> There's plenty of freshwater fish that are farmed around the world
Farmed fish are often fed on fish meal from the ocean - e.g. fish meal made from species that are not eaten by people. Between 5% and 10% of ocean fishing is used for such aquaculture.
Same same as the cattle example in Ireland being fed on imported animal feed.
That's only because that bycatch is almost free of charge in areas with significant ocean fishing fleets.
You can provide the right mix of proteins and fats from algal and insect sources, so this shouldn't be a barrier to increased adoption of fish farming.
(Scaling wastewater and disease management are perhaps greater challenges, but ought not to be intractable either)