It's not uncommon for me to find little mussel shells on occasion, small ones that would sit on a dime. I have not had any signs of otters, ever. Once, I had a bit of an outbreak of shells that, I assume, floated to the surface on mats of FA. My pond is quite a ways from otter habitat, but if I was closer to the constant running rivers/creeks...I would definitely be on the lookout for their presence.
Raccoons will also seek the mussels and leave the empties at the banks.
I am with you in thinking that their presence is a good sign of fertility and health, and believe that RES eat several types of mussels given that they are soft shelled enough to crack.
