I didn't read any of the links above... life's too short... but I can relate a couple of instances where I've seen stress affect the eating quality of fish.
Many years ago I hooked a 20kg Spanish Mackerel on a 6ft baitcaster with 6kg line... it took 45 mins to get the fish close enough to the boat to gaff... when I finally did he was stuffed, it was like realing in a Boral Besser block (cinder block)
Being such a big fish, I handed out large pieces to mates and neighbours. When I cooked it, a couple of different ways, it either curled up and/or went like rubber, it was unedible. I spoke to the others I'd given some to and they said the same.
A few months later I caught another 20kg specimen, but on a heavy 24kg trolling outfit and had him in the boat and bled in under 5 mins. He was good eating.
Also, when catching Herring sometimes I fillet the the bigger specimens and about 1 in 3 fillets will curl up and go rubbery... since the Mackerel episode I've always assumed these were the Herring that fought harder and longer.
But you don't know before you cook them which ones are going to curl up, or be rubbery, they all look and feel the same so I don't know how you would measure stress, at least in relation to eating quality.
