Why? If you have the money, the equipment, and the climate, what's stopping you from shifting agricultural production from one good to another on any scale you like? It's often as simple as the government saying "you know what, from now on, we're subsidizing beans instead of corn".
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".
> If you have the money, the equipment, and the climate, what's stopping you from shifting agricultural production from one good to another on any scale you like?
Then we will lack whatever was produced on the place where you those new ponds with huge amount of fish.
This argument does not work, because we are not limited by available space in total agricultural output. Just consider the Netherlands: Second largest food exporter despite the US being >200 times larger.
Most of the richer countries/trade unions have a large meat surplus that could be easily shifted to something else, too.
Fish don't care about soil quality or level ground. If anything fish ponds can benefit from height differences because that allows you to flow water through multiple ponds before pumping it back up
Obviously nations do have limited surface area and creating new agricultural areas for them would be to the detriment of forests and "nature"
There is a difference between 'can produce the food with the climate' and 'should produce the food with the climate'. Comparative advantage crops up yet. Iceland can grow bananas by magma but they are grown slower and have more expensive labor than tropical banana growing countries.
What's stopping us from shifting agricultural production, is probably the same that's stopping us from fixing climate change.