Given that Noah had to rescue 2 of a kind of every animal and fit them all in the ark, and given that he would've had to feed them while they were in his captivity:
If after the flood there were only mating pairs of each species in existence, what did the predators eat? If they ate any of their natural prey, the prey wouldn't survive, to be followed shortly by starving predators.
So once he set them free, what did they eat?