Why is 20 °C (68 °F) the famous cutoff for trout catch-and-release?
Because two curves cross there. As water warms, a trout's metabolic oxygen demand rises steeply while the water's capacity to hold dissolved oxygen falls — so a fought fish, flooded with lactic acid, needs more oxygen exactly when less is available. Below about 18 °C (64 °F) released trout recover reliably; in the 20–23 °C (68–73 °F) band, studies show post-release mortality climbing sharply even for fish that swim away looking fine, because death often comes hours later. The fish does not have to die in your hands for you to have killed it. That is why the ethic is a hard line, not a suggestion.
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