Bringing Stickleback Back to Mountain Lake — This Time From Three Sources

Should an animal population being reintroduced come from one source population or from several? Conservation biologists still argue about it. At Mountain Lake in the Presidio, biologists are trying both.

Threespine stickleback can live in salt water, fresh water or a mix of the two. They are named for the spines on their back, two of them more prominent than the third. NPS / Jessica Weinberg McClosky.

The first reintroduction

In 2015 Presidio Trust biologists moved 1,000 threespine stickleback (Gasterosteus aculeatus) into Mountain Lake from a single population in nearby Lobos Creek — the species' first reintroduction there. For years it worked. The fish were breeding within a year, and the population boomed. Their return was part of a wider restoration of the lake, and they matter to it: stickleback serve as hosts for the lake's restored California floater mussels (Anodonta californiensis).

Then, in 2020, came what may have been a disease-related die-off. Surveys since have found no stickleback. A few may survive, but the Presidio Trust is assuming the population is gone, or at least needs reinforcing.

Why it may have failed

No one can be sure why all or most of the fish died, but the biologists suspect low genetic diversity left them less able to withstand disease. The Lobos Creek population is isolated, cut off from the ocean and the larger marine stickleback population by military and residential development, and isolated populations commonly have little genetic diversity. The 2015 translocation, a genetic bottleneck, would have narrowed the gene pool further.

A bigger gene pool, the biologists reason, gives a population the raw material to adapt and stay fit when it meets stresses such as disease or drought.

The second attempt

So the next translocation will draw on three local populations: Lobos Creek, Crissy Marsh/Quartermaster Reach, and the Sutro Baths. The article, written in February 2022, said the collecting would happen that spring.

First the team wants to understand the genetic diversity of each source population. It also wants to use the chance to study how quickly the new population evolves in response to its environment; stickleback are a model organism for studies of evolution and population dynamics. The Presidio Trust has begun working with stickleback geneticists from UC Santa Cruz, UC Berkeley, San Francisco State University and McGill University.

The main aim is a lasting, resilient stickleback population in Mountain Lake. The work may also move conservation biologists a little closer to settling how many source populations a reintroduction should use.

Sources

• National Park Service, Bay Area Nature & Science Blog (Golden Gate National Recreation Area): "Biologists Seek to Reintroduce More Genetically Robust Stickleback Population at Mountain Lake," February 2022.