I have seen well over 100 chinook dead in the river that were fish that were already turning and hooked after they had "hardened" up. The fish were hooked, released because no retention was allowed and they fell to the bottom and died in droves. The game officer estimated that 400 chinook were killed from hook and release in the river in the first week of the season. Condition were not good at all, low water and warm water so that had a significant effect but the mortality rate was estimated at 80% or higher for released fish.

As for fly fishing, longer fight to land them, more stress and more tired fish should equal higher mortality.