Those fishing in the waters around the two Basel cantons are being advised not to eat the fish more than once a month.
The health departments of both cantons say the fish are contaminated by PFAS chemicals – which can be found in many household products.
Although their use is now strictly regulated and some are already illegal in Switzerland they linger in pollutants and landfill sites.
Experts say PFAS chemicals can cause birth defects, damage liver and kidneys and can limit the effectiveness of vaccinations.
Amateur anglers are being told to limit their intake of locally caught fish.
But fish bought in shops are subject to health and safety laws and are safe.
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