But, Sylvia - "dominion" does not address whether they (animals I mean) were immortal in eden prior to original sin, nor if they will be so in the supposed "new earth" - that is all speculation.
It is obvious with or without Stephen's reference that in today's world man can kill animals and that they are not immortal - the question was what was the rule in the garden and in the new system? (assuming of course that you accept the JW theology on both)