I never would have guessed that I'd be quoting Chick-Fil-A- I've never been there and most likely never will go- but their ads kind of sum up some recent research. Popular wisdom had long held that fish are the most efficient type of animal to raise. For one thing, unlike large animals such as cows and pigs, fish don't have a lot of bones: a larger share of their body mass is muscle, so a larger share of food you give a fish to make it grow turns into meat. However, this paper argues that chicken is more efficient than shrimp or fish aquaculture at turning feed into meat. One reason is that the stuff we give fish has to already be more nutritious: cattle turn grass, for example, into meat, and grass doesn't have a lot of protein in it. On the other hand basically what we feed fish and shrimp is other fish. That makes the whole process less efficient.
Note that no one is measuring any of the externalities of the production process or anything else so this is far from a final word, but it's definitely an important contribution. Feeding fish to fish never seemed like a good idea, but now there's evidence backing it up!