Hi Tony - you have made several references to "low" water and "shallow" water in your posts, but in principle this is good for DO levels as it increases the surface area to volume ratio. Temperature is obvioulsy a key issue, as would be low velocities.
Certainly the fight would increase the oxygen requirement of the fish significantly, but as I said, if it was borderline I'd be surprised if they were "ripping up the gravel". So if we accept the DO wasn't borderline, then the normal "hold them in the flow" recovery technique should have worked.
Of course what happens during the fight is that the white muscle is brought into play, and this operates anaerobically (without oxygen). The by-product of this form of respiration being lactic acid, which accumulates in the muscle - similar to cramp in humans. In this condition the fish wouldn't want to swim off powerfully, but they could limp to the nearest weedbed??
As regards algal blooms, you are right that algal die off could increase BOD. However, I didn't suggest it was always CSO's that caused low DO, but that organic enrichment was commonly the root cause. As you will know, algal blooms are most commonly caused by organic enrichment in the first place.
So where does that leave us? On the basis that fully recovered fish in normal DO conditions don't go belly up in the nearest weedbed, I would suggest that either the fish hadn't fully recovered, despite appearances, or the DO crashed sometime after capture.