In a medium pot, boil 4 cups water with the juice of the lemons, placing one lemon half in the water. Reduce heat to a simmering boil. Poach salmon filet in water by cooking, covered, for 7-10 minutes, or until done. Remove with spatula. Set aside to cool.
When salmon is cool, combine with celery, onion, mayonnaise, cayenne, tarragon, garlic, dry mustard, salt and black pepper in large bowl and mix well. Add dry potatoes slowly, while mixing until everything is moist.
Form patties and cook in preheated canola oil in a large skillet on medium-high heat until both sides are brown (3 minutes per side). Serve hot.