I caught my first Ultra Beast today -- it was great that I had a Beast Ball in stock, otherwise I would've missed it.
I read that you don't need to lower its HP since using a Beast/Vortex Ball on a UB has a 100% catch rate.
My question is: Do I need to have a Beast/Vortex Ball in my inventory first before I encounter any UBs, or not?
It just seemed convenient that I had Beast Balls when I encountered my first UB and didn't see any of them before I had any Beast Balls.