The sharks will leave on their own when the time comes.

Even if there is no food and the water temperature drops,
there is no way they will stay until they die.
I suspect there might be some kind of electronic device
used to lure the sharks,
but the water temperature won't be warm in October,
and since there is no food in this area,
they will leave on their own if they get hungry or cold.
The problem is that if, by some chance, they die off,
the city of Busan and the public officials will take the full brunt of the responsibility,
so I can understand why they are trying to send them away quickly.