Place the backbone and any scraps of flesh from a whole salmon in a saucepan with water to cover. Bring to a boil and cook for a couple of minutes, until the flesh is opaque. Drain; remove cooked flesh from bones and let cool. Mash cooked fish with a fork and moisten with a little mayonnaise, about ½ teaspoon horseradish for each ½ cup fish, and chopped parsley. Serve with crackers.