Enforcement is a solved problem - ships can be tracked by various means, ports where catches can be offloaded can be audited, crews can be arrested and ships impounded when in port. No one is going to overfish if they can't economically get their catch to market and get paid for their troubles. The issue is getting everyone to enforce the policies consistently - the leaders of different jurisdictions may not see eye to eye on what level of protection is optimal, and the people actually doing the enforcement may be willing to turn a blind eye to violations depending on circumstances.