Here is another article on the topic.
http://www.wired.com/wired/archive/12.05/fish.html?tw=wn_tophead_5

This proposal talks about a next-generation design, 174 feet tall and 270 feet in diameter, called the Ocean Drifter. Unlike its predecessors, which are fixed to the seafloor, this enormous cage will roam the seas, propelled by three electric thruster motors attached to the rig's steel equator. Powered by a diesel generator mounted atop the central spar and steered by software, it will venture hundreds of miles from shore. When the fish are big enough to sell, it steers itself into harbor, for harvest.

Interesting to note that this article cites food conversion to fish weight stats that are significantly different from the PBS program.