Applying a brine to salmon before smoking it removes moisture, improves flavor and preserves the fish against spoilage, according to Smoker Cooking. It's necessary to remove moisture from the fish to smoke it properly.
The basic ingredients of the brine are salt, sugar and water. Salt draws the moisture out of the salmon and changes the shape of the protein so the fish can retain moisture when the smoking process is finished. Sugar also improves the moistness of the finished salmon and prevents the growth of bacteria. Seasonings such as soy sauce or hot peppers can be added to the brine for enhanced flavor.