will survey Lower Penn Lake later this year to assess the number of fish and work on carp removal.

Though winterkill reports spring up with ice-out on the lakes, Burri said nutrient issues — caused by pesticides and fertilizers, manure and stormwater runoff in both urban and rural landscapes — make the problem worse.

"This is a symptom of a larger problem: nutrient-rich waters," he said.

Burri said people should report winterkills when they spot them because sometimes they're caused by pollution, such as an oil spill. Fishery staffers use the information for management and planning, such as whether to treat water or restock fish.

"But we want citizens to be on the lookout for anything because we don't want to miss the big one," he said. "And there will be some."

To report winterkills, call the Minnesota Duty Officer line at 651‐649‐5451.