The mergansers will likely wipe them out in short order. It depends on the size/set up/number of birds/number of fish/etc. I have my fish in a separate pond.
Lucky you JJRS. I have another year till I have baby's from mine. Already have 4 white manderian duck eggs and 9 wood duck eggs. I can't wait till they hatch.
In my experience, female hooded mergansers and smew will on very rare occasions lay at 1 year old but I have never had a 1 year old male fertilize eggs.