Fish stressed during capture release hormones that spoil their meat faster. A humane processing method that quickly spikes the brain and drains blood keeps sushi-grade fish fresh two to three times longer than standard handling.
There was this essay in the newspaper called If Fish Could Scream. It's all about how because fish don't have vocal cords, we give them less empathy than terrestrial animals like cows and chickens. Flopping around on the deck of the boat and releasing stress hormones, the adrenaline, the cortisol, the lactic acid, all that stuff makes the meat more acidic, which makes it more prime for the naturally occurring bacteria to grow. Where our process of basically spiking the fish in the brain, cutting the gills— Stop that acidification, you drain that blood, and the fish can last anywhere from twice to thrice as long fresh. The meat retains more of its actual water weight.
