Sources of Bighead Carp and Silver Carp Found in Chicago Urban Fishing Program Ponds

Planktivorous Bighead Carp Hypophthalmichthys nobilis and Silver Carp H. molitrix are two invasive species that pose a severe threat to native aquatic biota. Efforts have been made to inhibit Bighead Carp and Silver Carp expansion into the Great Lakes through hydrologically connected systems, but understanding their expansion to and from hydrologically disconnected systems could further reduce this risk. We assessed the origin of 23 Bighead Carp and 1 Silver Carp captured from Chicago Urban Fishing Program ponds using otolith chemistry. Otolith core δ ¹⁸ O and δ ¹³ C of captured Bighead Carp did not overlap with ranges of otolith δ ¹⁸ O and δ ¹³ C from Bighead Carp and Silver Carp from the Illinois River but was consistent with otolith δ ¹⁸ O and δ ¹³ C of Bighead Carp obtained from aquaculture facilities. Likewise, otolith core Sr:Ca of 19 captured Bighead Carp was higher than the range of otolith Sr:Ca from Illinois River Bighead Carp and Silver Carp. Due to inferred timing of introduction (likely age-0 or age-1 fish), these Bighead Carp may have been released incidentally via stocking sport fish shipments contaminated with the carp. Contrastingly, otolith chemistry results suggest that the captured Silver Carp originated from the Illinois River, although timing of introduction of this fish could not be inferred. Overall, our results indicate that nearly all fish analyzed did not originate from rivers within the Mississippi River basin and were instead likely introduced through stocking contaminated shipments of fish. Even though Chicago Urban Fishing Program ponds now pose a minimal threat for Bighead Carp and Silver Carp expansion into Lake Michigan, sustained urban pond monitoring and nonnative fish removal may limit or eliminate this alternative invasion pathway. Additionally, continued vigilance in understanding the origin of stocked fish and culture environment (e.g., polyculture) could decrease the likelihood of future invasive species introductions through contaminated sport fish stockings.