Let us not forget that the Bible makes a distinction between animals that are "clean" and animals that are "unclean."
That distinction is whether or not you are allowed to eat them.
If humans were vegetarians before the flood, how would Noah know the difference?
And afterward, since he had been a vegetarian for 600 years, why would he change his diet? Especially when there are so few clean animals to use? It would take years to create a viable stock large enough where you could cull individuals for food and not cause a genetic catastrophe.
And for that matter, why was Abel a shepherd?
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Rrhain
WWJD? JWRTFM!