Sound-emitting technology to test whether warmer Mille Lacs is affecting walleye

A new study is deploying underwater listening devices to find out why more young walleye in Lake Mille Lacs aren't surviving to adulthood.

The Mille Lacs Band of Ojibwe is using a technology called acoustic telemetry to track the movements of the juvenile fish.

Band scientists hope to test a hypothesis that warming lake temperatures are one of the factors contributing to the recent decline of the iconic lake's walleye population.

"What we're hoping to accomplish is to shed light on why there's elevated juvenile walleye mortality in Mille Lacs Lake," said Carl Klimah, fisheries biologist for the band's natural resources department.

Earlier this summer, researchers placed 61 acoustic receivers in a grid pattern about 3,000 meters apart throughout the lake.

This month, they will catch about 70 adult walleye, sedate them with an electrical charge and implant a transmitter tag in their bellies. The tags emit a sound picked up by the receivers when within range. Then the fish are released back into the lake.

Researchers will catch and tag 35 juvenile walleye in the fall and another 35 next spring, Klimah said. The tags allow researchers to glean a wealth of data about the fish's movements during different seasons, he said.

"We'll know roughly where that fish is, we'll know what depth it's at, we'll know what temperature it's occupying," Klimah said. "So there's a lot of information that comes with it."

That data will help band researchers test their theory that the lake's warmer temperature and clearer water — filtered by invasive species such as zebra mussels and spiny waterflea — are impacting young walleye.

Researchers want to know if walleye, seeking their optimal water temperature of around 68 degrees, are being forced into smaller areas of the lake. That would bring juvenile and adult walleye closer together and could increase the likelihood that larger adults would eat the younger fish, Klimah said.

Klimah said that to his knowledge, it's the first time the technology has been used on a large inland Minnesota lake. The band is partnering with the Great Lakes Indian Fish and Wildlife Commission and the Fond du Lac Band of Ojibwe on the study.

The Mille Lacs Band received a grant of nearly $200,000 from the U.S. Fish and Wildlife Service for the study, which runs through the spring of 2019.

However, Klimah said the technology can be used to continue studying Mille Lacs' walleye population into the future. He said the data will be shared with the Minnesota Department of Natural Resources, which co-manages the lake's fishery.

Walleye fishing on Mille Lacs Lake has been limited to catch-and-release only this summer. DNR officials have said the walleye population shows signs of improving health, but angling restrictions are needed to help it continue to recover.