Now that we have seen a shark using a shirt together with the Kraken (and the chapter has shown that, contrary to what the spoiler said, the shark hadn't been eaten by the Kraken), it seems like now it may be worth to post my theory about the Kraken.
I think that the Fishman put him there to block the path most used by pirates to reach the sea bottom (and therefore reach Fishman Island), and this was the way they found to stop pirates from reaching their island by this side of the Grand Line (there may be some kind of protection similar in the other side of the red line), and this was done to try and protect their island from pirates, since they are now without the protection that they had thanks to Whitebeard's name.
I suspected this could be the case since last week, but now that we saw that the Kraken was accompanied by a shark using a shirt (which could be a shark from Fishman Island sent there with the Kraken to stand guard protecting that place, and to give orders to the Kraken), and that the shark even tried to wake him up after Luffy knocked him down, my theory seems to have become more likely to be true.
On another matter, I wonder if Cariboo will use this opportunity to attack the crew while the monster trio is lost, the darkness should present several openings for him to attack. And it is worth to mention that he appears to be nowhere to be seem after Usopp awakes in the last page, he may already be planning something.