In uni we had classes about systems modeling and fishing population was one of the examples studied. Overfishing can cause these phenomenons not so muche because you actually fish 1bln unit and are 1bln down, but because even a modest amount of overfishing past a critical point puts your population on a crashing decrease due to reduced reprodution. So if under normal circumstances your next generation would be lets say 5bln crabs, because the reduced population makes it much (non linearly) harder to repro now you only have 0.5bln. And over a couple years everything is dried up even thought you maybe "only" overfished 200mln units.