Pole and Line methods do more for 'tuna sustainability' than other catch methods as they are a more selective method of fishing, leaving/returning juveniles to grow to spawning age and replenish the stock in the future...
I have just read a book about tuna fishing with lines, and the reason is that the tuna will flood its body with blood to escape capture, and turn the meat red. if it is caught with a line, they can quickly reel it in and then they smash its head in before it can do the blood thing, thereby getting the light coloured meat.
bloody gruesome I think.