Are you overfeeding them? Believe it or not that can cause an occasional mortality. Fish that eat too much can develop fatty livers (become functional diabetics) and be less healthy than fish that are fed less.
I've noticed since I've backed off on the feeding of my trout they are healthier, fight harder, and can survive catch and release better. I used to feed them twice a day but now only once a day to satiation. Makes for better quality water too with less weeds.
Impressive growth!
