ODESSA, Minn. — The cause of a late April die-off of turtles in the Big Stone National Wildlife Refuge will likely remain a mystery.

Laboratory tests performed on the carcasses of turtles collected by refuge staff are all inconclusive, according to Tina Shaw, public affairs specialist with the U.S. Fish & Wildlife Service.

The samples had been analyzed by the U.S. Geological Survey National Wildlife Health Center in Madison, Wis. Preliminary test results showed some bacterial activity, but the results were considered inconclusive, partly because the samples were greatly decomposed. Viral testing showed no conclusive evidence either.

Scott Simmons, refuge manager, said there is hardly a day that goes by without visitors to the refuge inquiring as to the cause of the die-off.

The die-off was reported to refuge staff by members of the public on April 30. Staff collected carcasses of both painted and snapping turtles from an approximate half-mile stretch of the Minnesota River in the refuge.

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Staff estimate that as many as 200 turtles may have died. Simmons noted it is unusual to have so many turtles die at one time in such a short stretch of river.

He said it’s not known how long the turtles had decomposed prior to staff being alerted to the die-off and then collecting samples. He said the time of decomposition is likely the reason the laboratory results are inconclusive.

Contamination, disease and even predation were considered as possible causes. Staff saw no clear evidence for any of these potential causes.

The refuge manager said there have been no further signs of turtle die-offs since the April 30 reports.

He said it is not known how the die-off may affect the overall population and reproduction of turtles within the refuge. Staff members are keeping a close eye on the turtle population, and a new biologist on staff will be monitoring how the population is doing.