Genetic Purity of California Golden Trout Populations 

in the Headwaters of Golden Trout Creek

 J. Cordes 1996

 For more information contact: jfcordes@ucdavis.edu 

Collaborators

    This work is progressing in collaboration with Dr. Graham A. E. Gall at UC Davis,  the California Department of Fish and Game (CDFG), and the U.S. Forest Service (USFS). Funding is provided by the CDFG and the USFS. 

Background and Significance of Study 

    California golden trout Oncorhynchus mykiss aguabonita were used extensively in the first half of the 20^th century to stock lakes and streams throughout the western United States (Pister 1991).  The source of these stocks have been attributed to an introduction of fish from either Golden Trout Creek (a tributary of the Kern River) or the South Fork Kern River (Stanley Stephens, California Department of Fish and Game, pers. comm..) to fishless Mulkey Creek (a tributary of the South Fork Kern River) prior to 1876. Twelve fish from Mulkey Creek were subsequently transplanted into Cottonwood Creek (1876) and from there into the Cottonwood Lakes (1891), which served as the source populations for stocks planted throughout the Sierra Nevada, including headwater Lakes draining into the Golden Trout Creek watershed (i.e. Johnson, Chicken Springs, and the Rocky Basin Lakes).

    Recent evidence based on protein allozyme analyses have indicated that the Cottonwood Creek and Cottonwood Lakes populations of California golden trout may have become introgressed with alleles from introduced rainbow trout O. m. mykiss, and that introgressed fish may have been subsequently transplanted into Johnson, Chicken Springs, and the Rocky Basin Lakes (Leary 1997). Thus there is concern that the original Golden Trout Creek population of California golden trout may be threatened with introgression by rainbow trout introduced into the headwaters of Golden Trout Creek. The objectives of the proposed study are to use nuclear DNA markers (microsatellites and single copy nuclear DNA) to investigate genetic variation and introgression in California golden trout populations in Golden Trout Creek, its tributaries, and its headwater lakes.

Materials and Methods

   Sample Collections. Fin clips samples of California golden trout will be collected by California Department of Fish and Game personnel from Johnson Lake, the Rocky Basin Lakes, Chicken Springs Lake, Stokes Stringer, and other appropriate sites and compared to golden trout samples taken from Golden Trout Creek by CDFG personnel in earlier studies (Gall and May 1997, Bagley et al. 1998) and rainbow trout samples from various hatcheries.

   Genetic Analysis. Whole genomic DNA will be isolated from fin clip samples using standard protocols. Microsatellite loci previously shown in this laboratory to be polymorphic in various lineages of the rainbow trout (O. mykiss spp.) complex will then be amplified from the isolated DNA samples via the polymerase chain reaction (PCR). Differences in the presence/ absence and frequencies of alleles will be used to assess population structure and introgression in California golden trout. A PCR-amplified single copy nuclear (scn)DNA marker that is diagnostic between California golden trout and rainbow trout will be used to independently assess levels of introgression.

Recent Results

   Results of this study were recently presented as a report to the California Department of Fish and Game Threatened Trout Committee (Cordes et al. 2001). In general, results indicate that all of the lake populations of California golden trout tested suffered reductions in genetic variability due to a number of population bottlenecks before being extensively introgressed with rainbow trout alleles introduced into the system.  Subsequent movement of introgressed fish into and/or reproduction in headwater creeks of the Golden Trout Creek (GTC) drainage has resulted in the presence of presumed rainbow trout alleles at low frequencies in Upper Stokes Stringer, in GTC below Stokes Stringer, in GTC at the mouth of Barigan Stringer, in GTC below Barigan Stringer, in GTC below little Whitney Creek, in middle Johnson Creek, in GTC at Groundhog Creek, and in the SFK above the Ramshaw Barrier. Given the generally low levels of possible introgression and their widespread occurrence in GTC, the use of rotenone treatments to eradicate hybrid fish is ill advised. The number of stream miles that would have to be treated is prohibitive, and a source population of uncompromised, genetically representative California golden trout has not been identified for restocking of treated waters. Although Volcano Creek has been suggested as such a source population, its reduced genetic variability and genetic distinctiveness from other GTC populations would not support its use as a founder stock. SFK populations located above the Ramshaw barrier have also been suggested as a source population for restocking, but our ascnDNA data show the presence of rainbow trout alleles in a sample from above the barrier, and calls for a more thorough investigation of hybridization in SFK headwater populations. In short, any attempt to eradicate the presence of rainbow trout alleles through rotenone treatments and subsequent restocking would probably result in California golden trout populations which are genetically less like the natural, pre-hybridized condition than the current populations.  The best way to halt further introgression of rainbow trout alleles into the system should include the removal of all fish from the Johnson, Chicken Springs, and Rocky Basin Lakes (already underway, Stanley Stephens, CDFG, pers. comm.) and discontinuation of all fish transplants into any of the lakes and creeks associated with the GTC drainage. There will hopefully be no new input of rainbow trout alleles into the system via the introgressed lake fish. Fortunately, the number of fish in GTC already carrying rainbow trout alleles is small, and these fish seem to exhibit the introgressed alleles at relatively few loci (a result of backcrossing with 'pure' golden trout). Unless these introgressed fish are highly reproductively successful, the RT alleles should tend to become diluted in the GTC populations (i.e. the alleles will become more spread out but less common than they are currently in the sampled locations). Eventually some of the alleles occurring at low frequencies may drop out of the system through genetic drift (random chance that an allele at low frequency in small populations will disappear because it doesn't get passed on to the next generation due to the vagaries of reproductive success of individual fish). This is a selectively neutral process and applies to neutral loci like the microsatellites. In the case of rainbow trout alleles that are under selective pressure, the deleterious alleles will tend to drop out faster, and any advantageous alleles will continue to persist. Limited removal of fish from and continued genetic monitoring of the hybridized reaches, particularly around the mouth of Barigan Stringer (which exhibited the highest levels of introgression) may be warranted to reduce the number of introgressed fish that can move and reproduce in the GTC system. 

Future Research

   We are currently conducting a similar study investigating California golden trout genetic variation and introgression in the South Fork Kern (SFK) watershed using the same genetic markers employed in the Golden Trout Creek study. Of particular concern is the preliminary evidence from the Golden Trout Creek study of introgression in SFK fish above the Ramshaw barrier, which was thought to have successfully protected California golden trout in the headwaters of the SFK.

Literature Cited

   Cordes, J.F., M.A. Blumberg, G.A.E. Gall, and B. May. 2001. Genetic status of California golden trout populations in the headwaters of Golden Trout Creek. Report to the California Department of Fish and Game Threatened Trout Committee. September 2001. 34 pp.

   Bagley, M., G.A.E. Gall, and B. May. 1998. Genetic analysis of 1997 trout collections. Report to California Department of Fish and Game, Threatened Trout Committee. August 1998. 18 pp.

   Gall, G.E., and B. May. 1997. Trout of the Kern River Basin: A genetic analysis of Little Kern River and Golden Trout Creek populations. Report to California Department of Fish and Game, Threatened Trout Committee. September 1997. 55 pp.

   Leary, R.F. 1997. Letter to Eric Gerstung, California Department of Fish and Game.  

   Pister, P. 1991. Golden trout (Oncorhynchus aguabonita). In Trout, J. Stoltz and J. Schnell (eds.). Stackpole Books, Harrisburg. pp.280-285. 

Last Updated: 11/04/02