Boil a small salmon in salted and acidulated water. Take up carefully and reduce the liquid by rapid boiling to two cupfuls. Cook together two tablespoonfuls each of butter and flour, add the reduced liquid, and cook until thick, stirring constantly. Take from the fire, add two tablespoonfuls of chopped capers, one tablespoonful of chopped parsley, the juice of a lemon, and one tablespoonful of butter. Pour over the fish and serve.