Watching an otter is a very difficult task. They are shy mammals with twilight or night activity.

But they are visitors to some biological pools in Southwest Portugal, as this photo shows, which shows otter waste on the shore.
Otters are clean animals, leaving their feces on the bank and not in the water, as they serve as a mark of territory. Other otters passing by easily realize that this biological pool already has an owner, another otter!

The obvious presence of the otter in the biological pool is yet another example that these places, designed and built to take a bath in natural water, are true nature-based solutions because they get this rare mammal on European level to accept the biological pool as its new habitat, to hunt frogs or simply to take a dip on a hot day, a task that otters enjoys as much as the true owners of the biological pool.