Think it's just down to pot luck pal to be honest,i've only had 6 boss out since game launched and none from Mickle or Ebro.
Whaaat only 6,i have 96 boss fish...ok its a bit luck too,but you must know how and where to fish dude...
Do you find that page helpful? Iv caught a few using it but I am struggling bit time with the boss fish. I just keep catching specimen instead.