The mortality rate is real. It's the reason fish for pay ponds don't allow release of any fish even if they would make more money when you pay by the inch. Myself I believe the biggest factor is squeezing fish, probably followed by overstressing them in warm water. Other factors like failing to properly revive a trout are likely to be more important than the net used but eliminating as many stress factors as we can is up to us as the fish cannot do anything for themselves.