Scientists hope to restore Marin’s pond turtle population

Samantha Kimmey
TURTLES: Researchers with the National Park Service, Sonoma State University and the San Francisco Zoo have been collecting data on western pond turtles in Point Reyes—including weighing the animals, measuring carapaces and eggs and tracking—as part of a project to reintroduce the reptile to areas in southern Marin like Redwood Creek, where they were recently discovered to be extirpated. As seashore biotechnician Tim Bernot weighed one specimen recently, he praised California’s only native freshwater turtle. “The exotic turtles will bite you. These are very gentle native turtles,” he said.  

Seashore biotechnician Tim Bernot pulled out a telemetry receiver on a recent Thursday morning, listening to the metronome-like clicks as he drove a park truck to a ranch pond to check turtle traps. The pace of the clicks, sourced from emitters glued to a female pond turtle’s carapace, tells him when the turtle may be out of the water and on dry land—in other words, whether she might be digging a nest, which are almost impossible for humans to find.

“If you were just to walk around, you would never find them. You have to catch them in the act. They’re super cryptic,” Mr. Bernot said.

The hunt for nests and eggs is part of a new effort to reintroduce western pond turtles to sites in the Marin Headlands, where the native turtles were recently discovered to be extirpated. A partnership between the National Park Service, the San Francisco Zoo and Sonoma State University aims to bring them back to the area by collecting turtle eggs in the northern reaches of the Golden Gate National Recreation Area that are managed by the Point Reyes National Seashore, where they still exist. 

The zoo will hatch the eggs, raise them for a year and then release them on the southern coast of Marin; Redwood Creek at Muir Beach and Rodeo Lagoon are the highest-priority sites for reintroduction. The project also presents an opportunity to learn more about pond turtles in Marin, where they are little studied.

“We just keep finding more and more,” Mr. Bernot said. “There are some big kings and queens here.” (He said the biggest turtle found so far was 42 ounces and nine inches.)

The western pond turtle, which historically ranged from Washington State to Baja California, is this state’s only native freshwater turtle. It features a subdued, mud-colored carapace, tan-and-black mottled skin and a dark underbelly with a light yellow stripe down the middle that darkens with age. It is considered a species of special concern in the state, although the U.S. Fish and Wildlife Service started a review in 2015 that could lead to a federal listing.

Like many species, the turtle has suffered from habitat destruction and development. It has been hit particularly hard in Southern California, said Nicholas Geist, a researcher from Sonoma State University who has spent many years studying the species. 

Pond turtles were a menu item in the late-19th century, when many thousands were harvested for restaurants. Foreign invaders like bullfrogs and red-eared slider turtles also pose problems; bullfrogs eat the young and the red ears, a Midwestern species often purchased as a pet and then abandoned, can claim habitat space, particularly the basking areas that are critical for the cold-blooded

Nest predation is also high. When a female wants to lay eggs, it leaves the comfort of the pond for dry land. After it finds a suitable spot—a turtle may nest in the same area for many years—it urinates to soften the ground and then uses its back legs to dig a hole perfectly sized to a half-a-dozen or so eggs. (The number of eggs varies somewhat from region to region.) 

Afterwards, the turtle plugs up the nest with a mixture of straw, dirt and leaves. (Turtles do not nurture their young, which are left to fend for themselves once they hatch.) If left undisturbed, the plug dries up and the nest becomes almost imperceptible. But other animals can smell the urine and eggs, so as many as 99 percent of nests are destroyed by predators like raccoons and foxes, typically within the first day or even first few hours of nesting. 

And when the eggs do hatch, they’re so small—just about a fifth of an ounce at birth—that they make a tasty snack for small mammals, bullfrogs and water birds. Young turtles are especially vulnerable until they are at least two or three years old, when their carapaces are wieldy enough to deter many predators. But if they make it past that age, they can live to be 60, 70 or even 80 years old.

Golden Gate National Recreation Area does not conduct routine western pond turtle surveys, but in the late ‘90s, surveys around the headlands found low numbers. At sites like Redwood Creek and the Rodeo watershed, the numbers were in the single digits. More recent surveys in 2014 and 2015 in those same areas found no turtles at all.   

