When I'm not flyfishing for the hucho, I use the calm winter days to fill up my fly boxes.
With this fly in particular.
I found this fly, hanging in a branch at the Rena river in Norway, it was around a size 14 and I immediately tied it onto my leader. The fluffy CDC fibers give the fly a pretty good visibility for the angler.
I landed several large grayling that afternoon and list a strong brown trout in the night who was sipping this fly in directly unter the river bank.
Apart from the Rena river, this fly all works great at our Bavarian streams as well.
Concerning the tail fibers, I found out that green is most successful at my waters, but this can vary depending on your river.