I've seen a video of cormorants eating some pretty darn big fish which causes my concern.
You've seen cormorants on your pond?
I've been lucky; haven't seen one yet. Did have two ospreys show up one morning at daybreak and just stand on my pier as if they were resting after a long flight. They soon took off again. I didn't observe them after any fish.
I do dye my big pond to reduce visibility (otherwise you can see all the way to the bottom) and feel this helps dissuade diving birds.