If you shoot a big fish with a bow regardless of the damage(its estimated health damage being full red like all other fish) the fish just shoots off at high speed not dying. Most of the time for the second you can see it, the model is bouncing up and down usually on top of the water before it disappears.
Tested rivers, oceans, ponds. Assuming various big fish by locations.
Testing I did when i was able to find one in a pond small enough the model couldn't get far away from bouncing, i put 3 pred arrows for 400+, 600+, 400+ (all full red damage bars) to this one big fish and it never died, just bounced to the other side of the pond each time.