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Transcript
California Trout’s Paiute Cutthroat Trout Recovery Project Working Outline Purposes To restore and delist the Paiute Cutthroat Trout (PCT) from the Endangered Species Act To inspire and engage California Trout’s donors in order to increase their financial support To expand and deepen conservation partnerships with agencies, NGOs, and foundations To pave the way for opening California Trout’s Tahoe/Truckee regional office this year Background1 PCT are listed as “threatened” pursuant to the Endangered Species Act and are native to just a single drainage, Silver King Creek, in eastern California. Silver King Creek is a tributary of the East Fork of the Carson River. PCT historical distribution is exceedingly limited. They are thought to have existed in only 14.7 kilometers of habitat from the base of Llewellyn Falls downstream to Silver King Canyon and including three small tributary creeks in the drainage, Tamarack Creek, Tamarack Lake Creek, and the lower reaches of Coyote Valley Creek downstream of barrier falls. PCT are thought to occupy a total of 18.6 km of stream with a core habitat of 12.9 km. California Department of Fish and Game (DFG) population assessments found approximately 1,020 adult fish in the six streams currently inhabited by PCT in the Silver King Drainage. While most populations are stable, they remain heavily fragmented and have no chance of interbreeding without human intervention, thus reducing the effective population size and seriously limiting the genetic viability of the species. The biggest threats to the persistence of PCT are 1) alien trout, 2) loss of genetic diversity, and 3) habitat loss. Alien trout. Alien trout are the principal threat to the continued persistence of the PCT. They impact PCT through competition for resources and habitat, predation, and by hybridization. The introduction of non-native rainbow, golden and Lahontan cutthroat trout into the historic range of the PCT below Llewellyn Falls has resulted in the extirpation of PCT. PCT are subject to both competition and predation from alien trout. PCT readily hybridize with rainbow trout and Lahontan cutthroat trout, resulting in loss of genetic integrity and phenotypic distinctiveness. Loss of genetic diversity. A study by Cordes et al (2004) found PCT to be the most genetically limited and narrowly distributed native trout in the state of California. Genetic distances between the current populations of PCT show that there are three genetic groups within PCT, but Cordes et al (2004) surmise that this differentiation is due to founders effects and genetic drift, which are more representative of PCT’s stocking histories than natural variation within the subspecies. Most of the transfers consisted of This section is drawn from Dr. Peter Moyle’s State of Salmonids draft life history account for Paiute Cutthroat Trout. 1 small numbers of fish and the creeks with similar genetic strains had similar stocking histories (Cordes et al 2004). Nielsen and Sage (2002) found no distinctive alleles differentiating PCT from LCT, but did see that PCT had gone through a genetic bottleneck. It is likely that the first isolation of PCT when it diverged from LCT represented a genetic bottleneck and that subsequent stocking and culling of stocks to eliminate hybridization has further amplified the situation. Additionally, there is no population that currently possesses all of the alleles known to PCT, so further transfers to maintain what is left of genetic diversity may be required. The loss of genetic diversity due to small populations and lack of metapopulation connectivity combined with the loss of genetic diversity due to introgression from other species of trout represent the largest threats to PCT. Habitat loss. Right now (2008), the 23 km of stream habitat in which PCT persist are in reasaonably good condition, with limited or no grazing affecting the stream banks and light human use. However, this could change if grazing allotments are renewed or if a catastrophic fire swept through one or more of the basins. Thus the limited habitat by itself represents a reason for careful management because conditions can change so quickly. Conservation The populations of Paiute cutthroat trout are small but reasonably stable because they are entirely located in streams in remote national forest lands (Toiyabe, Sierra, and Inyo NFs). The habitat closest to their native range is all in the Carson-Iceberg Wilderness as well. However, additional protection is needed by expanding their range back into their historic habitats. Part of the management actions listed in the 2004 USFWS Recovery Plan is removal of non-native trout in the waters between Llewellyn Falls and Silver King Canyon. This was to be done using the piscicide, rotenone, which has resulted in years of lawsuits and no progress in restoring PCT below Llewellyn Falls. Opposition to the poisoning of the creek comes in part from anglers who value the healthy fishery of wild (though non-native) trout that thrives in the excellent habitat below the falls. There is also concern that endemic invertebrates may occur in that reach and rotenone is toxic to them as well as fish. Additionally, the area is habitat for two species of amphibians that are candidates for listing under the Endangered Species Act, the mountain yellow-legged frog (Rana mucosa), and the Yosemite toad (Bufo canorus); there is fear that the rotenone treatments could harm them, though the USFWS plans include removal of any amphibians found in the reach prior to treating the creek (USFWS 2004). Recent reviews of toad sampling indicate that most toads