The effects of overfishing are still reversible, that is, if we act now and act strongly.
When fish stocks decline and and fisheries become commercially unviable 1 the damaged stock gets some rest and generally struggles along on a pathetic level compared to it's pre-fishing level, but doesn't go biologically extinct 2. A damaged system is struggling and shifting, but can still be active (e.g. filled with jellyfish instead of cod).
If we want to we can reverse most of the destruction. In some situations it might only take a decade, in other situations it might take many centuries. Yet in the end we can have productive and healthy oceans again as is shown in many examples around the world. We do however need to act on it now, before we cross the point of no return.
Every long-term successful and sustainable fishery, near-shore or high-seas, needs to be managed according to some basic ground rules: