Like most generalizations, this one has a few caveats.
Regular (western style) fish farming is sometimes efficient at producing food, depending on the feed that is being used. That's mainly because the goal is to produce a specific high value output (eg. salmon) rather than maximising protein output.
Fish farming, as practiced in some areas, can be far more productive than raising chickens, pigs or any other meat production.
Fresh water fish farming (tilapia, carp, catfish etc) can often work very well as part of an integrated farming strategy.
For example, putting fish in paddy fields helps to eliminate crop pests (better yield with less chemicals) while converting bugs and algae into fertilizer (less chemicals and better yeild) as well as generating fish. Ducks are pretty good for this too.
Similarly, farm ponds used to hold irrigation/stock water can benefit from fish (converting bugs and algae into fertilizer) and produce a fish crop too.
in August 2008