When I can start mining DarkFish?
You can already mine it, pool list is available.
Has mainnet been launched yet?
Yes.
The distributional rewards have planned to up and down at different phase, why is that?
Rewards are slowly growing up to 18 so that everyone has the time to catch up - first miners do not have any significant advantage.
Current reward is 14 - 7 for miners and 7 for masternodes.