Interesting article:

http://espn.go.com/outdoors/bassmaster/columns/b_col_bt_bass_biology_0902/1439201.html

Part of the article:

Studies have shown that playing a fish to complete physical exhaustion produces greater changes in its blood chemistry. And it may take days for a fish played to complete exhaustion to fully recover. These stress-related changes are magnified in warmer water tem- peratures, not to mention that confinement in a livewell during a hot summer tournament may put those fish in an even more stressful environment, further worsening their condition and delaying recovery even more.

While these physiological changes are invisible from the outside, they are taking place inside the fish's body. And these changes may not manifest themselves for several hours — or even days — after a fish has been released. What's important to realize is that a fish's blood chemistry may not return to normal levels in time to prevent physiological functions from completely breaking down. Meanwhile, infections may spread and become lethal. Even fish that are severely stressed will show few outward symptoms and will act "normal" when handled and released, swimming away as if there was nothing wrong.