Calculate the maximum sustainable amount of fish that can be caught/harvested, limit the world-wide production to that and let the market handle the rest.

Those limits would have to be enforced useing armed boats however - else I don't know if there would be an effective way for those huge swimming fish factories to just catch everything until the ecosystem collapses.

Then you'd also need a mechanism so make it socially balanced, or we'd pretty much ban low-incoming fishing societies from consuming their own product.

My humble opinion, not an expert.