Cathedral Sierra Nevada Bighorn Sheep Augmentations
The National Park Service and California Department of Fish and Wildlife (CDFW) will reintroduce a herd comprising up to 25 Sierra Nevada bighorn sheep (hereafter bighorn or sheep) into the Cathedral Range of Yosemite National Park starting in March 2015. Up to 20 of these animals will wear Global Positioning System (GPS) and Very High Frequency (VHF) collars at any given time in order to monitor their movements throughout the range. In addition to collars, field surveys will be conducted by NPS and CDFW personnel to monitor the herd.
From 2015-2019, NPS and CDFW conducted three separate bighorn releases in the Cathedral Range. The first release occurred from March 26 - April 3, 2015 and consisted of 13 bighorns (10 ewes and 3 rams). The second release occurred on November 2, 2016 and consisted of five rams. The third release occurred on October 24, 2017 and consisted of two rams. During the 3 releases, 8 total helicopter landings took place: 5 in the Washburn Lake area in 2015; 1 on the SW side of Mt. Lyell and 1 on the Parson's Plateau in 2016; and 1 on the Parson's Plateau in 2017.
The most recent surveys conducted in 2019 concluded that four bighorns currently occupy the Cathedral Range. Three of the bighorns are adult ewes and one is a yearling. At this time, there are no functional collars in the Cathedral herd, which limits monitoring efforts to ground-based surveys. Three of the four existing ewes were born in the Cathedral herd and they have not been collared. The one ewe that was transported and released into the Cathedral Range carries a collar, but the collar no longer functions.
Population declines in the Cathedral Range are attributed mostly to severe winter conditions in 2017 and 2019. High snowpack levels hinder foraging and can cause malnutrition and death; while increased avalanche activity can cause direct mortality. Members of the Science Advisory Team are currently engaged in discussions about how to strategically conduct future augmentations/releases to continue growing and monitoring the herd.
Additional population augmentations will occur 2020 to 2024. Each of those years, up to 5 ewes or rams will be translocated to the Cathedral Herd. The source herd will likely be Mt. Langley, but could be from other herds if necessary. The maximum number of sheep reintroduced into the Cathedral Herd will not exceed a herd size of 25 animals (up to 15 ewes and up to 10 rams).
Augmentations will be completed using the same methods as past efforts. The crews use net-guns operated from a helicopter. Net-gunning involves deploying a net on an individual sheep, tying their feet together and covering the bighorn's eyes with a mask. Research has shown net-gunning to be the safest alternative for bighorn captures with 2-3% accidental mortality. After a bighorn is immobilized, it is harnessed and flown out of the capture site to a processing site on the east side of the Sierra Nevada. During processing, veterinarians and biologists perform health assessments on each individual, take blood and hair samples for genetic analysis, and affix GPS and VHF collars for monitoring purposes. The sheep are then loaded into large boxes and transported by truck north to a location close to the release site. Once there, biologists transport all boxes of sheep to release locations in Yosemite's Cathedral Range via helicopter. Biologists release the sheep and are flown out of the backcountry. Alternatively, sheep are harnessed and flown directly to the release site where they are released by the helicopter crew, negating the need for the biologist, boxes, or vehicular transport.
Helicopter flights will generally be short in duration (10 minutes or less) and as short as possible when transporting sheep and crew members. Over the next five-year duration of the project (through 2024), a maximum of 24 landings over 7 days will be required for augmentations in the Cathedral Range.
The release sites are located near Parsons Plateau in the Cathedral Range. The sheep have selected the Parson's Plateau as their main wintering area since the initial reintroduction in 2015. Summer range includes extensive habitat throughout the Cathedral Range and in the headwaters of the Merced River. There is potential for rams from the Cathedral herd to migrate and breed with the existing Mt. Gibbs herd farther northeast and vice-versa.
Monitoring methods and protocols will be consistent with those identified in the Sierra Nevada Bighorn Sheep Recovery Plan. Field crews will conduct surveys throughout each summer and fall to monitor the sheep and assess movement and mortality. Twice yearly surveys will continue for at least 15 years. The GPS collars will send out location fixes once per day and can last up to 3 years. Once the collar battery dies or nears depletion, collars may be replaced depending on monitoring needs for the herd. GPS collaring of multiple bighorns will continue for at least 15 years due to Endangered Species Act delisting requirements. VHF collars send out a short-range radio signal that help crews and air support locate the sheep. Batteries on the VHF collars have a much longer life and generally last throughout the lifetime of a sheep. Due to herd movements and dynamics, we will attempt to collar all rams and multiple, but not all, ewes.
Because of the wet winters of 2017 and 2019, the Yosemite bighorn populations are precarious. The previous MRA did not allow for any predator control actions. For 2020-2024, mountain lion predation on bighorn sheep may trigger killing that lion if it threatens sheep. A single kill by a lion does not necessarily trigger killing that lion. A single lion may have a devastating effect on a small sheep population, and a decision to kill the lion would have to occur quickly to prevent such a loss. If a mountain lion is discovered to be preying on bighorn sheep, that lion may be killed according to the guidance and limitations discussed in "A Strategy for Managing Predation on Sierra Nevada Bighorn Sheep", Gammons et al. (2019). This document provides a thorough analysis and convincing argument that, in the Sierra, killing mountain lions to protect the remaining bighorn herds is justified if done with the due consideration and restraint outlined within.