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Walleye wired for science

Jul 04, 2018 10:11AM ● Published by Editor

By Vivian LaMoore of the Mille Lacs Messenger - July 3, 2018


Scientists have been keeping an eye on the walleye population of Mille Lacs Lake for decades, yet there is much that remains a mystery. The Department of Natural Resources of both the state and the Mille Lacs Band of Ojibwe have, for the last four years or so, recorded a declining population of walleye and have yet to figure out why. While the population is currently in recovery mode, there is much to be learned about the often elusive walleye.

A first-ever acoustic telemetry (fish tracking) study is underway that aims to reveal factors impacting juvenile walleye survival. The Mille Lacs Band of Ojibwe DNR has been setting up to begin a year-long (possibly longer) acoustic telemetry study of tracking fish patterns in season.

Both the state and the Band biologists have stated the survival rate of the young walleye has been in a decline due to walleye year classes that have been below average. This study will be able to track the young walleye for a better understanding of why the survival rate is so low.

Carl Klimah is the fisheries biologist for the Mille Lacs Band of Ojibwe DNR. “I think it’s important for people to understand that the mystery of juvenile walleye mortality is solvable. First, we will figure out what is happening, and then we will figure out what can be done about it,” Klimah said.

Biologists from both the state and the Band have long said there are many factors likely contributing the decline in the walleye population in the past few years. While it is important to realize the population is recovering, it is also important to understand why the population plummeted in the first place.

Some of the known contributors are the increasing water clarity and temperature along with a rise of aquatic invasive species like spiny water flea and zebra mussels. Yet there are unknown factors that may increase the mortality of juvenile walleye between their first and second winters.

Using sophisticated underwater listening equipment and fish-implanted tags, the Mille Lacs Band DNR is about to launch a first-of-its-kind telemetry study on Mille Lacs Lake to gain valuable data they hope will unlock the mystery of why so many of the lake’s young walleye are dying.

Testing a hypothesis

According to Klimah, walleye are cold-water fish that like a water temperature of around 68 degrees Fahrenheit for optimal growth and survival. The average daily temperature of Mille Lacs Lake has increased 1.8 degrees Fahrenheit since 1990.

“As water temperature increases, the optimal thermal habitat becomes more constrained, forcing more walleye into a smaller area,” Klimah said. “One hypothesis we will test with this study is whether the environmental conditions of the lake are bringing adult and juvenile walleye into closer proximity, potentially leading to higher predation of juvenile walleye by adult walleye.”

Klimah calls this “the squeeze.” In simpler terms, as water temperatures at the surface increase, walleye are perhaps seeking colder, deeper water than before. More walleye are being squeezed into a smaller area. When the juveniles, young of the year and other species of bait fish are in a concentrated area along with adult fish, the predation factor increases, i.e. cannibalism.

To test the hypothesis, Band DNR is conducting the telemetry study in partnership with the Great Lakes Indian Fish and Wildlife Commission. The study is made possible by a $199,075 grant to the Mille Lacs Band from the U.S. Fish and Wildlife Service. GLIFWC, the Fond du Lac Band of Lake Superior Chippewa and USFWS are collaborators on the grant.

How it works

The study will use 61 fixed-location receivers placed around 3,000 meters apart in every kind of habitat known in Mille Lacs Lake, including but not limited to mud flats, rocks, sand, etc. Walleye will be surgically implanted with acoustic transmitters (fish tracking tags) that provide data on what depth and temperature the walleye are occupying. In addition, they will also be tagged with a green external identification tag and then released into the water. In total, 70 adult walleye greater than 18 inches and 70 juvenile walleye between 8-10 inches will be tagged and released before study completion. In the first part of the study this July, tribal biologists will implant acoustic transmitters into 70 adult walleye. Then in the fall (when water temperatures are cooler) they will implant acoustic transmitters into 35 juvenile walleye (one year old fish). In the spring of 2019, another 35 juvenile walleyes in the same size range will be tagged and released. This will allow scientists to track one full year worth of juvenile movement data as the juvenile acoustic transmitting tags have limited battery life. More scientific research is also being conducted currently by the Band on these juvenile acoustic transmitting tags, and a hatchery study is under way to evaluate them further (See upcoming issue on the hatchery study).

When the walleye (implanted with acoustic transmitting tags) swim within range of an acoustic receiver, information will be recorded. Based on previous published research experience, the study team expects the Mille Lacs Lake acoustic receivers to register a tagged fish approximately every 30 to 60 minutes.

Each of the transmitters has a unique identifier. The data to be collected will include water temperature, pressure, home range of the fish, how far the fish has traveled, the habitat, light and growth of the fish. All of the data collected will be used to test the thermal habitat and squeeze hypothesis.

“As biologists, we develop a lot of scientific hypotheses about what is happening in the lake, but it is really exciting to deploy this type of technology to gather rich data about walleye movement and behavior in Mille Lacs to ultimately uncover meaningful insights,” Dr. Aaron Shultz, inland fisheries climate change biologist for GLIFWC, said. “We hope that this data can inform future management and conservation plans for Mille Lacs Lake.”

Each of the implanted fish will also be tagged with a special green tag different from the tags the state DNR uses. The tags will have a phone number on them. Klimah has requested that if anglers catch a tagged fish to contact the Band DNR office at (320) 532-5690 to help offer data that can be used in the study.

The study team will survey the acoustic receivers this fall and begin collecting data. The acoustic receivers are marked with GPS coordinates so the team can locate them via boats. They will then use BlueTooth technology to transfer data from the acoustic receivers directly to onboard computers.

This telemetry study is planned to go from July through next spring. Juvenile walleye will be tracked for one year due to limited battery life of the tags used on smaller fish. The adult tags may last for three or more years, providing ongoing opportunity to learn from the activity of the fish. If future telemetry tagging studies are conducted in subsequent years, the data will continue to build a more complete picture of fish movement in Mille Lacs Lake over time.

Researchers from the Mille Lacs Band and GLIFWC will compile data into a findings report at the conclusion of the study period.

The Mille Lacs Band hopes to continue to use this technology after the conclusion of the grant study. The Band also may explore collaborations with other managing agencies.


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