Fishing is a blood sport. Even doing everything right, you can expect to kill some fish (either right away or after you release them). However, using to small a tippet, using barbed hooks, excessive handling, playing to long, warm water or maybe fish weakened by being in the truck to long on the way from the hatchery can all contribute.
By the same token, using heavier tippet whenever possible is a good idea. I always have 6x and 7x, just in case, and end up using it once or twice a year becasue it does cover a lot of sins. But in general 4x and 5x are plenty, especially if the water is slightly discolored or high. It allows you to play the fish to hand much more quickly, and release them much sooner. Just as using barbless hooks allows you to release them more easily.
The science tells us that barbed vs barbless, that 5x vs. 6x , that treble or single hook are individually statistically insignificant. However, taken in total I find that I kill fewer fish using heavier tippet, barbless single hooks and avoiding taking the fish out of the river as much as possible.
But even doing everything right some fish will die, it is something we all need to acknowledge and get over. But keep that in mind when you have one of those extraordinary 60 fish days, chances are statistically pretty fair 4-5 of those fish will die.
Then again if you plan to harvest them it doesn't matter.