Biologists Seek to Reintroduce More Genetically Robust Stickleback Population at Presidio’s Mountain Lake

Threespine stickleback are small fish capable of living in saltwater, freshwater, or a combination. They're named for the spines on their back, two of which are more prominent than the third. Presidio biologists reintroduced them into Mountain Lake for the first time in 2015.

NPS / Jessica Weinberg McClosky

February 2022 - Should a reintroduced wildlife population come from one source population, or from several? This question is actively debated among conservation biologists. Presidio Trust biologists tried the former approach in 2015 when they first reintroduced threespine stickleback (Gasterosteus aculeatus) into Mountain Lake from a population in nearby Lobos Creek. Now, after what may have been a disease-related die-off in 2020, they will draw on three local stickleback populations for a second reintroduction.

For years, the initial stickleback reintroduction to Mountain Lake was a resounding success. Biologists confirmed reproduction within a year after 1,000 fish were translocated. The population boomed. The species’ reestablishment was part of the holistic restoration of the lake. The stickleback play a critical role as hosts for restored California floater mussels (Anodonta californiensis). But since the 2020 die-off, surveys haven’t turned up any stickleback. While it is possible a small population remains, the Presidio Trust is assuming extirpation, or at least the need for augmentation.

Threespine sticleback in Mountain Lake in 2017. Biologists can’t be sure of why all or most of them died off in 2020. But they suspect the fish may have been less resilient in the face of disease due to low genetic diversity. They were sourced from a single, isolated population in Lobos Creek.

© Fue Her / Photo 7067157 / 4-14-2017 / iNaturalist.org / CC BY

Biologists can’t be sure of why all or most of the stickleback died. But they suspect the fish may have been less resilient in the face of disease due to low genetic diversity. The Lobos Creek stickleback source population is isolated—disconnected from the ocean and the larger marine stickleback population by military/residential infrastructure—and low genetic diversity is a common issue for isolated populations. This assumed low genetic diversity would have been amplified by the bottleneck of the 2015 translocation. Presidio biologists hypothesize that establishing a more genetically robust population at the lake will help. A bigger gene pool should provide the necessary raw material for evolutionary adaptation and fitness when confronted with stressors like disease or drought. That’s why this spring, they will collect stickleback to translocate not just from Lobos Creek, but also from Crissy Marsh/Quartermaster Reach, and the Sutro Baths.

As they begin, biologists want to be sure they understand the genetic diversity of those source populations. They also want to seize the opportunity to study the population's rapid evolutionary response to its new environment. Stickleback are a model organism for such studies of evolution and population dynamics. To that end, the Presidio Trust has initiated a collaboration with stickleback genetic researchers from UC Santa Cruz, UC Berkeley, San Francisco State University, and McGill University. The team’s main goal is to establish a more resilient long-term population of stickleback at the lake. But perhaps their efforts will also bring the conservation community a small step closer to resolving the “how many source populations for a wildlife introduction?” debate.

For more information

• Contact Presidio Wildlife Ecologist Jonathan Young

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