When I started fishing in 1960, we killed every fish we caught. Over a half-century later, even though there are 1000 times more anglers, there are more fish and larger trout in Colorado waters. A big reason for this is the number of folks practicing Catch and Release or Catch and Choose the Fish to Release.
Over the years we have learned more about what stresses fish and techniques for proper fish handling. Fish released properly have a 90% chance of survival
This link describes the best practices for handling fish to increase their chance of survival after release: