Health Checks Show Growth of Reintroduced Western Pond Turtles as Tracking Project Ends

By Aquatic Ecology Intern Emma Force, Golden Gate National Recreation Area

A partnership of several institutions has helped reintroduce western pond turtles to Muir Beach and Rodeo Lake and Lagoon in Golden Gate National Recreation Area. The turtles were tracked using radio telemetry for several years to better understand how they've been using their habitat.

NPS / Emma Force

October 2022 - Starting as a joint reintroduction program with Sonoma State University and the San Francisco Zoo, Golden Gate National Recreation Area and partners have been egg collecting, rearing, releasing, and monitoring western pond turtles since 2016. The program aims to re-establish the turtles in southern Marin County, where they had been extirpated since the 1990s.

Biologists conduct turtle trapping at the end of each summer to perform health checks on reintroduced turtles.

NPS / Emma Force

As part of the post-release monitoring portion of this project, Golden Gate biologists have been using radio telemetry to track the movements of released turtles. We also conduct annual health checks of the turtles at the end of the summer through trapping. Trapping for the 2022 year occurred in late-August through early-September. Unlike previous years, this year's health check marks the official end of the radio telemetry monitoring program.

During the two weeks of trapping at Muir Beach we were able to process 28 of the 37 tracked turtles present in the area, yielding a 76% capture success rate. The graph below shows the growth (weight in grams) of six turtles from release years, 2017 (turtle numbers 7 & 9), 2019 (turtles 20 & 24), and 2020 (turtles 117 & 137).

Growth (weight in grams) of six turtles from release years, 2017 (turtle numbers 7 & 9), 2019 (turtles 20 & 24), and 2020 (turtles 117 & 137).

At Rodeo Lake and Lagoon, trapping was much more difficult due to heavy vegetation and the geography of the site. We could not effectively target the traps in areas where turtles were most likely located. Because of this difficulty, we were only able to successfully trap two out the 26 suspected turtles believed to be in the area, yielding a success rate of 8% after one week of trapping effort. Both turtles found within the Lake and Lagoon had indentations on their shell, which is the probable result of attempted predation from otters within the area. One week following trapping, we caught one more turtle in a seine net during tidewater goby sampling and processed it as usual. Although trapping in the Rodeo area was not as successful as Muir Beach, it was still more successful than in 2021, when staff only caught one turtle after two weeks of effort.

Data collected from telemetry will be sent to USGS to be analyzed and mapped to better understand western pond turtle habitat use throughout the year. Western pond turtles are of conservation concern across their range, so understanding them better may aid in their protection. And while telemetry has concluded, annual health checks will continue in the coming years to track the reintroduction program’s success.

For more information

• Contact Golden Gate National Recreation Area Aquatic Ecologist Darren Fong
• SF Zoo Western Pond Turtle Project video
• Get involved with turtle monitoring through the Marin Municipal Water District turtle observer program

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