Restore Rare Frogs in High Mountain Lakes to their Natural Condition
High elevation aquatic ecosystems in Yosemite National Park have experienced a 95% decline of the once common and endemic Sierra Nevada yellow-legged frog (SNYF), due in part to predation by non-native trout. Historically, high elevation aquatic ecosystems (above 5,000 feet) did not contain fish due to impermeable barriers (e.g. waterfalls) in Yosemite Valley for the Merced River and Hetch Hetchy Valley for the Tuolumne River. The practice of fish planting started in Yosemite in the 1870s and continued through 1991. In 2000-2002, non-native trout were found in 9% (245 of 2655) of all Yosemite lakes, and in 54% (112 of 209) of lakes suitable for both trout and SNYFs. This project would seek to restore populations of the endangered SNYF to 20 - 30 lakes and meadows over the next 7 - 10 years.
Standardized surveys for SNYF presence/absence and abundance will be conducted annually at 30 - 50 sites within the park. Survey sites include locations with potential source populations and locations where previous restoration efforts have occurred. Aquatic ecosystems selected for restoration are near, or adjacent to, existing frog populations where natural recolonization is likely. Whenever possible, sites containing brook trout are preferred for removal since they are not native to the surrounding Sierra Nevada region.
We propose to remove non-native fish from 10 - 20 lakes or lake complexes primarily using gillnets. All captured fish are removed by hand, identified, measured, and counted. All carcasses are then either deposited into woven canvas sacks (biodegradable) and sunk in the deepest portion of the lake to retain the nutrients to the lake system or placed in dry-sacks and hiked out and deposited in bear proof dumpsters. Fish are removed from inlet and outlet streams of the lake using a battery-powered electrofisher device. Pack stock will be used to transport gear and frogs, with the possibility of one or two helicopter flights per year contingent on approval of a minimum requirement analysis. Removal of non-native fish is expected to take 3 - 4 years per site and a site is declared fishless after two consecutive winters of no fish captured.
Adult frogs are gently captured by hand and dip net at selected donor sites. Between 20 - 100 adult frogs will be collected depending on what the donor site can handle losing in one year without detrimental effects. The numbers we translocate will depend on balancing the objective of creating the largest founding population at a new recipient site while minimizing impacts to the donor population. The frogs are removed from the water and placed back in the water inside a mesh holding pen (roughly 1 meter squared). They are measured, weighed, and often microchipped ("PIT tagged"). In the past, frogs have been treated with an antifungal drug, but it is not likely that this treatment will be necessary. The handling process takes less than 10 minutes per frog (usually 5 minutes). The capture is usually done in the late afternoon and processing (weighing, etc.) takes place thereafter. The frogs spend the night in the holding pen, if they are being transported by helicopter, they are loaded in the morning into individual containers for transport. If the frogs will be transported by stock support or by backpack on a human, they may or may not overnight in a holding pen. If they are transported by backpack on a human, they will be hiked out at night (timed to give the most favorable climate conditions for the frogs). If they are transported by pack stock, they will be moved in insulated coolers (with cool packs) and will likely be packed out in the early morning.
The goal of this restoration effort is to re-establish self-sustaining frog populations in the park in strategic habitat areas. The SNYF is a keystone species, and with their reintroduction, aquatic insects will rebound, bird species will increase in abundance and wildlife observations improve.