Yes you have to, but only on any profits you make, so probably not for most people, or very little.
It will be very hard to keep a track of, as each time you mine you will have gained capital, what is the price of that capital and what percentage of that is profit? I guess you would work it out purely from a mining - electricity cost, then deduct the costs of computers, miners, coolers etc that you bought later.
There is software out there that can be used to help traders work out their tax to be paid, I guess that should work for mining too, luckily (kind of) the price of bitcoin is so low that most people are losing money and are tax exempt. Don't forget to write that off next year as a capital loss though!