Larval fish community of the urbanized Chicago River

Abstract

Urbanization has drastically altered the riverine systems upon which cities once depended, leading to significant declines in suitable freshwater habitat and, consequently, biodiversity. The Chicago River exemplifies this transformation, with the added distinction of having been engineered to flow across a continental divide. Over the past 50 years, improvements in water quality have been closely associated with a transition to a fish community that is unexpectedly high in species richness. The goal of this research was to assess which fish species spawn in the Chicago River by capturing larvae. Light trapping conducted in the North and South Branches of the Chicago River between 2020 and 2022 yielded 2211 larval fish representing 24 species, identified using 16S rRNA Sanger sequencing. Peak abundance of larvae occurred in July, coinciding with water temperatures around 23 °C. Species considered pollution-intolerant were more common in the South Branch, whereas pollution-tolerant species were common in the North Branch. Larvae from families Atherinopsidae, Centrarchidae, and Leuciscidae (i.e., native minnows, formerly in the family Cyprinidae) were more abundant in the South Branch, suggesting that a combination of suitable substrate, submerged vegetation, and water quality must be present, potentially in backwater areas such as unused barge slips. As habitat restoration activities continue, more species are likely to exhibit successful reproduction in the system. These findings suggest that other heavily modified urban river systems may support resilient, biodiverse fish communities.