Study Finds Chicago River Dyeing Does Not Disrupt Fish Behavior

Researchers tracking aquatic life observe no agitation or harm from the vegetable-based dye used in the annual St. Patrick’s Day tradition.

A study led by the Shedd Aquarium and partners tracked over 80 fish species in the Chicago River to examine the effects of the St. Patrick’s Day dyeing tradition.
The vegetable-based dye, used since 1962, showed no toxic effects or behavioral changes in fish during or after the dyeing process, according to researchers.
Environmental groups remain concerned about the symbolic message of introducing foreign substances into the river, despite its improved health and biodiversity.
The Chicago River now supports over 80 fish species, up from fewer than 10 in the 1970s, highlighting significant ecological recovery.
Advocates argue for a reevaluation of the dyeing tradition to promote better environmental stewardship of the waterway.