We have 5 River Otters that inhabit our lake this season. While the are fun to watch, they can be a detriment because they eat a lot of our fish. Earlier this year, one otter was seen eating 4 fish (12" rainbow trout) in one sitting. Over the course of the year, this can put a huge dent in the trout population.
Here are excerpts from an online source:
Otters also have a pivotal role in river ecosystems. They are predators, meaning they help control the populations of food species they prey upon. This affects the ecosystem as a whole, and as a result, their presence is a signal that the ecosystem is healthy.
For example they can’t do well when the waters contain pollutants including chemicals or oil. This is due to the severe damage that it creates for their coat. They have been able to adjust to new environments due to their land being taken over. However, it is important to note that these animals need adequate land to be able to survive. This includes when they are in captivity. Without it they will experience high levels of stress.