Should I be concerned?

Mallards will and do eat small fish. They break off the fins and remove enough
scales to kill the larger fish. If the pond is deep enough the ducks don't have
a good chance grabbing fish. Scales are almost all calcium and you may notice
the female with more fish in her mouth.
I was hit early in the season when the water was cold and the fish slow. The
tails were chewed off ranchu to comets, it didn't matter what size pond. The
deeper the safer; 24" plus.
Last season when the water was lowered for cleaning, ducks ate every top
quality fry and killed the adults. It wasn't until I saw the female with a huge
ryukin in her mouth that I solved the mystery. The ducks were so tame I could
get within reach of their necks.