Researchers at the University of Waterloo found that small fish in Canadian rivers are accumulating fentanyl, methadone, and antidepressants from treated wastewater flowing into their habitat — the first documented evidence of this pattern in wild Canadian fish. Current wastewater treatment plants don't filter pharmaceuticals, so aquatic organisms downstream absorb these drugs, and prior research suggests this can alter fish behavior, reproduction, and development. The study's significance is that it proves the drugs are actually bioaccumulating in real fish populations, not just present in water samples, raising questions about long-term ecological impacts and whether wastewater treatment standards need updating to address this emerging contaminant class.