True that that's a non-trivial problem, but seaweed farming is actually something that we could massively scale up. With vertical farms, a ton (metaphorically speaking; more than that literally) of food can be grown in a small surface area of ocean. Would be much more efficient for feeding livestock than relying on farmland that could be growing human food. (Although as an aside I'm personally in favour of a decent amount of natural grazing space and time for farmed animals, so I'd like to see a balance.)