SAN FRANCISCO (CBS SF) — Crews took drastic measures to kill alien invaders in the San Francisco Presidio’s Mountain Lake, and in order to prevent another non-native fish issue, the park will set up a nearby “goldfish amnesty” pond where people can drop unwanted aquatic critters, while keeping the native fish safe.

The poisoning began Wednesday morning when crews dumped nearly 50 gallons of the pesticide Rotenone the lake.

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“We want to restore the native San Francisco ecosystem here,” said Terri Thomas, Presidio Trust Director of Conservation. “In order to do that we need to get all of the fish out.”

For decades the lake has been a dumping ground for pet gold fish, bass and carp. These invasive species have overrun the lake leaving behind a smelly polluted mess.

Thomas said they well introduce the native species now like the Three-Spine Stickleback.

“We’re put in our native fresh water mussel that’s going to keep the lake clean.”

The native fish won’t be put in the lake immediately. They’re going to wait until next spring to make sure everything is clear.

After the poison pour, dead and dying fish were floating up by the hundreds. Small boats raced around scooping them up. Other workers are simply netting and tossing them on the shore.

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Dr. John McCosker of California Academy of Sciences says they well study the DNA of the carp.

“You can tell by examining the genetics of these carp that some came from different areas, probably Europe maybe or Asia …who knows?”

There’s no debate that the carp have got to go.

“These carp are bottom feeding, muck feeding fishes that disturb the habitat and make it impossible for native fishes to survive,” said McCosker.

Those that aren’t studied will serve another purpose. The castaway carp will be used as fertilizer in other parts of the park.

The work is part of an $12 million restoration project.

In the future, a covered pond nearby will allow humane drop off of species that don’t belong in the lake.

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