in the Silver King basin are western toads (Bufo boreas), although a few hybrids with Yosemite toads may exist. No pure Yosemite toads have been found in the Silver King Creek basin (William Somer, DFG, pers. comm.). It is unlikely that the candidate species amphibians occur in high densities below Llewellyn Falls because non-native trout predation is one of the factors in their decline throughout their range. The current distribution of PCT and mountain yellow-legged frogs overlap in Silver King Creek basin. The two organisms co-evolved in the basin, so it is likely they can coexist. The two species also co-occur in all four of the out of basin populations of PCT (N.F. Cottonwood Creek, Cabin Creek, Sharktooth Creek, and Stairway Creek). The Recovery Plan includes treatment of Tamarack Lake (in the upper Silver King drainage) to rid it of trout for the benefit of the two amphibian species (USFWS 2004). The many unauthorized transfers both of PCT and the non-native trout that threaten them have been both a scourge and a savior. By 1924, the PCT in their native reach (below Llewellyn Falls) were already introgressed with LCT, rainbow trout, and golden trout. In 1949, another unauthorized transfer introduced rainbow trout above the falls. If it had not been for the 1946 stocking in Cottonwood Creek and introduced populations within Silver King basin in Fly Valley and Four Mile Creeks, PCT could well have been lost. Introductions, hybridization, and culling have occurred repeatedly throughout the 20th century. Table 2 below shows the complicated stocking history of PCT in the last 150 years and identifies management actions that have taken place during that time. Current PCT populations are fairly stable, but the lack of genetic diversity of existing stocks, extirpation from their native range, lack of connectivity among populations, and small effective population sizes continue to hamper recovery efforts. The 2004 PCT Recovery Plan lists reintroduction of PCT to their native range below Llewellyn Falls and eradication of non-native salmonids there as one of the criteria for delisting the species. Efforts on the part of CDFG, US Fish and Wildlife Service, and US Forest Service to eradicate alien trout in lower Silver King Creek with piscicides have been blocked by litigation for several years, but USFWS is in the process of completing an environmental scoping document to move forward with a chemical treatment. If treatments are successful, then the process of restocking PCT in lower Silver King Creek can begin. Further conservation plans by CDFG and USFWS include monitoring and maintaining all existing populations of PCT and their habitat as well as continued protection of all existing PCT populations from alien trout incursions. The limited historical range of PCT and the fact that this habitat is publicly owned and suffers from very limited degradation gives the species a good chance of recovery. However, the small geographic range and limited genetic diversity of PCT make it vulnerable to inbreeding depression, stochastic events, and illegal introductions of alien trout. Restoring PCT to their historic habitat will more than double the number of fish in the Silver King basin, provide greater connectivity of habitat, restore the PCT as the principal aquatic predator to re-establish natural processes, and help to isolate PCT in the basin from the threat of non-native trout introductions. In short, PCT have a good chance at recovery if alien trout can be kept out of their, their range can be expanded, and their habitat protected. Trends: Short term: PCT populations have remained fairly stable since 1998. They currently inhabit more miles of stream than they did historically, but the populations are heavily fragmented and cannot interbreed. Non-native trout exist below Llewellyn falls and 56 years of restoration and conservation efforts could be unraveled by a single illegal introduction of non-native fish into current PCT habitat. Long term: Though PCT have a far more limited distribution than LCT, their habitat is in reasonably good condition and is all on public land (US Forest Service) which simplifies recovery efforts considerably. Their dependence on humans for reproduction is limited, though continued transfers will be required in order to maximize the remaining genetic diversity in the populations. Monitoring and removal of any alien trout must continue indefinitely to protect genetic integrity of the remaining fish. Climate change may pose a threat to PCT populations in the future, but the alpine nature of their native habitat could potentially buffer them from the effects of warming or loss of snowpack. The out-of-basin populations at Cottonwood and Cabin Creeks may be more at risk because of the arid nature of the White Mountains. Status: 2. PCT have a high likelihood of extinction in their native range within the next 50 years without continued intense monitoring and management. All populations are small and isolated, so therefore subject to illegal introductions of alien trout and local natural and man-made disasters. PCT were listed as endangered under the Endangered Species Preservation Act of 1966 on March 11, 1967. However, they were subsequently downgraded to threatened under the Endangered Species Act of 1973 (on July 16, 1975) to facilitate management activities and to allow limited recreational fishing (USFWS 2004). The 2004 USFWS Recovery Plan’s goal is to restore PCT to their native range in Silver King Creek and continue to monitor and protect all existing populations. The status determination (Table 1) is for the effective wild populations, including introduced populations.