How do river otters function regarding water quality assessment in their habitat?

Answer

They act as bioindicators because they are sensitive to persistent toxins.

Otters are described as bioindicators because their presence is a good sign that the water is relatively clean, as they are sensitive to persistent toxins that accumulate high on the food chain.

How do river otters function regarding water quality assessment in their habitat?
habitatwildlifeUSAdistributionriver otter