Three-Spined Stickleback Fish Returns to San Francisco's Mountain Lake

The endless stream of cars created a white noise, drowning out the lapping of water in San Francisco’s Mountain Lake at the southern edge of the Presidio. The cars zipping along nearby Highway 1 were a constant reminder of its status as an urban lake, one of the few freshwater bodies of water left in the city.

On Thursday, Presidio Ecologist Jonathan Young ignored the din of traffic as he stood on the lake’s eastern shore, addressing a group of volunteers about the important milestone they were about to mark in the lake’s 2,000-year history.

“This is a really cool opportunity for us to restore the health of Mountain Lake,” Young said earlier in the day. That morning, he and other researchers used nets to capture hundreds of tiny fish known as three-spined stickleback from nearby Lobos Creek. The tiny fish were once the only native fish in the lake - until the highway was built in the 1930s, cutting off their route back and forth to the creek. Non-native trout and carp wiped out the remaining stickleback in the lake decades ago.

Several years ago, Presidio managers embarked on a project to restore the health of the lake which suffered from runoff from the highway, and the havoc non-native fish like carp and bass and wreaked on its native species. Ecologists finally used a toxic solution to kill the lake’s fishy interlopers, returning the lake to a blank slate.

“It’s sort of a new 21st Century concept of restoring an urban ecosystem,” Young said.

But as Young and his team scooped up dozens of the tiny stickleback from the creek, it marked a new phase for the long-beleaguered lake. In the afternoon, a couple dozen people stood on the lake, ready to release the stickleback back into the lake for the first time in 50 years.

“Because highway one is there, there’s a huge barrier,” Young said. “They can’t get back on their own so this is sort of our chance to trans-locate them there.”

After letting the group known the importance of the task at hand, Young began scooping the squirming fish from a bucket and into small plastic cups. The assembled group which included Presidio staff and volunteers, and members of the public began pouring the cups into the water — the tiny fish quickly disappearing into the waters of Mountain Lake.

Soon the fish will have company; ecologists plan to soon reintroduce other of the lake’s native species including the Pacific chorus frog, the Western Pond turtle and the California Floater mussel. Young said the mussel needs the stickleback to survive.

“The mussel actually requires this fish to complete its lifecycle,” Young said.

Once the group had release all 150 of the fish, Young stood at the river’s edge gazing out across its waters. The fish had disappeared from sight — embarking on a new life in their old home. Across the water, the rush hour traffic was beginning to bog down as Young packed up the empty buckets and headed for home.

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