This method will work on any small oily fish, so any of the bonito or bonito-like fish discussed here, as well as Spanish mackerel, king mackerel, really larger regular mackerel, and small skipjack or blackfin tuna.
Dissolve the salt an sugar in the water. Soak the bonito fillets in this brine, in the fridge, for 4 to 12 hours; 8 hours is my sweet spot.
Remove the fish from the brine and set on a rack over a tray, skin side down. Let these rest, uncovered, in the fridge up to a day or two, but at least 1 hour. You can also let them rest at cool room temperature in a breezy place, like under a ceiling fan or near an open window, for an hour or two.
Get your smoker ready. I prefer alder wood here, but any fruit wood is nice, too, apple and cherry especially. Slick the skins of the bonito with some oil and lay them on the smoker grates. Smoke over low heat, as low as you can go, for up to 3 hours, or until they are nicely bronzed. I set my Traeger at 165F for this.
Remove the fish from the smoker and let cool to room temperature before vacuum sealing and storing. They will keep in the fridge a week, and up to a year in the freezer. Or, eat them hot!