
How does dyeing the Chicago River on St. Patrick’s Day impact fish?
By Craig Nigrelli (Anchor), Evan Hummel (Producer), Jake Larsen (Video Editor)
- It’s a long-time Saint Patrick’s Day tradition to dye the Chicago River green, but does it harm fish or change their behavior? As far as scientists can tell, the answer to those questions is no.
- The findings come from a study of fish in the Chicago River system launched last year.
- Researchers are tracking the aquatic activity of more than 80 fish they’ve tagged for study.
Full Story
It’s a long-time Saint Patrick’s Day tradition to dye the Chicago River green, but does it harm fish or change their behavior?
Media Landscape
See how news outlets across the political spectrum are covering this story. Learn moreBias Summary
- The Chicago River has been dyed bright green to mark the city's St. Patrick's Day celebrations.
- Members of the Chicago Journeymen Plumbers Union Local 130 initiated the dyeing tradition 63 years ago.
- The dyeing occurs right before the annual downtown St. Patrick's Day parade.
- A second St. Patrick's Day parade is scheduled for Sunday on Chicago's South Side.
- No summary available because of a lack of coverage.
- The Chicago River was dyed green on March 15, 2025, to celebrate St. Patrick's Day, marking a 70-year tradition.
- Members of the Chicago Journeymen Plumbers Union Local 130 sprayed dye into the river, attracting thousands of spectators.
- The dyeing event began at approximately 10 a.m. Local time, using a nontoxic, secret formula.
- St. Patrick's Day events attract people of all backgrounds, celebrating Irish culture and the contributions of St. Patrick, who brought Christianity to Ireland.
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As far as scientists can tell, the answer to those questions is no.

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How do they know?
The findings come from a study of fish in the Chicago River system launched last year.
Researchers are tracking the aquatic activity of more than 80 fish they have tagged for study.
During last year’s parade, scientists say none of the tagged fish sought shelter as the water turned green, suggesting it is not impacting their daily habits or harming them.
How do they monitor the fish?
Biologists can detect movements by the fish through acoustic receivers placed in various parts of the waterway, allowing them to learn how fish respond to habitat restoration, flooding, sewage overflows and seasonal changes.
Why are environmentalists concerned?
The dyeing of the river has concerned environmentalists since its inauguration in 1962, in which oil-based dye kept the water green for nearly a month. Since then, a vegetable dye, which the Illinois Environmental Protection Agency says is nontoxic to wildlife, has been used.
Researchers now say the tagged fish appear to be unbothered by the dye, but other things like heavy rain and wastewater overflows lead them to escape to areas where oxygen levels aren’t depleted.
If fish are unable to leave a contaminated environment, it often kills them in massive numbers over a very short period of time.
Researchers said they have yet to witness any fish killed from the dye.
How is the biodiversity along the river?
The group also says that the river is the healthiest it has been in the past 150 years, with all kinds of wildlife, including birds, turtles, beavers and more than 80 species of fish in and around the river’s waters. Those 80 species are up from fewer than 10 species of fish in the 1970s.
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Why are environmentalists still opposed to the tradition?
Despite the findings, environmentalists still question whether dyeing the river is the best example of being good stewards of the Earth, and many remain opposed to the tradition.
[CRAIG NIGRELLI]
IT’S A LONG-TIME SAINT PADDY’S DAY TRADITION TO DYE THE CHICAGO RIVER GREEN BUT DOES IT HARM FISH OR CHANGE THEIR BEHAVIOR?
AS FAR AS SCIENTISTS CAN TELL, THE ANSWERS TO THOSE QUESTIONS ARE, NO.
THE FINDINGS COME FROM A GROUNDBREAKING STUDY OF FISH IN THE CHICAGO RIVER SYSTEM LAUNCHED LAST YEAR.
RESEARCHERS ARE TRACKING THE AQUATIC ACTIVITY OF MORE THAN 80 FISH THEY TAGGED.
DURING LAST YEAR’S PARADE, SCIENTISTS SAY NONE OF THE TAGGED FISH SOUGHT SHELTER AS THE WATER TURNED GREEN, SUGGESTING IT’S NOT IMPACTING THEIR DAILY HABITS OR HARMING THEM.
BIOLOGISTS CAN DETECT THE FISH MOVEMENTS THROUGH ACOUSTIC RECEIVERS PLACED IN VARIOUS PARTS OF THE WATERWAY ALLOWING THEM TO LEARN HOW FISH RESPOND TO HABITAT RESTORATION, FLOODING, SEWAGE OVERFLOWS, AND SEASONAL CHANGES.
THE DYEING OF THE RIVER HAS CONCERNED ENVIRONMENTALISTS SINCE ITS INAUGURATION IN 1962, IN WHICH THE OIL-BASED DYE KEPT THE WATER GREEN FOR NEARLY A MONTH.
SINCE THEN, A VEGETABLE DYE, WHICH THE ILLINOIS ENVIRONMENTAL PROTECTION AGENCY SAYS IS NON-TOXIC TO WILDLIFE, HAS BEEN USED.
RESEARCHERS NOW SAY THE TAGGED FISH APPEAR TO BE UNBOTHERED BY THE DYE BUT OTHER THINGS LIKE HEAVY RAIN AND WASTEWATER OVERFLOWS LEAD THEM TO ESCAPE TO AREAS WHERE OXYGEN LEVELS AREN’T DEPLETED.
IF THE FISH ARE UNABLE TO LEAVE A CONTAMINATED ENVIRONMENT, IT OFTEN KILLS THEM IN MASSIVE NUMBERS OVER A VERY SHORT PERIOD OF TIME.
RESEARCHERS SAY THEY HAVE YET TO WITNESS ANY FISH KILLED FROM THE DYE.
SCIENTISTS ALSO SAY THE RIVER IS THE HEALTHIEST ITS BEEN IN THE PAST 150 YEARS.
WITH ALL KINDS OF WILDLIFE FROM BIRDS, TURTLES, BEAVERS AND MORE THAN 80 SPECIES OF FISH, UP FROM FEWER THAN 10 IN THE 1970S.
DESPITE THE FINDINGS, ENVIRONMENTALISTS STILL QUESTION WHETHER DYEING THE RIVER IS THE BEST EXAMPLE OF BEING GOOD STEWARDS OF THE EARTH AND MANY REMAIN OPPOSED TO THE TRADITION.
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Media Landscape
See how news outlets across the political spectrum are covering this story. Learn moreBias Summary
- The Chicago River has been dyed bright green to mark the city's St. Patrick's Day celebrations.
- Members of the Chicago Journeymen Plumbers Union Local 130 initiated the dyeing tradition 63 years ago.
- The dyeing occurs right before the annual downtown St. Patrick's Day parade.
- A second St. Patrick's Day parade is scheduled for Sunday on Chicago's South Side.
- No summary available because of a lack of coverage.
- The Chicago River was dyed green on March 15, 2025, to celebrate St. Patrick's Day, marking a 70-year tradition.
- Members of the Chicago Journeymen Plumbers Union Local 130 sprayed dye into the river, attracting thousands of spectators.
- The dyeing event began at approximately 10 a.m. Local time, using a nontoxic, secret formula.
- St. Patrick's Day events attract people of all backgrounds, celebrating Irish culture and the contributions of St. Patrick, who brought Christianity to Ireland.
Bias Comparison
Bias Distribution
Left
Right
Untracked Bias
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