In a 4-quart container, stir together salt, maple syrup and water until the salt is dissolved. Add the fillets so they are submerged. Cover and refrigerate for 3 hours.
Remove fillets from the brine, rinse thoroughly, pat dry and place, skin side down, on a cooling rack over a sheet pan to catch any drips. Continue to dry the fish in the refrigerator for at least 24 hours or until the skin is tacky to the touch.
Bring a smoker to 160°F. Put the fish onto the smoking racks, skin side down, about ½ inch apart. Smoke until the fish is cooked through and darkens in color, about 2 ½ - 3 hours. The fish can be stored in an airtight container for up to a week.