maybe i didn't get the story straight, but i heard they poisoned all the fish before draining the lagoon, to cleanse the lagoon of invasive species. i guess it was the kind thing to poison them first so they did not have to drown in alien air. i guess the poison naturally can't discriminate between the natives and the invaders. and i guess the humans cant either. so they kill them all and see who comes back naturally. see above: welcome the native invaders.