I saw video of a test site, but I don’t know if it is being done on a large scale. I gather they use a double net, the inside one to keep the fish in, and the outside one to keep out “biologicals” that would eat fish.
Other than the pontoons and the nets, there is a small boat to both move the net as needed, to store the fish food, and as a place to sleep for the divers who pull maintenance on the nets.
Something like this is needed, as the overfishing is really starting to become a problem. Countries like Japan have zero self control, even if it eventually destroys the entire Japanese fishing industry, and some countries are so vigorously poaching each others fish that it is leading to conflict on the high seas.
A problem for as long as I can recall. Many years ago I was something of a 'Save the Whales' type because that wasn't really about hugging whales but about a need for some international agreements regarding all harvesting of the oceans. Since then there have been some agreements and, I think, all nations with a coast have claimed fishing rights out to 200 miles.