A few years ago, before I'd bought a smoker or had any experience at all, I came back from a Florida fishing trip with a big load of mackerel. We took it to a local BBQ joint, I prepped it and provided the wood and they did the smoking. It was fantastic and I ate most of it myself.
This recipe came from online somewhere (sorry that I didn't save where). I cut all the fish up into moderately sized pieces, removed any dark flesh and followed the recipe to a T.
For the brine:
1 gallon of warm water
2 cups sea salt
¾ cup brown sugar
¼ cup granulated white sugar
½ cup fresh squeezed lemon juice
4 cloves of garlic (roughly chopped)
1 Spanish onion (roughly chopped)
3 tablespoons coarse black pepper
1 tablespoon red pepper flakes
1 teaspoon ground ginger
1 teaspoon ground allspice
In a food safe plastic container, combine all of the above listed ingredients and whisk. Whisk until all the salt and sugar has dissolved and the water gets to be around room temperature.
Place the fish fillets in the brining solution and make sure that they lay flat, and are completely covered. I use a couple of plates on top of the fish to hold them under the water properly. Cover and place in the fridge for 5 to 6 hours. This brining solution and time is for two large fillets from a kingfish in the 30-pound range. If you have a smaller kingfish (say 15 pounds or so) then drop the brining time by an hour or so.
During the brining time check the fillets once or twice to make sure that they stay covered by the brine.
After you have brined the fish properly you need to set up a rack and a fan in your kitchen area and place the fillets on the rack with the fan running for at least an hour, or dry to the touch. This will help develop the pellicle on the fish and will keep the fish from drying out when smoked.
I gave them some cherry wood to use and I don't know what their methodology was, but here's what the recipe called for:
The key here is to keep the temperature between 160 to 180 degrees for the entire smoking period. Smoking two fairly large fillets will take about three hours at this temperature. Don’t open the lid! Keep an eye on the temperature gauge and adjust your vents as needed. When you get around the two and a half hour mark, check it once. You should see a beautiful, colorful smoked fish that is still moist and very aromatic. If you like smoked fish that is still very moist then pull it now. If you like it a little drier because you want to make a dip or just like it drier, then let it go for the full three hours.
If I had a bunch of similar fish to smoke, I'd do it like this -- but I'd do it myself this time.