Yes they can, they are farming a fish called Cobia in Queensland Australia that is a sashimi grade fish. The farming process is surprisingly good too, they worked with a local company and sorted out a crop based land grown food pellet for the fish to remove a large amount of reliance on pulling their food source from the ocean.

http://www.abc.net.au/news/2016-04-01/cobia-to-kick-start-mu...

Farm raising tuna is difficult. Not sure if it is done commercially yet and, even if successful, I would guess wild caught tuna will still be highly prized in Japan. It is really the Japanese market that is and will keep driving the high end of the fish market.

It's a little more complicated than just 'farmed vs wild-caught', as a lot of tuna is wild-caught, then transferred to floating pens to fatten up for up to a year. Penned tuna are neither truly wild, nor considered farmed as there is no breeding stock.

Sure. You'd have to be pretty lucky to find a wild salmon in a sushi place - and if you did it'd probably be twice the price.

A somewhat recent development in packing fish is automated cutting and vacuum packing - alive to wrapped in less than 30 (or so) seconds.

This is afaik also being done on factory ships, with fish like cod. Still rather new (a few years?) and expensive - but a great way to treat fish. Many of the advantages of flash freezing without the down sides.