Skin them, remove all fat, chunk them up and cook in a paella with seafood. Mergansers are SO fishy that I will not shoot them knowingly. But if you screw up and kill one, you can do this paella dish (rice, veggies, shrimp, sausage, etc) and you will not notice the fishiness too much. Most of the nastiest taste is in the skin & fat, like a coot.
I love all of God's creatures, properly prepared.