There is no date to be sure that the white population migrate all the way to SA...more research is needed in that area.
There are more realistic theories than cage diving, such as present tuna fishing methods for one ie netting whole migrating populations of young tuna then bringing them inshore to be feed up to make the best $$$!
Just think of the volume of feed that is dropped into these nets every week compared to the small amount of food involved with cage diving!?
Then take into account the removal of the whites natural food source....then when they manage to track down where their missing food (-they have more of a right to it than us) they find it relatively close to shore....where we live....then we think there is a shark problem