The illegal drug’s main byproduct, benzoylecgonine, caused more robust effects than cocaine itself. Wastewater treatment plants often don't fully process such metabolites, so they are frequently found in bodies of water at higher concentrations than their parent drugs
I thought this was dumb at first, but then realised these researchers must have procured some lab-grade cocaine for testing purposes.
Imagine how far the salmon would have swam id they’d been given all the cocaine they said they did.