Around 2,500 fish — mostly brown trout — were found dead in a major fish kill on Rush Creek near Lewiston, Minn. in late July. Photo courtesy of Carl Berberich / Minnesota Trout Unlimited.

The deaths of the fish remain unexplained.

State agencies are continuing to investigate what might have killed thousands of fish in a popular southeastern Minnesota trout stream earlier this summer. 

The Minnesota Pollution Control Agency said some 2,500 fish — mostly brown trout — died in Rush Creek near Lewiston in late July and there's reason to believe the deaths didn't occur naturally. 

The MPCA, the Department of Agriculture and the Department of Natural Resources have yet to pinpoint the cause of the fish kill. 

Last week, the MPCA said state agencies are awaiting results of water quality tests and also gathered samples of the macroinvertebrate community from four different locations near the fish kill. 

According to state investigators, over 100 landowners within the 10-square mile area upstream of the fish kill are also being asked to provide detailed information about manure and pesticide applications on their property. 

A 27" brown trout washed up in apparent fish kill on Rush Creek near Lewiston, Minn. in late July. Photo courtesy of Robert Meyer / Minnesota Trout Unlimited. 

While fish kills can occur naturally due to diseases or low oxygen levels, fish kills occurring after rainfall are often indicative of manure, pesticide or fertilizer runoff.

Toxic spills or discharges of wastewater or stormwater have also been linked to fish kills. According to the MPCA, fish kills driven by rain events often have less-discernible causes. 

In a statement Wednesday, the MPCA said state agencies have a long history of working together on fish kill incidents where a cause is not immediately known. 

"Our agencies are committed to conducting a thorough investigation and will collaborate to analyze data and details gathered from numerous sources," the agency said. 

John Lenczewski, executive director of Minnesota Trout Unlimited, said it's possible an exact cause won't be determined. Lenczewski is among trout anglers raising concerns about whether or not state regulations are doing enough to protect Minnesota's freshwater. 

"This isn't an isolated incident," he said. "Unfortunately, it's sort of a pattern," Lenczewski told Bring Me The News last month. 

"We all want clean water," he continued. "So this should be a concern to everyone, not just trout anglers." 

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