The Cisco Squeeze

A streamlined, silvery fish that swims in large schools in Minnesota lakes, the cisco faces challenges in some of those waters as changes in climate and land use alter its habitat. A Minnesota Department of Natural Resources study of three lakes aims to find out what cisco need and how to help them thrive.

Members of the salmon family, cisco depend on cold, well-oxygenated water, says DNR native fish biologist Brett Nagle. Broadly distributed across the northern two-thirds of the state, Minnesota cisco exist at the southern edge of the species’ North American range, making them “more vulnerable to changes in the environment, and vulnerable to extirpation,” he explains.

In the past 50 years, the area has seen warmer and wetter climate conditions and land use changes including development and deforestation. Biologists, Nagle says, are noticing that many lakes where cisco live are changing too. Longer, warmer summers are raising surface water temperatures while deeper, colder waters are losing oxygen—and habitat.

“Even if a lake has historically had good cisco habitat, a big problem cisco populations are facing is that deeper layers of cold water are losing oxygen,” he explains. “In the summer months, when the lake is stratified into layers, the surface is too warm. They want to move deeper, but that deeper layer doesn’t have enough oxygen, so they’re getting pressed into a smaller and smaller range of depths they can survive in. Biologists who work on cisco call this a squeeze.”

Cisco populations are variable and hard to track, but some trends are clear. Across their range, cisco are gone from some lakes where they once swam, and where they remain, their coldwater habitat is being squeezed.

To get a better handle on what drives a healthy cisco population in a lake, scientists are taking a close look at three Minnesota lakes of different sizes, depths, and conditions: Elk Lake in Itasca State Park, Lake Carlos near Alexandria, and Ten Mile Lake near Hackensack. The three are among 25 water bodies in the DNR’s Sentinel Lakes Program, a long-term monitoring effort begun in 2008.

Using specialized equipment like vertical gill nets and hydroacoustic surveys to monitor massive cisco schools, biologists are learning more about the fish’s needs and how management might help them.

Because cisco, also called tullibee or lake herring, can signify something about a lake’s health, they are sometimes considered an indicator species. And they are a key part of the food chain in waters where they live.

“They are a really fatty and nutrient-rich forage base for large predators like walleye, northern pike, and lake trout,” says Nagle. “In a lot of our lakes in the northern two-thirds of the state, cisco are important forage for the game fish people enjoy so much.”