A number of dead turtles found lined up on the shore of the St. Croix River last week had been accidentally killed by the Wisconsin Department of Natural Resources, the agency said.

DNR crews set several hoop nets on the bottom of the river early this month to monitor the St. Croix's catfish population. When they checked the nets on Aug. 21, they found that about 60 turtles had been entangled. The surveyors were able to set most of the turtles free, and watched them swim away, the agency said in a statement.

The crew placed all the turtles that didn't immediately revive on a sandbank, facing the water, in the hope that they would eventually come to and slip away into the water.

"DNR staff checked on the turtles a day later and noticed many swam away," the agency said in a statement.

About 20 turtles died.

Boaters on the St. Croix noticed the eerie sight of a line of lifeless turtles on the shore, near Scandia, Minn., and posted videos. KARE 11 was the first to report on it.

The DNR said that the agency started the catfish survey in 2020 and this was the first time that large numbers of turtles had been caught in the nets. The nets have been removed. Most of the entangled turtles were spiny softshell turtles, which are common in the St. Croix and have a healthy population, the agency said.

No endangered or threatened turtles were caught or harmed, the agency reported.

National Park Service staff removed the dead turtles from the sandbank over the weekend.