Zach:
You ask an interesting question that does not have a simple answer. "Ducks" can encompass a lot of birds that are not strictly ducks and the question of what will eat a fish is also a question of size of prey. So, to try to answer your question, Puddle ducks such as Mallards, Teal, Pintails etc are not fish eaters per se, but will eat fry opportunistically, that is if it happens to be caught in their rooting around they are happy to have it. As such, they are not strictly fish eaters but will cause a certain amount of predation. Diving ducks such as Mergansers on the other hand are pretty good at catching fish - small fish, minnows and fry. The real problems are Cormorants, which some people will lump in with the ducks. They are very efficient predators and seaching this site will yield lots of info on how much damage they do and what people do to deal with them. They feed heavily at the top of the chain and will take fish up to 15" long by many reports. If you think you are having a problem, can you identify what you are worried about?
Jim
