Hans, I have swum with the fish. I used to scuba dive and I know well of the color loss at depth and water turbidity as relates to color in open and fresh (lake & river)water. I didn't specifically take herl down with me but was fascinated by the environments.

Bottom line for me is that all of this means very little to us as fishers. Herled flies seem to do very well compared to flies sans herl but as is the case with much of the discussions, until fish can talk, we will never know why they take one fly and not the other. It really is only slightly of interest to me. I like the colors of herl, they work and that is enough for me.

Happy Trails!
Ronn