One of the biggest issues with the fundamental concept of fish farming is that many of the fish we like to eat are carnivores. So to truly sustainably farm salm

One of the biggest issues with the fundamental concept of fish farming is that many of the fish we like to eat are carnivores. So to truly sustainably farm salmon, you would have to farm a bunch of bait fish to use to feed the salmon (in practice I believe the bait fish are usually wild caught). And indirectly you would need to be farming a bunch of algae or seaweed or something to feed the bait fish. Compare that to farming cows or chickens, where you just need to farm a bunch corn to feed them (or if you have enough land available you can make them free range). That extra level of indirection makes farming much more inefficient, which is why essentially all of our domestic animals we use for food are herbivores. Of course this isn't true of all fish, tilapia being the prime example of a commonly eaten herbivorous fish. And shellfish, being filter feeders, are very easy to farm and actually tend to be beneficial for the surrounding ecosystem. EDIT: interestingly, according to https://ourworldindata.org/seafood-production https://ourworldindata.org/seafood-production in terms of tonnage the amount of wild-caught fish has remained relatively steady since 1990 while the amount of farmed fish has steadily increased and is now more than 50% of our source of fish. There's actually some very interesting data in that link