Why science fiction and not plain old fish farming?
We go through this whole process and the result is half as much fish as we started with, and it's worse for you.
Salmon farming, for example, can be incredibly efficient - Norwegian salmon farms typically get 1 kg of fish for every 1.15-1.2kg of feed. That's an amazing conversion factor.
That said, there are serious welfare concerns with fish farming. Even in Norway, which has some of the highest standards in the world, sea lice are a huge problem, and the mechanical and chemical treatments used are horrible. Another issue is the welfare of "cleaner fish", which are places in cages to eat lice from salmon - AFAIK there are still no regulations around their welfare, and mortality rates are shocking[0]
https://sciencenorway.no/animal-welfare-fish-farming-salmon-...
Anchovies, sardines and other palm-sized, schooling fish are caught in the ocean and processed into fishmeal and oil to feed to other fishes and crustaceans that are raised in aquatic farms around the world.
https://www.washington.edu/news/2018/06/14/key-ocean-fish-ca...
Every year, millions of tons of wild fish, like sardines and anchovies, are caught and processed into fishmeal and fish oil, which is used to make feed for farm-raised species.
https://www.seafoodwatch.org/ocean-issues/aquaculture/wild-f...
Similar international agreements have worked relatively well to cut down on things like chemical weapons and anti-personnel mines.