Scientists at the Metropolitan Water Reclamation District of Greater Chicago (MWRD) are going to need a bigger boat as aquatic biologists at the MWRD reported catching the biggest smallmouth bass in over 51 years of MWRD fish monitoring on the Chicago Area Waterway System (CAWS).

The fish was caught on Oct. 30 in the Cal-Sag Channel near the MWRD’s sidestream elevated pool aeration (SEPA) station in Blue Island. It weighed 3.9 pounds (1,770 grams) with a length of 19.7 inches (500 mm). In 2022, the same MWRD crew also caught a trophy size largemouth bass over four pounds on the same waterway.

The collection of larger game fish is generally a positive indicator for local water quality and ecosystem health. Many game fish species are sensitive to pollutants, temperature changes, and low dissolved oxygen levels. The presence of larger specimens suggests that the conditions can support the high energy demands needed for fish to grow large. 

“This smallmouth bass, like thousands of other fish, are finding inviting habitat and suitable conditions in the Chicago Area Waterway System,” said MWRD President Kari K. Steele. “We are proud to report this amazing record setting find because it provides a tangible example of a thriving water environment due to our excellent treatment process. In addition, it highlights the District’s clean water initiative and our daily work protecting our precious water resources.”

The MWRD first began studying fish in 1974, studying abundance, composition, diversity and the general health of the fish. At that time, they documented only 10 fish species. Today, that figure has ballooned to 77 in the CAWS, a sign of improved water quality. This includes more than 30 game fish species and 62 species that are native to Illinois. Catch rates have also steadily increased, from an average of 22 fish per hour in the 1970s to 436 fish per hour in the 2010s at electrofishing sites on the Cal-Sag Channel.

This year, the MWRD’s Monitoring and Research Department conducted 22 fish surveys across area waterways from June to October 2025, collecting, identifying, weighing and measuring over 8,000 fish, nearly all of which were released back into their habitats. They also carried out 686,784 hourly measurements at 20 different locations in the area waterways to monitor dissolved oxygen levels, which are essential for aquatic life and chart the steady improvement in water quality. They also collected water samples from 30 locations in the CAWS as part of the MWRD’s Ambient Water Quality Monitoring Program. At each of the 384 sampling events, there are between 15 and 17 individual sample bottles filled. These water samples are sent to MWRD labs where approximately 24,312 individual chemicals were analyzed. To learn more about the MWRD’s water quality monitoring work, visit gispub.mwrd.org/awqa.