Shred the codfish and soak for 30 minutes in cold water. Drain. Boil fish with potatoes in water until potatoes are tender. Drain and mash the fish and potato mixture together; add butter and beat until light. Sir in beaten egg yolks and ketchup. Fold in stiffly beaten egg whites and turn into a greased 1 1/2 quart casserole dish. Place in a pan of hot water and bake at 375F for 20 minutes. Serves 4 to 6.