New study shows fish spawning in Chicago River hits a high point

Chicago — Crappies, bluegills and bass appear to be benefiting greatly from improved habitat and conditions in what was once considered the most polluted body of water in Illinois.

But those popular species aren’t alone.

Austin Happel, a research biologist at Shedd Aquarium and author of a new study that looks at fish currently reproducing in the Chicago River, reported that at least two dozen species have successfully spawned in the urban waterway.

Happel and his team set nighttime light traps at 10 locations in the river over a three-year period to collect more than 2,000 larval fish – undeveloped, and often microscopic, stages of fish that emerge from eggs.

DNA samples from Happel’s traps revealed 24 different fish species are 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,” Happel explained. “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.”

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Along with sport fish like crappies, bluegills and bass, other species that are reproducing include forage fish – gizzard shad, golden shiners and spotfin shiners. The Shedd study also uncovered larvae of brook silverside and mimic shiner, which are “generally thought to be pollution-intolerant, and thus their presence was a pleasant surprise,” Happel noted.

A  paper on the study was recently published in Journal of Great Lakes Research. According to the Shedd research team, the study showcased some differences across sites that could suggest opportunities to augment spawning habitats for fish.

Other highlights of the research include:

• The peak abundance of fish larvae occurred in July, coinciding with water reaching temperatures in the low 70s.

• 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 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.

• Happel is also researching how floating wetlands, which have been installed in the river by local nonprofit Urban Rivers and Shedd Aquarium over the last several years, affect fish reproduction.

Happel said that, overall, the study “offers hope for other heavily modified urban waterways and might offer insights on how to best support fish communities to spawn.”

As noted in the paper on the study, the Chicago River represents a “highly modified and degraded system that has exhibited an increase in biodiversity thanks to over 50 years of remediation efforts.”

In other words, the river has undergone positive transformations.

The Chicago River was originally channelized to aid in the removal of waste water and storm water. It was hydrologically controlled by dam structures, and connected to Chicago’s sewer system in the late 1800s and early 1900s.

While the North Branch of the river generally follows its historical path, the North Shore Channel was added in 1910 to aid in maintaining a southward flow. The South Branch historically flowed toward the northeast, meeting the North Branch before flowing eastward into Lake Michigan.

Today, the South Branch is fully channelized and connected to the Chicago Sanitary and Ship Canal, allowing southwestward flows.

Historically, the river’s sediment  was heavily contaminated with pollutants and silt, remnants of industrial and urban wastes.

Since passing of the Clean Water Act of 1972, water quality of the Chicago River has vastly improved, with strong reductions in total organic carbon, nitrogenous compounds, and suspended solids.

Importantly, the new study by Shedd Aquarium staff demonstrates how restoration can work to bring wildlife back.

“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,” Happel added.