Investigating Great Lakes Fish Migrations

Shedd’s Director of Freshwater Research Karen Murchie, Ph.D. and her collaborators are conducting research to better understand suckers, answering questions about their spawning behaviors and their lives in the lakes. These findings will help us make conservation and management recommendations that could ease challenges not only for suckers, but for a range of migratory fishes that call the Great Lakes home.

Who are the Suckers in your Neighborhood?

When you hear the word "sucker," what do you think of? Some might envision a tasty lollipop, while others think of two fish species that share this common name: remoras, which are a marine species, and algae-eating plecos, which are popular tropical aquarium fish. The suckers here in the Great Lakes have fabulously fleshy lips and pectoral fins that stick out like airplane wings when pausing in the water, two traits that set them apart.

Found in all five of the Great Lakes, white suckers (Catostomus commersonii) and longnose suckers (Catostomus catostomus) are the most widespread and the focus of Shedd’s research. Collectively, these species have been dubbed “the wildebeests of the Great Lakes” for the sheer number of individuals that move from the lakes into creeks and rivers to spawn in the spring.

One single tributary of Lake Superior had over 82,000 white suckers and longnose suckers during the migration! Beyond sustaining their population, sucker spawning breathes life into creek and river food webs. Their eggs and other waste provide nutrients that help algae grow faster and aquatic bugs grow bigger. Other fish feast on sucker eggs and eagles, osprey and wolves take advantage of the large volume of fish in shallow waters. The spawning run typically lasts a few weeks before the fish return to the lake they came from.

White suckers and longnose suckers face challenges that are common to many migratory fishes in the region, including habitat fragmentation and climate change. What we learn from them can be applied to improve conditions for other fish species.

Exploring the Impact of Climate Change on Fish Migrations

What environmental cues trigger suckers to leave the Great Lakes and begin their commute up tributaries to spawn? Is climate change affecting this natural process? 

Since 2017, Dr. Murchie and a team of volunteers have been documenting the timing of sucker migrations in tributaries of Lake Michigan and Lake Superior (in Illinois, Wisconsin and Michigan) in relation to stream conditions.  

The results published in fall 2024 showed that water temperature is the strongest predictor of when spawning will begin. Shifts in climate can cause creeks to warm earlier than usual. If suckers spawn before the rest of the ecosystem is ready to benefit from their arrival, it can have far-reaching consequences that remain to be seen. 

Understanding Sucker Behavior: During Spawning Season

Unlike migratory Pacific salmon that die after spawning, suckers live to spawn again and again. Using tiny microchips — like those we use to tag our pets — Dr. Murchie has tagged white suckers to determine if these fish return to the same location year after year to spawn. Murchie has confirmed that adult suckers typically return to a preferred spawning site each year. Since suckers can live for decades, results from this study can be used to inform future habitat restoration projects so the paths from lake to spawning sites are healthy, connected and clear.

Understanding Sucker Behavior: Outside of Spawning Season

We know that suckers move in large groups into tributaries to spawn, but where do they hang out the rest of the year? Do fish that spawn together, stay together year-round? To find out where suckers go and what ecological role they play beyond the spring migration window, we’ve been tagging white suckers and longnose suckers with acoustic tags during their spawning runs.

Monitoring the movement of suckers is done through a network of acoustic receivers across the Great Lakes. Whenever a tagged fish swims close to one of the acoustic receivers, that information is recorded and later uploaded in a database. The Great Lakes Acoustic Telemetry Observation System (GLATOS), established by the Great Lakes Fishery Commission (GLFC), allows researchers from both the United States and Canada to share tracking infrastructure to unravel the mysteries of Great Lakes fishes.

The battery-life of the tags lasts three years, so researchers can put together travel logs for each individual tagged fish to learn how and where they moved in that time. So far, we’ve learned that suckers that spawn together don’t stay with each other year-round!

What Do Suckers Sound Like?

Imagine putting your ear underwater during sucker spawning, what would you hear? When white suckers and longnose suckers spawn, two or three males gather around a female and begin to vibrate their bodies and tails vigorously while releasing eggs and sperm. The result is a swishing sound as loose gravel and sand move about under the spawning fish. Eco-acoustics allow researchers to record and analyze this underwater audio.

This technology has played a big role in understanding coral reef ecology and marine mammal communication, but has rarely been applied in freshwater. By recording characteristic spawning sounds from both species of suckers, researchers can pinpoint where they were, even in muddy waters. This tool can be very helpful in restoration initiatives that improve habitat connectivity.

Great Lakes Fishery Commission FishPass

Around the Great Lakes, dams and culverts can impede fish movement. Only 14% of the tributaries in Michigan are free-flowing waterways with no barriers. While dam removal can improve fish migration corridors, it can also allow invasive species in the Great Lakes to gain access to locations they have previously been cut off from.

On the Boardman (Ottaway) River in Traverse City, Michigan, the Great Lakes Fishery Commission is building an innovative solution called FishPass that will replace a deteriorating dam. The new system will sort and selectively pass native fishes like suckers while blocking harmful invaders like sea lamprey. 

Murchie is among several collaborators on the project to assess the genetics of suckers using the river before and after the completion of FishPass, as well as contributing her extensive knowledge of migratory fishes on the Science Advisory Team.