Something to consider: a resource which is being exploited is one which people will want to conserve. A resource which is not being exploited is one which there is little impetus to conserve.
My point here is that if wild fish are not being harvested, nations and individuals will have little reason to protect them from the numerous other threats they face (pollution, habitat destruction, etc.). Whereas if they are being harvested, then sustainable harvest and protection from other threats is in the interest of all parties involved.
The trouble now is short-sightedness by fishermen and governments, and ineffectiveness of many governments. They harvest unsustainably because they aren't thinking about ten years or twenty years down the line. I'm speaking here primarily of the developing countries, but this applies to some wealthy countries as well.
So, what's the answer? There are three tacks, really, which I think should all be used:
First, the approach from above: tighter international regulation. Most of the wealthy nations are major importers of seafood; this gives them leverage to demand that sustainable harvest techniques be implemented and enforced in other nations.
Second, the lateral approach: education of commercial fishermen, policy-makers, and others about the consequences of unsustainable fishing. This has worked well here in the US; there was a lot of initial resistance to the heavy regulation of commercial fishing, but the fishermen (and lawmakers from states where fishing is important) have come around and largely realize the importance of sustainable harvest and work with the regulatory agencies and fisheries biologists instead of constantly fighting them.
Thirdly and most importantly, the approach from below: everything rests on the consumers. Vote with your dollars. Make it clear that you will only purchase fish that is sustainably harvested. This provides a clear impetus to nations and fishermen to implement those techniques.
Just some of my thoughts!