Minnesota officials on Monday were trying to determine what killed thousands of fish in Lake Owasso.
There's no evidence of a chemical spill or toxins, and tests show that oxygen levels are normal, said Harland Hiemstra, a spokesman for the Minnesota Department of Natural Resources.
Lake Owasso lies within Shoreview and Roseville in the northern St. Paul suburbs. The lake is managed primarily for muskies, he said.
Lake resident Les Hassler said he's counted thousands of dead fish since it froze over last Tuesday. Hundreds of fish could be seen belly-up inches beneath the new ice on Sunday.
About 90 percent of the dead fish DNR workers have counted are small panfish, with a few bass and walleyes, Hiemstra said. Residents have pulled out a few dead muskies and large walleyes, too.
"We're not too alarmed by it," Hiemstra said. "It's part of a natural process."
Hiemstra discounted speculation by some residents that herbicides used to kill aquatic weeds are to blame. He said the DNR doesn't treat lakes for weeds, and while it issues permits for lakeshore property owners to do so in limited circumstances, the last treatments would have been in August.
"I'm sick to my stomach for the lake," said John Newman, a board member with the Twin Cities chapter of Muskies Inc., which helped stock the lake. "I'm also real worried (because) this lake is connected through pipes to other lakes. So the best thing we can do right now is find out what caused this, and hopefully keep it from happening in any other lake."