Chicago River Study is Tracking Fish Movement in These Busy Waters

Efforts in recent years to clean up the Chicago River system have led to increases in fish numbers and diversity in this waterway. But in this highly urban environment, do fish have the conditions they need to thrive?

An ongoing study is monitoring fish in the river using a system of sensors and receivers, in other words, a tracking system that allows fish to tell us what habitats are important to them, according to Austin Happel, research biologist at the John G. Shedd Aquarium, who is leading this effort.

While the level of contaminants in the Chicago River is lower than it’s been in maybe a century, the environment still poses challenges to fish living in the river system. It’s life in the big city—busy, noisy and sometimes polluted.

For example, the Chicago wastewater system is designed for stormwater and sewage to travel in the same pipes. When big storms hit, the combined sewer overflow system can become overwhelmed and untreated sewage can leak out, or in several locations, is pumped into the river to alleviate pressure on the system. Water quality plummets, at least temporarily.

Another challenge is structural. In places, this hectic, urban river flows right up against high rises, some that make up the Chicago skyline. Rather than having a natural shoreline, which can provide fish habitat, most of the downtown river’s edge is comprised of steel walls.

Efforts to improve fish habitat have led to the development of the Wild Mile in the river’s North Branch Canal in addition to floating wetlands in the South Branch. Shedd Aquarium, along with Urban Rivers, a non-profit organization, have created a floating “ecopark” of tiny islands complete with wetland plants native to Illinois. The underwater root systems are meant to provide safe areas for spawning and for young fish and other river organisms.

It’s not clear the extent to which fish are using this habitat, so Shedd Aquarium is collaborating with Illinois-Indiana Sea Grant, Purdue University, and the Metropolitan Water Reclamation District of Greater Chicago to answer questions about where river fish go and when, including how they respond to combined sewer overflow events and when and how they are making use of the Wild Mile.

“We have put listening devices in strategic locations that allow us to not only track where fish are, but how quickly they move through the system and maybe what might cause them to move and relocate to other areas,” said Happel.

In late spring of 2023, the researchers set up 32 receivers in the river and equipped 80 fish with sensor tags, including largemouth bass, common carp, some panfish species and walleye. The acoustic telemetry receivers can hear and identify the unique sound emitted from each tagged fish.

The first question that the researchers needed to answer before collecting data was whether the tracking system would work accurately in parts of the river. Some features of the Chicago River that pose challenges for fish were also a concern for the study itself, including the steel walls and the general busyness of the downtown portion of the river.

“One of our main concerns was boat noise,” said Luke McGill, a Purdue master’s student. “The Chicago area waterway system has pleasure boaters, but then also has large barges and architectural tour boats, so we were concerned that all that boat traffic would drown out the sounds of the tagged fish.”

Another question was whether the river’s steel walls would create echoes that throw off the flow of data—maybe creating false detections. As it turns out, neither the noise nor the steel walls posed a significant problem, so receivers were placed where it made the most sense to answer critical questions.

After collecting a few months of data, Happel and McGill have several interesting stories to share of individual fish behavior. For example, there’s one bluegill that regularly makes a 14-kilometer (8.7 mile) trek in a period of about 18 hours.

“He’s done it five times,” said Happel. “It’s really interesting to see such a little fish make such a huge journey many times, back and forth, while other bluegills in various parts of the waterway are basically staying put.”

Several fish have left the river, moving through the Chicago Harbor Lock that opens into Lake Michigan. But, interestingly, one walleye, after being gone for a few months, once again, was detected in the river.

The researchers are also seeing some impacts from combined sewer overflows. In Bubbly Creek, an infamous section of the river on the city’s southside that is still recovering from many decades of pollution from Chicago’s meatpacking industry, the fish population dropped last summer when, in response to a large rainstorm, raw sewage was pumped into these waters.

“As sewage entered Bubbly Creek, the dissolved oxygen in that area dropped to zero so we saw a lot of fish leave,” said McGill, who is focusing on the movement of largemouth bass in the river for his master’s thesis. “For the most part, largemouth bass have not come back even though the dissolved oxygen level has increased.”

As the monitoring enters its second year, some tag batteries will soon die, and new fish will be added to the study, which will continue for at least another year. Happel hopes the study will help answer a range of questions with regards to the Chicago River fish, from understanding what they do in the winter to how they cope with busy boat traffic.

“There’s a slew of questions that we’d like to answer with this data,” he said. “We’re hoping it will shed some light on what parts of the river are important to fish during different events that might happen over the course of a year.”

---

Irene Miles has been a communicator with Illinois-Indiana Sea Grant for 23 years, writing about program research and outreach activities. She has a master’s degree from the University of Illinois Urbana-Champaign and a bachelor’s degree from Loyola University Chicago.