Northerns are best. Put cleaned fish in cold water for 1 hour or more. Then put in jars. Add 4 cups vinegar and 5/8 cup canning salt. Put in refrigerator for 1 week. Drain. Mix and bring to a boil 1/2 box mixed pickling spices, 4 cups sugar, 4 cups vinegar. Cool before putting on fish. Put layers of onion and fish. This is ready to use in one week.