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Impact of triploid Rainbow Trout and naturalized Rainbow Trout (Oncorhynchus mykiss) on recovery of Lahontan Cutthroat Trout (Oncorhynchus clarkii henshawi) in the Truckee River watershed
AuthorKirchoff, Veronica Sue
AdvisorPeacock, Mary M.
Biochemistry and Molecular Biology
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Historically the Lahontan cutthroat trout (Oncorhynchus clarkii henshawi, LCT) occurred throughout the Truckee River basin, supporting important commercial fisheries and was extirpated in the 1940s due to water diversions, predation, competition and hybridization with non-native trout. To provide angling opportunities, Rainbow trout (Oncorhynchus mykiss, RBT) has been planted, and there is a robust naturalized population throughout the Truckee River. Recovery efforts are underway to reintroduce the threatened LCT back into their native habitat in the Truckee; however, planting LCT sympatric with naturalized RBT can support hybridization between the species and hamper LCT recovery. Since 2004, in an effort to limit hybridization, 90% of the RBT stocked are non-reproductive triploid RBT. Over 3,400 trout samples were collected in the Truckee River and its tributaries from 2007-2010. These trout were identified as pure LCT, pure RBT, LCT/RBT hybrids or triploid RBT using bi-parentally inherited markers that differentiate between RBT and LCT and microsatellite markers that revealed triploidy in a proportion of the RBT. A mitochondrial marker was sequenced in hybrids to determine the maternal contribution to hybridization and to look at spawning success. The highest level of hybridization was found in 2008 from samples in the river tributaries. This correlates to the time period when fry stocked in 2005 and 2006 would reach sexual maturity. Backcrossing of hybrids with RBT was detected, and a low level of introgression indicates that hybridization has been occurring in the river for multiple generations. Mitochondrial sequences show that LCT is successfully competing for spawning gravels; however continued stocking of LCT without the removal of the naturalized RBT will likely lead to a hybrid swarm. Triploidy was successfully identified in the hatchery supplied known triploids; despite high levels of stocking of trpRBT, less than 10% of the RBT sampled in the Truckee River were identified as triploid. The diploid RBT samples represent the naturalized RBT population in the river. The genetic population structure of the naturalized RBT was investigated using 11 microsatellite loci to look for potential RBT eradication units allowing for LCT reintroduction. Barriers along the Truckee River contribute to developing population structure, but these barriers are transient, and structure varies year to year. No clear eradication units or regions of the river to potentially isolate a translocated LCT population from RBT encroachment were identified. Six of the 11 microsatellites cross amplified and showed variation in LCT. Comparison of the LCT and HYB sampled in the Truckee River to the LCT strains stocked indicates that the Pilot Peak Strain of LCT has a higher survivorship in the Truckee River compared to the contemporary Pyramid Lake or Independence Lake strains. Reintroduction of LCT into the Truckee River is possible, but would require the eradication of the reproductive RBT and extensive monitoring to detect hybridization.