"Catch and release" anglers kill the fish inadvertently at a later time. The fish may die from damage from hooks or be attacked by predators while they are helplessly hooked. While "humane slaughter" for other animals killed for their flesh is a marketing myth, there is not even this pretence when fish are killed. They instead struggle and gasp while they slowly suffocate to death.
One paper in this review found that, out of water, rainbow trout took 11 minutes to stop moving at room temperature, and an incredible 3½ hours at 2°C.
"Striking the cranium of a fish with a priest (club) can cause immediate death or unconsciousness and, if accurately administered, is humane. However, fish sometimes recover consciousness after percussive stunning."
"Hypothermia and asphyxia¹ are not recommended for euthanasia of fish captured recreationally, due to variable and extended times required to cause insensibility and death."
"Kestin et al (1991)² reported that rainbow trout took 11.5 min to lose all movement at 20°C and 197.6 min at 2°C."