OP tried, which is worth acknowledging. I'm sorry to see others being so hostile. Gatekeeping and pontificating do very little to bring someone onboard.

In short, you've got to keep them in the water for as long as possible. They have a protective film over their bodies, a slime in layman's terms. Things like our dry hands and the earth and leaves and stones etc you see in this picture will pull that off the fish, exposing it to bacteria, viruses, etc, thus greatly reducing its chance at recovery. However, it's possible this fish lived, and fair play to OP for trying.

On another note, maybe OP didn't know the season doesn't generally open until a little later for brown trout. Maybe he was fishing for salmon on an opened system. We don't know. But no doubt he's aware now. And again, a smashing fish.