The documentary talks about this. The salton sea has one of the world's largest tilapia populations. Mass die offs occur, and the dead fish acts as fertilizer. This causes algae blooms to form, which provides an ample food source for the tilapia, and they have a population explosion, rinse and repeat. That said, it sounds like the water is getting saltier all the time, so eventually, it will just be a dead sea if things continue the way they are going.