The Fish San Francisco Put Back in Its Last Natural Lake, Twice
San Francisco has one natural lake left. Mountain Lake sits in the Presidio, between Lake Street and the Presidio's south border and between 8th and 12th Avenues, and it is the only natural lake in the entire 80,000-acre Golden Gate National Recreation Area. Mountain Lake | SF Beautiful Mountain Lake Trail | Presidio Trust Mountain Lake | Presidio Trust
Under the surface lives a three-inch fish called the threespine stickleback, named for the three spines along its back. The species can live in fresh water, salt water, or both. Biologists Seek to Reintroduce More Genetically Robust Stickleback Population at Presidio's Mountain Lake | National Park Service
In 2015, Presidio Trust biologists brought a thousand stickleback to the lake from Lobos Creek. The fish were reproducing on their own within a year, and the population boomed. Biologists Seek to Reintroduce More Genetically Robust Stickleback Population | National Park Service
In 2020, a die-off that may have been disease-related hit the population, and surveys afterwards turned up no stickleback at all. Biologists suspected a thin gene pool — every one of the 2015 fish came from a single, isolated source creek — and assumed the lake had lost them. Biologists Seek to Reintroduce More Genetically Robust Stickleback Population | National Park Service
The lake's California floater mussels depend on the fish. Their larvae, called glochidia, cannot swim; they need a host fish to carry them. In 2017, biologists kept about 300 stickleback overnight in tanks with hundreds of glochidia, which clamped onto the fish's fins. The fish were back in the lake within 24 hours, and in as little as two weeks the larvae dropped off onto the lakebed to begin their lives as filter feeders. The stickleback were unharmed. Stickleback Help with Mountain Lake Mussel Reintroduction | National Park Service
In spring 2022 the stickleback were released again, this time from three local populations — Lobos Creek, Crissy Marsh/Quartermaster Reach and Sutro Baths — to build a more genetically diverse population, in a collaboration with stickleback geneticists at UC Santa Cruz, UC Berkeley, San Francisco State University and McGill University. The Park Service's Mountain Lake page, last updated in October 2024, records the three-spined stickleback among the native species the Presidio Trust has reintroduced to the lake. Biologists Seek to Reintroduce More Genetically Robust Stickleback Population | National Park Service Mountain Lake | National Park Service
This weekend, walk the paved loop around the lake off Lake Street at 8th Avenue, or the full 2.6-mile Mountain Lake Trail, which runs from the Broadway Gate to Baker Beach — easy, with gravel, paved and boardwalk sections, about an hour. Stand at the south shore and look into the shallows; the fish are three inches long and the water is green, so give your eyes a minute. Keep pets out of the water: the Trust has kept a drop box by the south shore since 2015 for unwanted aquarium animals, and fishing is not permitted at Mountain Lake. Mountain Lake Trail | Presidio Trust Mountain Lake | Presidio Trust
Sources
- Mountain Lake Trail | Presidio Trust — San Francisco's last surviving natural lake; the 2.6-mile easy trail from the Broadway Gate to Baker Beach, its surface and walking time; restrooms and access.
- Mountain Lake | Presidio Trust — the only natural lake in the 80,000-acre Golden Gate National Recreation Area; the paved stretch around the lake and paved restroom access; street parking on Lake Street; the 1939 highway fill and the released pets; the 2015 amphibian-and-fish drop box; fishing not permitted.
- Mountain Lake | SF Beautiful — the lake's street boundaries (between Lake Street and the Presidio's south border, between 8th and 12th Avenues).
- Biologists Seek to Reintroduce More Genetically Robust Stickleback Population at Presidio's Mountain Lake | National Park Service — published February 2022; the 2015 translocation of 1,000 stickleback from Lobos Creek and the reproduction confirmed within a year; the 2020 die-off and the empty surveys; the low-diversity hypothesis; the 2022 second reintroduction from Lobos Creek, Crissy Marsh/Quartermaster Reach and Sutro Baths and the genetics collaboration; the three spines and the fish's tolerance of salt, fresh and mixed water.
- Stickleback Help with Mountain Lake Mussel Reintroduction | National Park Service — the California floater mussel reintroduction, the glochidia and their need for a host fish, the roughly 300 stickleback held overnight and returned within 24 hours, and the larvae dropping off within about two weeks.
- Mountain Lake | National Park Service — page last updated 10 October 2024; the western pond turtle, Pacific chorus frog and three-spined stickleback reintroduced to the lake.
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