New research has found 24 species of fish spawning successfully in the Chicago River, a network of waterways running from Lake Michigan through the heart of downtown Chicago.
Such a high number of reproducing fish species suggests the once severely polluted US river is now able to sustain and support resilient, biodiverse animal populations.
Researchers, who published their findings in the Journal of Great Lakes Research, say the recovery indicates that conservation efforts to restore the health of the river are working.
Chicago River: what lies beneath
To find out which fish species are reproducing in the river, scientists from the city's Shedd Aquarium selected 10 sites along the North and South Branches (two sections of the Chicago River).
At each site they installed light traps, illuminated devices used to attract larvae. The team collected 2,211 fish between 2020 and 2022.
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DNA analysis of the larvae revealed 24 species, including black crappie, bluegill and largemouth bass, as well as gizzard shad, golden shiner and spotfin shiner.
Some of the most exciting discoveries were juvenile brook silverside and mimic shiner fish, species generally thought to be pollution-intolerant. Their presence suggests significant improvements in water quality and habitat.
The study found differences between sections of the river. The South Branch supported more pollution-intolerant species, while the North Branch was dominated by those that tolerate poorer conditions. Researchers say this may be because the South Branch comprises a more habitable combination of submerged vegetation and riverbed substrate, as well as better water quality and flow.
There are also several sheltered backwater areas in the South Branch, such as the unused barge slips in Bubbly Creek (the nickname given to the South Fork of the South Branch), which provide safe nursery grounds for young fish rich in food.
“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,” says Dr Austin Happel, research biologist at Shedd Aquarium and lead author of the study.
The findings highlight how even heavily modified, urban rivers can bounce back with sustained restoration, adds Happel, who next plans to explore how floating wetlands, installed in recent years by Shedd Aquarium and the local nonprofit Urban Rivers, may also be contributing to successful fish reproduction.
Top image: light trap on South Branch of Chicago River. Credit: Shedd Aquarium
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