Bullfrog Removal in the Tuolumne River Watershed
Bullfrogs (Rana catesbeianus) are a non-native species in the western U.S., including Yosemite National Park. They are highly carnivorous, opportunistic feeders that eat anything they can get their mouths around, including: insects, reptiles, and small birds and mammals - including bats. Bullfrogs currently infest several ponds within SMG area and have been previously detected along the mainstem of the Tuolumne River, just outside of the park on Stanislaus National Forest lands.
The proposed eradication effort would occur over a five-year period (2019-2024) and would focus on removal of Bullfrog egg masses in the spring, removal of adult frogs and tadpoles in the spring and summer, and removal of metamorphosing frogs in early fall. Bullfrogs often congregate and breeding sites have been mapped in the SMG area, as well as the mainstem Tuolumne River. Egg masses would be found, identified, and removed using a fine-mesh net such as a paint strainer. Adult and metamorph frogs would be removed through the use of gigs, traps, air guns (using non-lead ammunition), hand-grab, and nets. If tadpoles hatch, the park would would remove them with nets, or minnow traps concentrating on late summer when water levels are lowest and tadpoles are concentrated (bullfrog tadpoles require two years to metamorphose into adults at the elevation of the SMG area). Removal of frogs would occur primarily at night, when bright headlamps can be used to dazzle/stun the frogs, allowing park crews to approach the animal for capture (using hand-grab, spears, pellet guns). Removal of egg masses and tadpoles would occur primarily during daylight hours when they would be most visible.
Repeated visits to the breeding ponds and locations are planned throughout the summer seasons in order to maximize the number of frogs and tadpoles removed. Past efforts (2005-2019) in Yosemite Valley have resulted in an estimated >95% reduction in the bullfrog population in the first two years. Further application of these methods is expected to result in complete elimination of bullfrogs from the project areas.
As part of a monitoring component that would measure the response of native amphibians/reptiles to bullfrog eradication efforts, the project also includes visual and eDNA surveys for amphibians and turtles, monitoring of turtle nests using remote trail cameras, trapping and marking (scute notching and GPS/radio tracking) of Western pond turtles throughout the project area. The project also includes the placement of song-meters in the project area, as needed, for Bullfrog detection. These actions are not discussed in detail here because they are covered under an existing programmatic categorical exclusion for Wildlife Management Routine Activities (PEPC 78040). They are also covered by the Minimum Requirements Analysis attached to this project's compliance.
Bullfrog removal effort occurring outside of the park along the mainstem of the Tuolumne River on USFS lands are being coordinated with and approved by Stanislaus National Forest before implementation.
This project will complete the eradication of non-native bullfrogs from the Tuolumne River Watershed, with focus on the Swamp Lake/Miguel Meadow/Gravel Pit Lake (SMG) area of the park and the mainstem Tuolumne River in and outside of the park on federal lands, through removal of adults, tadpoles, and egg masses. This effort builds upon funded efforts in 2005-2006 which removed a vast majority of the frogs, and 2007-2011 base-funded efforts to prevent resurgence of the bullfrogs in Yosemite Valley, but is now being conducted in a new (backcountry) area of the park. It is expected that complete removal will require concentrated efforts to locate and catch the last individuals, and to prevent reproduction through removal of egg masses.