That's a very complex question. I'm not a cod scientist. Dr. Worm may have a better answer for that.
What I can say is that fishing was a major cause in the collapse, but maybe not the only cause. The recovery was limited by environmental conditions. There have been a number of years where the ocean was not in the same favourable condition for cod, which would prevent that. From what I understand of the problem, there are a number of causes, including environment, bycatch in some fisheries, and some prey that cod were reliant on that were also not recovering in the ecosystem.
The important lesson it taught us is that when we stop putting pressure on the system, it doesn't come back right away. I think that's the important lesson we have to see. When we stress an ecosystem, we cannot expect it to come back to it's natural state in five or 10 years. Sometimes we just screw it up and sometimes it may take decades to come back. That's why we want to be very careful with what we're doing currently across the country, since we know that the ecosystem is very fragile.