Combine the salt, sugar, sumac, and rosemary and rub it onto the fillets. Let the fillets sit in the fridge for 4–5 hours, depending on the thickness of the fillets. After the fillets have cured, rinse them clean and pat dry with a paper towel. Let the fillets sit out for 30 minutes to an hour to form what is called a pellicle. The fillets should be slightly tacky. Fire up your smoker and set the temperature to 160 degrees. If you are using a Traeger grill or other style pellet grill, the smoke setting should be good. Smoke over Alder for four hours. If you prefer your smoked fish a bit drier you can go longer.