Smoking fish is fairly easy and very rewarding. You can break it down into three steps, if you like. In order they are: Brining; Forming a pellicle; and, Smoking.
Brining helps prevent the fish from drying out during the smoking process. It also does double duty as a way of partly seasoning the fish. A typical fish brine is mostly salt, sugar and seasonings. "Cures" aren't very common, and neither is a lot of acid. Because of the cell structure, you're usually better off brining fish in a stronger brine for a shorter period, than a weaker brine for a longer -- as you might use for chicken or pork.
There are thousands (if not more) recipes for pre-smoking fish brine on the internet. Just use the obvious terms and google will find hundreds of them for you. In the meantime, here's something very basic you might want to use the first couple of times.
A BASIC RECIPE AND TECHNIQUE FOR SMOKING FISH
Brine (recipe follows)
Dry rub, if desired.
6 cloves garlic
1 gal water
2 cups table salt (non-iodized)
1-1/4 cup brown sugar, or 1 cup sugar and 1/4 cup molasses
2 tsp white pepper
Prepare the brine by cutting the onion into eight pieces. Smash the garlic cloves.
Combine the onion, garlic, water, salt, sugar, and pepper in a kettle, and heat to the point where the salt and sugar completely dissolve, and to begin the process of infusing the brine with the other flavors. More simply, bring to a boil, reduce to a simmer, and hold at the simmer for five to ten minutes. While the brine heats, slice the lemon into 5 or 6 slices.
Remove the brine from the heat, and add the lemon slices. Allow to cool completely. The cooling time will complete the infusion.
Meanwhile cut the fish into serving size fillets. (In your case, you'll want to size them for convenient handling -- 6 oz ought to be about right -- you'll be breaking them down later for the pie). It's important to keep your pieces and/or portions fairly consistently sized.
Put the fish in a glass, plastic or ceramic container, and cover with brine so it is completely immersed. Alternatively, you can use a large plastic bag. (You can use stainless steel, but only if you absolutely
must. Under no circumstances use any other type of metal.) You may include some or all of the onions, garlic and lemon slices if you wish. Most of their flavor is already in the brine, but "it couldn't hurt."
Cover the fish or seal the bag and brine in the refrigerator. Alternatively, if the fish is very well chilled it may be brined on the counter in a room temperature solution for about three hours. Alternatively, you may use some ice cubes in the brine so that it's cold.Tip: Make the brine short a couple of ice cube trays worth of water. After it's cooled to room temp, add the cubes.
4 oz pieces will brine in about 45 minutes. 6 oz pieces will take about an hour. Note that brining time is controlled by the weight of the individual piece and not by the total weight of the fish in the solution.Tip: Add 25% to 33% more time for oily fish (such as the types you suggested).
When the brining period is over, you need to dry the fish and form a pellicle. Remove the fish and set it on a rack elevated above a catch pan (to catch brine which drips off). Do not rinse the fish before drying, and do not blot with a paper towel. Set the pan in a cool palce, and arrange a fan so to blow over the fish to speed drying. Every fifteen minutes, turn and rearrange the pieces so they dry more or less evenly. After about an hour the dried brine should form a "pellicle." (The pellicle is a smooth, slightly tacky surface, which may appear clear but is usually slightly cloudy. It helps "hold" the smoke and any seasoning you may use to the fish, and will form a nice crust on the finished product.) And, at this point the fish may be held in the refrigerator for a convenient smoking time.
If you have the space, and can tolerate, unwrapped raw fish in your refrigerator, you may put the fish on its rack in the refrigerator -- uncovered. The fish will dry more slowly but very thoroughly. Figure a couple of hours to the pellicle.
Prepare your smoker to run at around 200F (cook chamber temp) or a bit lower. If you can get your smoker to produce smoke at a lower temp, you'll have to adjust smoking times accordingly.
Season the fish if you like. Just remember that the fish is already salty.
When the smoker is fully prepped, running at a stable temperature, and making smoke you may add your fish. Ideally, you'll keep both the cook and fire chambers closed during the entire cooking period. If it isn't clear, there's no need to turn or rotate during the cooking period -- unless your pit runs very unevenly. At roughly 200F, pieces in the 4 oz - 8 oz range will take between 1 hour and 1-1/2 hours. The touch test is better than a probe. The fish should be fairly firm and push back. It should not feel mushy (too rare), and it should not be very flakey (too done). If you must use a probe, you're looking for an internal in the 135F to 145F range.
Hope this helps,