Illinois' lead biologist on invasive fish, Steve Shults, had an interesting take on the number of bighead carp being found in Chicago lagoons, the latest caught by Gregory Hamilton from the Columbus Park lagoon on Friday.
Nobody knows for sure how the bigheads have ended up in at least three Chicago lagoons, but here's Shults' best assessment:
There have been several collections of bighead carp in various lagoons in Chicago, and we even collected a bighead carp in a Springfield park pond a few years ago. The most plausible explanation is that these lagoons have been stocked with catchable size channel catfish in the past from suppliers which also used bighead carp in their rearing ponds. Some of these carp were likely stocked inadvertently into these ponds during the catfish stockings.
There have been reports from other states with catfish stockings, so this seems most likely. There may have been releases of these fish from local sales prior to live sales being prohibited a few years ago. Contaminated bait may also be responsible as bighead carp fry can be difficult to distinguish from other baitfish species (although I wouldn't suspect those to be normally used as bait in the situations found in park district ponds).
The item of note is that since there are no natural connections with these ponds, the fish didn't get there by swimming through the river or canal, etc. There shouldn't be concerns for an invasion into Lake Michigan provided we continue to inform the public to be vigilant about not moving these invasive species.