You need to release them.
However, the genome percentage doesn't matter. So long as you have the minimum for release, you're good to go. The game ONLY tracks that the animal was released and not the genome percentage it was released at. If you release a 50% genome animal in Challenge mode, it will still be 100% in Sandbox mode.
It's just another annoyingly tedious grind, especially for the more expensive species like Spinosaurus.