Shred the codfish and soak 30 minutes in cold water, or according to directions on the package.
Boil fish with potatoes in salted water until potatoes are tender.
Drain and mash the fish-potato mixture; add butter and beat until light.
Stir in beaten egg yolks and catsup.
Fold in stiffly beaten egg whites, and turn into a greased 1 1/2-quart casserole.
Place in a pan of hot water and bake in a 375° oven for 20 minutes.