It's got to be black goat, don't you dare put bucktail under it to stop it wrapping! fish don't know what goats are.
Only 3 strands of crystal flash!
Aye? Yesterday, 3 fish were caught on 12cm rapalas in the 2 pools below the one you're fishing on Saturday..
Substituting materials is a pretty standard part of my tying, and especially when it comes to salmon flies it's a necessary part-it'd be mad to use nice long 12cm hair on a 14! But sometimes I, and others, make changes for other reasons. Sometimes it's to prevent fouling, make the fly swim better, increase durability or make a version that fishes differently in some way.
There are few perfect flies, we can always tweak and adjust patterns. Although the old adage that perfect is the enemy of good often rings true and we can "improve" our way out of a good fly. A new set of eyes can often spot things that the inventor may have missed. Take the Clouser minnow, probably the most successful fly ever, it might be near perfect, but a lot people prefer Lefty's Version and Bob Popovic's Jiggy might actually be a better design than either of them. All arrived at by changing and adapting what existed before. Perhaps salmon fishing would have developed more if there wasn't such a blind faith in the past.