Keith Lusher 08.21.26
For years, wildlife officials have worked to keep invasive silver carp from reaching Lake Michigan. A discovery made last week shows the fish may be getting closer.
Sampling crews removed hundreds of juvenile silver carp from the Marseilles Pool of the Illinois River on Aug. 13 and 14, according to the Illinois Department of Natural Resources. The fish were only 4 to 6 inches long, but it was their location that caused concern.
The Marseilles Pool is approximately 80 miles downstream from Lake Michigan. Although adult invasive carp have previously traveled that far upriver, biologists had never found silver carp that young so far north during 16 years of monitoring.
The discovery suggests the fish may have spawned somewhere farther upriver or in a tributary feeding into the Illinois River.
Silver carp need moving water to reproduce because their eggs must remain suspended in the current until they hatch. The young fish can then drift for miles before reaching the size of those collected in the Marseilles Pool. Flooding and warm water temperatures this summer may have helped create favorable spawning conditions.
However, officials said the discovery does not yet prove silver carp have established a reproducing population in the area. Biologists generally consider a population established only after finding both adult fish and young carp in multiple stages of development.
Still, the presence of hundreds of juveniles marks a notable change in the river.
Crews are now expanding their search across the upper Illinois River and its tributaries to determine how far the spawning event may have reached. They will use electrofishing to target backwaters where young carp often gather, while scientists will study the microchemistry of the captured fish for clues about where they hatched.
Monitoring will also increase in the Dresden Island Pool. Located about 50 miles from Lake Michigan, the pool is considered the leading edge of the adult invasive carp population.
The discovery comes at a difficult time for efforts to keep the fish out of the Great Lakes. Construction remains halted on the Brandon Road Interbasin Project near Joliet, where a series of electrical, acoustic and bubble barriers is planned to stop invasive carp while still allowing barges to pass.
The U.S. Army Corps of Engineers issued a stop-work order July 24 while the project undergoes an administrative review. However, no date has been announced for work to resume.
Silver carp have not reached Lake Michigan, but officials now have a new sign that the threat may be moving closer.
