The numbers look pretty insane, you can raise many tons of fish in relatively small volumes of water (several hundred kg of fish per year per cubic meter). You just gotta build the ponds/tanks/cages, and the infrastructure to filter the water, supply the oxygen and deliver the feed.
Barring some planetary-scale cataclysm, most of Europe and the US are at no real risk of starving. There are other countries that are at a real risk, but the map doesn't make a clear distinction between "red as a matter of convenience" and "red because they physically can't do it".
Then we will lack whatever was produced on the place where you those new ponds with huge amount of fish.
Most of the richer countries/trade unions have a large meat surplus that could be easily shifted to something else, too.
Obviously nations do have limited surface area and creating new agricultural areas for them would be to the detriment of forests and "nature"