About 40 years ago my bud got me started on smoking salmon. Have used many different smokers and a myriad of brines since then. I have settled on a method that is simple and produces a superior product every time, no fail.
Smoked Salmon
Brine: I use a dry brine consisting of dark brown sugar, canning salt and garlic. The ratio is: 4 cups of dark brown sugar, 1 cup of pickling salt and 10-15 cloves of garlic (you can substitute dry garlic products, 1 tsp per clove). This makes enough brine for about 10-12# of fillets. That's about 6 sockeye or coho fillets.
Salmon: I used to catch my own, but the MFWICs turned a world class fishery here in Washington into a mere shadow of what it used to be. It is actually cheaper now for me to buy it. I wait til sockeye or coho go on sale, say $7.99 a pound. Steelhead works too as does Chinook, but I'd rather save those for the grill. Pretty much all salmonoids work well including trout and char (brookies, lake trout, etc) as long as they aren't oily like Pink salmon.
Blend the brine ingredients. Use a non metallic container. Put a layer of brine on the bottom of container. Add fish, skin side down. Cover that layer in brine. Add new layer of fish, flesh side down. Add layer if brine. layer flesh to flesh, skin to skin. Refrigerate over night. The brine will become liquid.
In the morning, rinse off the fillets/chunks with cold water, just enough to remove any chunks or blobs of remaining brine. Dab dry with paper towels. Place fish on racks, skin side down and sprinkle with fresh ground pepper. Let the fish set at room temp till the pellicle (a translucent skin like layer) forms, 4 to 6 hours or so.
I now use a Bradley smoker with auto feed and temp control. Use what you have. Preheat smoker to 130*. Add racks of fish. Get the smoke going. Use Alder and Apple. I use alder for ~ 1 1/2 hours then apple for ~1 1/2 hours. Go another 2 hours without smoke for a total of ~ 5 hours. I then run the temp up to ~ 160* to kill any remaining bacteria. About one more hour. I let the smoker cool off overnight and in the morning, I have a bunch of salmon candy.