I have a complete different take than all the answers above...

It has everything to do with Omega 3's, 6's, etc with scents, I'll say that much.......

But when it comes to steelhead, many will say scent doesn't matter.. I beg to differ. They're just as much a whore as salmon are but steelhead tend to be more active feeders throughout their freshwater life whereas salmon become way more finicky especially as they reach spawning maturity...

Steelhead:
1. Presentation
2. Smell
3. Color

Salmon:
1. Smell
2. Presentation
3. Color

I've had all too many times in my life fishing where a seperate egg cure or scent will turn a steelhead bite back on after you've taken the over achiever's out of the hole... Persistence, patience and lot of wasted time in my life has helped me define those answers...

Keith