The reason for the local extirpation is not exactly known, but development and habitat intrusion are the likely culprit. For instance, at Muir Beach, old middens indicate that pond turtles had a long presence in the area, said Darren Fong, an aquatic ecologist with G.G.N.R.A. But intrusions like a large parking lot built many decades ago took away habitat. The turtles “had a somewhat large freshwater lagoon, and that, of course, is no longer,” he said. 

Once populations get too low, they often can no longer sustain themselves, said Mr. Geist, especially for a species that has a tough time procreating.

Mr. Fong said a confluence of factors gives the turtle reintroduction project a strong shot at success. The zoo has reintroduced western pond turtles to Mountain Lake in the Presidio, where only one of 54 turtles released last summer is known to have died. They won’t be old enough to breed for another four or six years, but the zoo says the turtles are growing and appear healthy. And G.G.N.R.A. recently completed a restoration of Redwood Creek, providing hope that the turtles may now find the site more suitable.

The first part of the project required surveying G.G.N.R.A. lands managed by the seashore and Point Reyes to find existing turtle populations, which happened last year. It required a good amount of legwork, because no one had previously identified turtle sites in the area. The effort revealed both cause for celebration and concern, said Dave Press, a wildlife biologist for the seashore. He said there are a few sites in the park where pond turtles are living, and traps have found small juvenile turtles—a sign that the animal is having some reproductive success.

But surveys have also found threats, like bullfrogs and the red-eared slider. And Mr. Geist said that despite the presence of some young, “you see mostly adults. They’re not doing great.”

This year, researchers like Mr. Geist, Mr. Bernot and Ellen Gallanty, an intern with the San Francisco Zoo, are tracking the turtles to find and protect nests, as well as record as much data as they can. So far the researchers found two intact nests. They discovered others, but too late; predators had left only the remains of shells, sometimes found with remnants of fresh yolk.

With both nests, the researchers carefully removed the plug, measured and weighed each egg, placed a temperature gauge inside the crevice and replaced the eggs precisely in the arrangement in which they were found. Unlike chicken eggs, turtle eggs contain a pouch of oxygen for the little ones to breathe, so putting the eggs back incorrectly could kill the embryo. “It’s another example of how fragile the turtle nest is,” Ms. Gallanty said.

The eggs, which hatch after 90 days, will incubate at the zoo. There, the turtles will live for one year in large indoor ponds under UV lights. Their habitat was created to mimic the outdoors, so they can amble amidst rocks and hunt live food, but it also imitates summertime conditions. The turtles will grow to be as large as 2 or 3-year-olds, since in the wild they grow much more slowly in the winter. 

“We want to get them larger than the mouth of a predator,” said Jessie Bushel, the zoo’s director of conservation. She said the zoo expects to release about a dozen next year, all of which will be tracked with microchips and transponders.

Aside from the reintroduction project, the collaboration is providing an opportunity to learn more about pond turtles in coastal Marin. One major research interest for Mr. Geist are sex ratios, as the sex of the turtle is related to the incubation temperature. Cooler temperatures tend to produce males and high temperatures females. “I call them cool boys and hot girls,” Mr. Geist joked.

In Lake County, where Mr. Geist has conducted much of his research, the population tends to favor females—not surprising, given the hot summers there. At the beginning of the project, he wondered whether the cooler coastal temperatures would produce males, or whether turtles here might adapt to a different tipping point, so to speak, to keep the population in balance.

Early survey work tentatively indicates that there are more males than females in the area—not an ideal condition for reproduction. “Not to put it bluntly,” Mr. Geist said, “but you don’t necessarily need a lot of males.”

Researchers are also exploring whether the nesting season here differs from that in inland areas. The first “pulse” of nesting, as researchers put it, took place in June, but there is evidence from other coastal areas that Marin may get a second shot. The limited measurements so far also show that eggs here, at least based on the two nests, are larger than in Lake County, but each nest had fewer eggs.

Next year, researchers hope they will have a better understanding of the turtles—and therefore be able to protect and measure more nests, providing more babies for the reintroduction and more data for research. “The first year is always a learning experience.” Mr. Geist said. “We are trying to learn their behavior.”