I smoke several hundred pounds of king salmon every year and have it dialed in pretty damn good.
In my opinion, because of the oil content, larger kings are best for the smoker. Kings under 8# and cohos are better cooked than smoked. Trout and smaller salmon certainly can be smoked but the best will be 8# or bigger kings. That said, because they are firmer and less oil content, those smaller salmon or trout are excellent smoked and made into a cream cheese spread.
Start by cutting fillets into portion size pieces. 1 1/2”-2” wide. The belly portion is by far the best, so I try to keep as much of it as possible intact when filleting. A lot of times I will not smoke the tail ends and just use those for normal cooking, they tend to be drier and not bad but not great smoked. Rinse and dry. Place on a sheet pan with at least a 1” rim. Do not do this on a flat cookie sheet or you will have a giant mess. Sprinkle each piece with garlic powder and a healthy dose of black pepper. Other seasoning will also work if wanted. Lemon pepper, Cajun or something along those lines.
Mix 2 cups packed dark brown sugar with 3/4 cups of kosher salt. Add approximately one heaping TBSP to each portion and pack down so it’s more than and 1/8” but not more than 1/4” thick. Cover with plastic wrap and place in fridge for a minimum of 24 hours and up to 36 hours for thicker pieces.
Quickly rinse each piece from excess salt/sugar. Dry with paper towels and place back on sheet pan. If needed or wanted, add more black pepper or seasoning. I then return it to the fridge for 8-12 hours to let the pellicle form. Basically just let the surface become tacky. I know it can be done at room temp quicker but I personally prefer keeping it cold in the fridge.
Smoke at a max temp of 175*, adding a handful of a fruit wood, aspen or hickory chips every 20-30 minutes, I do not recommend mesquite, it has a harsh smoke flavor. If your grill doesn’t have a thermometer, if you can place your hand flat on the lid for a count of 5, that’s about the right temp. Also after about a half hour, sometimes a liquid comes to the surface, blot that dry. After about 90 minutes when the surface is dry, add a small sprinkle of brown sugar. Smoke for about 3-4 hours total. The best I can tell you is experience will teach you when it’s done. My experience has been that if it looks done while on the smoker, it’s too done and will be somewhat dry. Pull it when you think it’s about 30 minutes short of looking done, after it cools and the surface is dry, then it will look done.
All I ever use to smoke is a regular old charcoal Weber kettle grill. I have the larger 26” and start with 8-10 briquettes all shoved to one side. I add charcoal as needed to maintain temp. I prefer a grill over an automatic smoker because I feel that I have more control over the amount of smoke, which I prefer a heavier smoke.
After removing from the smoker, place back on the sheet pan and place back in the fridge uncovered overnight or until completely cool. Bag or cover once completely cooled. It is ready to eat at that point but it is much better after 2-3 days in the fridge. While smoking the surface dries out some. After being covered for a couple days, the surface rehydrates and the smoke flavor permeates the rest of the fish. If freezing, I would wait until that point. But smoked fish will keep perfectly well for 2-3 weeks in the fridge.
Freezing does nothing good for fish ever, so I always try avoid it. If I do freeze it, I wrap each piece in a couple wraps of plastic wrap and then vacuum seal. I do not ever trust a vacuum bag and wrap all meat or fish in plastic wrap before. That way the plastic wrap gets frozen tight to the meat and if the vacuum fails, at least there’s a second barrier from air getting to the meat/fish.
When serving smoked fish, I remove the skin and grey stuff under the skin. Then slice it into bite size pieces and let it get to room temperature. The flavor and texture improves greatly when it’s not straight out of the fridge.
One other thing that I often see overlooked with the larger trout and salmon is the cheeks. They are the filet mignon of fish. Carving them out is well worth the effort. They are excellent smoked as above but need less curing and smoking time. Or they are also excellent blackened. Similar texture as a scallop.
And another thing is that if you bleed your fish when first caught, it has a much better flavor. Plus it keeps your cooler cleaner and less mess when filleting. I cut the gills and drag them along side the boat for 10 minutes, then immediately on ice.