Otters act as ecological early warning systems
Just as sea otters are linked to the health of a kelp forest, so too are river otters linked to the health of riparian habitats. River otters need clean water and an abundance of prey, and so they are sensitive to disruptions in river systems.
The River Otter Ecology Project puts it like this: "Because they are top predators [at the pinnacle their food chain], and utilize both water, and land environments, otters are ideal environmental indicators — canaries in the mine, so-to-speak. Any contaminants that enter their food web become more concentrated with each step, resulting in the highest concentration at the top. They are among the first species to disappear from a polluted watershed. This applies to every species of otter, in every nation."
It may be an increase in pollutants, or a loss of prey due to habitat destruction, or habitat fragmentation along waterways, but one thing is for sure: when otters disappear, we know right away that something is off kilter in the ecosystem.