SPIRIT RIVER UV2 YAK

Incredibly long, natural hair possessing a translucent sheen much like polar bear. Has no equal as a natural material for large saltwater patterns such as billfish flies. An affordable alternative to polar bear for steelhead/ salmon flies and streamers. BILL
Here is what Drew Chicone said about it in a chapter from his book. You can check his web site to. saltyflytying.com You will learn a lot from Drew. He is not only a great angler but excellent tier.
Hi Bill,
In my opinion Yak hair is the best material there is for large baitfish patterns … Perfect for just about everything in saltwater. I have published several Yak hair patterns and have been writing about it’s attributes for years now. Here’s a chapter from my book and a few pic’s and blurbs on why I love the stuff;
It’s no secret that my favorite material to use for tying large baitfish patterns is yak hair.
When you are using a material without tips like yak hair or a synthetic like Fiber, you can cut a section so you have two flat ends. The section will contain multiple fibers or hairs that are the same length. You can make the section longer by pulling a few hairs or fibers from the ends. This will make the entire section longer and taper the ends. 
The pattern uses yak hair mostly for its translucent qualities, but also because its length and wiry texture allows you to create larger flies. The natural fibers are great for blending because the strands of hair can be counted easily. This allows you to create flies that are more uniform in color and fullness. The coarse hair becomes supple when wet and has a lifelike quality when stripped. 
I like yak hair because it is just a touch thicker and stiffer than most synthetics when it is dry, but quickly softens when it saturates with water. Yak hair’s long natural fibers are somewhat translucent, and the lighter colors react to UV light well. It comes in a variety of colors, and most importantly it is easy to separate individual fibers from the bunch and they blend well.