Yes that's correct, genome editing would eliminate instinctual predation and contraception would maintain ecological balance.
It would indeed require a very significant time and resource investment so global governmental cooperation would be necessary (which is why I say it's far-fetched; I think it's unlikely that we'll pass the ideological hurdles in creating a consensus to make it happen )
At least at first, this would likely be technologically out of reach. Micromanagement of the oceans would require pretty advanced nanotechnology, which of course is still in its infancy and what we can do with it is barely known.
This is not my original idea by the way. You can read more from what I think is the earliest outline of it, from David Pearce, here, if you're interested:
http://www.abolitionist.com/reprogramming/index.htmlLots of good content on that group of sites in general, regardless if you agree with his ideas.
Thanks Your posts on this subject have raised some ethical questions that I sometimes sweep under the "don't want to think about it" door mat in my dinged up brain.