"With some improvements, do you think this could be a sustainable way of harvesting tuna in the future?"
No. This is absolutely not the answer to harvesting tuna. How can this be viewed as sustainable if you are still taking wild fish and harvesting them? The only way it could be sustainable is if you raised them from egg, which to my knowledge has been tried without much success and if at all successful only it certainly is not enough to satisfy demand.
I (and indeed many other biologists) would argue that this method is worse than rod and reel, hook-and-line fisheries like those in Atlantic Canada. You are talking about super high-tech fishing fleets (some use spotter planes and helicopters) with voluminous nets that round up thousands and thousands of pounds of fish at a single time. Sure, the fish don't die on the spot, but they certainly will once they've been sufficiently fattened up. Bottom-line is that they all wind up on a plate. To my knowledge, there's also no reproduction occurring in the nets that would otherwise offset the harvest mortality.
IMO, the only method of improving this fishery is to eliminate it.
Edited by ComeFromAway, 11 October 2017 - 10:04 AM.