New study on fish reproduction suggests ongoing recovery of the Chicago River

A new peer-reviewed research paper, authored by Shedd Aquarium and published in Journal of Great Lakes Research, assesses the fish species that spawn in the Chicago River to sustain and support resilient, biodiverse fish populations. Utilizing light traps at 10 locations in the Chicago River between the years 2020 and 2022, over 2,000 larval fish were collected, and their DNA revealed 24 different fish species present.

“This research represents the first documentation of larvae from fish species occurring within the Chicago River, indicating that recovery and restoration efforts allow at least 24 species to successfully spawn and reach the larval stage,” said Dr. Austin Happel, author of the study and research biologist at Shedd Aquarium. “Our previous studies showed that improvements to water quality in the river increased biodiversity, and now we have more positive news as populations are proliferating locally within the river.”

Species that are reproducing in the Chicago River included popular sport fish such as black crappie, bluegill and largemouth bass, and forage fish such as gizzard shad, golden shiner and spotfin shiner. Further, the study uncovered larvae of brook silverside and mimic shiner, which are generally thought to be pollution-intolerant, and thus their presence was a pleasant surprise. The peak abundance of larvae occurred in July, coinciding with water reaching temperatures in the low 70s (degrees Fahrenheit).

The study showcased some differences across sites that could suggest additional opportunities to augment spawning habitats for fishes. Most notably, species considered intolerant of pollution were more common in the South Branch, yet species more tolerant to pollution were common in the North Branch of the Chicago River.

The finding suggests that some combination of suitable substrate, submerged vegetation, and water quality and flow are most likely present in the South Branch. Particularly, there’s a possibility of backwater areas, such as unused barge slips in Bubbly Creek, which serve as important nursery habitats and offer protection from predation and abundant food. Ongoing research by Happel focuses on how floating wetlands, which have been installed in the river by local nonprofit Urban Rivers and Shedd Aquarium over the last several years, affect successful fish reproduction.

“As people continue to care about and conserve this critical freshwater ecosystem, we hope to see increases in the number of fish spawning in the system,” said Dr. Happel.

Overall, the study offers hope for other heavily modified urban waterways and might offer insights on how to best support fish communities to spawn. Further, it demonstrates how restoration can work to bring wildlife back to local waters. This is important as healthy fish populations mean healthier aquatic environments, which ultimately benefits the people that live and work around them.

Shedd Aquarium works with partners and community members to reimagine, research and rewild the river. This ongoing effort involves creating new habitat with manmade floating wetlands, restoring existing habitat with volunteers, creating additional access through recreational opportunities, collecting data to monitor wildlife populations and more. Support for this work could include visiting the aquarium, volunteering for an action day, paddling for a good cause or donating to Shedd.

The findings from this research have been published in Journal of Great Lakes Research under the title, “Larval fish community of the urbanized Chicago River.”