I was re-reading Chapter 609 today, but (http://mangastream.com/read/one_piece/39543275/18) these words by Keimi made me think a little.
Fishman Island on the whole seems rather peaceful, much different then the war-torn hellhole I thought it was going to be after Whitebeard died. Although there hasn't been any menntion of another Yonkou owning the Island, I kind of wonder if the Merman and Fishman rulers might be paying off groups like the New Fishman Pirates & maybe van der Decken's crew to try and kill off as much people coming to the Island, so they can handle the ones that do survive and try to stir the shitpot with more ease.
I mean, the three Princes seem like decent people, but it is kind of understandable if none of the Yonkou took over and/or Marco and Co got defeated by Blackbeard and he doesn't care about them, and you don't want scum like Caribou killing them off or Shabondy slavers coming back.
Again, probably way too early in, but you never know.