The only problems I know of with the smaller hooded mergansers is they will eat small fish and they will lay eggs in or take over wood duck houses. So I chase them away for those two reasons. Hooded mergs will sometimes just lay eggs in a wood duck nest and let the female wood duck take care of them. I've also seen them chase the wood ducks away. I've checked some of my wood duck houses and if I don't chase them away the nests will sometimes contain over half merganser eggs.

Gotta get back to fishin!