Mountain Lake Park in the Richmond District is part of the Presidio run by the National Parks Service.
Its mission is to keep preserves the way nature made them.
The 4-acre pond is one of the few natural bodies of water in the city.
However, over the years people have dumped invasive fish, turtles and other creatures in the lake from home aquariums.
Crews are using nets to scoop out some of the invaders. So far, they have scooped out at least 500 young carp and a largemouth bass.