As salmon anglers, we often get too preoccupied with fly pattern. We always ponder which fly to use and what colour this should be. Of course, fly pattern is important but in real terms do we place too much emphasis on this? The old adage that the fly catches the angler and not the fish can often ring true. 
Salmon fly size as well as the depth it is fishing is just as important if not more important than the pattern itself. Often the size of the fly and the depth at which it fishes go hand in hand.
During the early spring months when salmon fishing in Scotland often the water temperature is low and river levels high. In such conditions, many of the fish will be lying close to the riverbed. It is therefore important for the fly to get well down in the water column. In cold conditions, salmon prefer to conserve their energy and so are not keen to move quickly over long distances. It is therefore imperative that the fly is moving close to where the fish are lying. Also when the water is higher a bigger fly is used. In such conditions, a weighted tube fly coupled with a sink tip or intermediate line would be perfect for the job.
During the summer months, the water is usually much lower and warmer. In such conditions, the fish are usually lying higher up in the water column and more active. When it is warmer salmon can be happy to chase a fast-moving fly long distances before taking it. If the water is low and clear, then often smaller dressed flies work better. Having a subtle approach with lightly dressed doubles and trebles can work well coupled with floating lines when salmon fishing in Scotland during the summer months.
In autumn much depends on water and weather conditions. The weather can be very variable during the September and October in Scotland. During some years a long hot summer can stretch well into September. If this is the case, and the water is low, smaller flies and surface tactics can often work well. Conversely, if the water is high and cold then the larger flies fished that bit deeper (like in the spring) can produce the best results.
The depth the fly fishes, as well as the size of the fly, are just as important as the fly pattern itself. As a salmon angler, it is important to consider these parameters when it comes to changing flies and altering tactics.