As Riginslinger said up there, too many people.

You cannot trust commercial fishermen to fish sustainably. Their job is to catch fish, not conserve them, and they will deplete a resource and still keep fishing it as long as better-and-better methods make it possible. When the resource collapses, they will, if they can, move to another fishery, which is probably already being fished. With similar results.
Nowadays, fishermen cross the largest oceans to fill a ship with fish. They fish Antarctic waters for the fish they have renamed the Chilean Sea Bass, the deepest water for Orange Roughy. Fish that our fathers never heard of. Because the fish they HAD heard of, are gone.