I'd be inclined to let it play out, even though mergansers eat fish. Then again, I'm not sure how many/what sizes of fish HMs can eat in a given amount of time.
I did find he following:
"The Hooded Merganser finds its prey underwater by sight. The merganser can actually change the refractive properties of its eyes to enhance its underwater vision. In addition, the nictating membrane (third eyelid) is very transparent and probably acts to protect the eye during swimming, just like a pair of goggles."
How clear is the pond? If visibility is very high, then the HMs could be efficient fish-hunting machines. I'll be interested to hear what you end up doing.
