This is the king of the insects on most western rivers, it's BIG. The nymphs do live 2-4 years in the water so it's not a bad plan to use nymphs that are #10-#8 for general prospecting in streams that have salmon fly nymphs. The fully mature insects are a solid #4. If you find yourself dealing with selective trout use various patterns to find out what they like. With such large flies the trout will become wise to popular overfished patterns.