Fish have very low blood pressure due to the 2 chambered heart, and don't bleed that much anyway.

Once we were fishing in the BWCAW and my brother caught a northern, and it was bleeding from the gills. We put it on the stringer and figured that it would be dead before dinner. Nope, so we figured we would save it for breakfast. Bye the time we got around to eating that fish it had completly healed and fully recovered... on a stringer.

Someone did do a study on hooking mortality and growth rates of rainbow trout. They intentionaly throat hooked trout and lip hooked trout. They found that you should use circle hooks and cut the line. There were only a few circumstances were leaving the hook in affected fish growth. Otherwise most of the fish recovered and grew normaly. If I can't get a hook out I gladly cut the line.