Movement of small‐bodied fishes from Lake Michigan into Chicago Area Waterways: Insights from otolith chemistry

The Chicago Area Waterways System (CAWS), an artificial connection between Lake Michigan (LMI) and the Illinois River watershed, has served as a conduit for invasive species and is a potential pathway for viral hemorrhagic septicemia (VHS) to spread from the Great Lakes into the Illinois River drainage. Although some fishes are known to have moved from LMI into the Illinois River via the CAWS, the rate of fish passage from the lake into the CAWS is unclear. Stable carbon isotopic signatures (δ¹³C) in otoliths differ between lake‐ and CAWS‐resident fish and were used as a natural tag to identify individuals that may have moved into the CAWS from LMI. The objectives of this study were to (i) estimate relative frequency of individuals with otolith δ¹³C indicative of prior residency in LMI among small‐bodied (<150 mm total length) VHS‐susceptible fishes collected from several locations in the CAWS, and (ii) to assess differences in relative frequency of probable immigrants from LMI among CAWS locations that differ in distance from the lake. Thirty‐six percent of small‐bodied fishes collected from the CAWS were identified as potential immigrants based on otolith δ¹³C. The percentage of individuals that had otolith δ¹³C suggestive of prior residency in LMI was higher among fish collected from the Calumet River compared to the Chicago River and North Shore Channel and decreased with increasing distance from lock and dam structures at the three entrances to the CAWS from the lake. Movement of small‐bodied fishes from LMI into the CAWS appears to be common and may represent a pathway for VHS‐susceptible species to transport the virus into the CAWS and potentially beyond.