I don't think you would have to look at it as a question of retroactivity or retrospectivity. What the criminal law is always trying to do is prohibit human beings from undertaking certain actions. If there are beluga whales that are already pregnant by virtue of the human action of handlers and other people at Marineland, that's activity that has already happened.
What will be criminalized from the coming into force are human actions that seek on purpose to facilitate the breeding, not the birthing. The birthing of a baby beluga by its mother is not breeding or impregnating, through the criminal law lens. The criminal law is focused on what human beings are doing.