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Species with large geographic distributions often exhibit complex patterns of diversity that can be further complicated by human activities. Cutthroat trout (Oncorhynchus clarkii) are one of the most widely distributed freshwater fish species in western North America exhibiting substantial phenotypic and genetic variability; however, fish stocking practices have translocated populations outside of their native range and may have obscured intraspecific boundaries. This study focuses on cutthroat trout populations representing three distinct evolutionary clades that are found intermixed within a contact zone between the Bonneville and upper Snake River watersheds in the western United States. We used mitochondrial and microsatellite genetic data, as well as historical stocking records, to evaluate whether populations of cutthroat trout in the contact zone are native or are introduced. We found significant genetic differentiation and fine-scale genetic population structure that was organized primarily by watershed boundaries. While we detected increased genetic diversity in some areas in close proximity to the greatest number of stocking events, the highly organized population structure both within and between areas of the contact zone indicates that the populations are native to the watersheds. Intermixing of distinct evolutionary lineages of cutthroat trout appears to be the result of historical connections between paleodrainages. Our analyses provide a context for understanding how genetic data can be used to assess the status of populations as native or introduced., Competing Interests: The authors have declared that no competing interests exist.

Background: For wide-ranging species, intraspecific variation can occur as a result of reproductive isolation from local adaptive differences or from physical barriers to movement. Cutthroat trout (Oncorhynchus clarkii), a widely distributed fish species from North America, has been divided into numerous putative subspecies largely based on its isolation in different watersheds. In this study, we examined mtDNA sequence variation of cutthroat trout to determine the major phylogenetic lineages of this polytypic species. We use these data as a means of testing whether geographic isolation by watershed boundaries can be a primary factor organizing intraspecific diversification., Results: We collected cutthroat trout from locations spanning almost the entire geographic range of this species and included samples from all major subspecies of cutthroat trout. Based on our analyses, we reveal eight major lineages of cutthroat trout, six of which correspond to subspecific taxonomy commonly used to describe intraspecific variation in this species. The Bonneville cutthroat trout (O. c. utah) and Yellowstone cutthroat trout (O. c. bouvieri) did not form separate monophyletic lineages, but instead formed an intermixed clade. We also document the geographic distribution of a Great Basin lineage of cutthroat trout; a group typically defined as Bonneville cutthroat trout, but it appears more closely related to the Colorado River lineage of cutthroat trout., Conclusion: Our study indicates that watershed boundaries can be an organizing factor isolating genetic diversity in fishes; however, historical connections between watersheds can also influence the template of isolation. Widely distributed species, like cutthroat trout, offer an opportunity to assess where historic watershed connections may have existed, and help explain the current distribution of biological diversity across a landscape.

Prioritization of efforts to maintain biodiversity is an important component of conservation, but is more often applied to ecosystems or species than within species. We assessed distinctiveness among 27 populations of rainbow trout (Salmonidae: Oncorhynchus mykiss) from British Columbia, Canada, using microsatellite DNA variation (representing historical or contemporary demography) and morphology (representing adaptive variation). Standardized genetic scores, that is, the average deviation across individuals within populations from the overall genetic score generated by factorial correspondence analysis, ranged from 1.05 to 4.90 among populations. Similar standardized morphological scores, generated by principal components analysis, ranged from 1.19 to 5.35. There was little correlation between genetic and morphological distinctiveness across populations, although one population was genetically and morphologically the most distinctive. There was, however, a significant correlation (r = 0.26, P = 0.008) between microsatellite (F ST) and morphological (P ST) divergence. We combined measures of allelic richness, genetic variation within, and divergence among, populations and morphological variation to provide a conservation ranking of populations. Our approach can be combined with other measures of biodiversity value (habitat, rarity, human uses, threat status) to rationalize the prioritization of populations, especially for widespread species where geographic isolation across distinct environments promotes intraspecific variability.

Redband trout (Oncorhynchus mykiss ssp.) in southeastern Oregon inhabit high-elevation streams that exhibit extreme variability in seasonal flow and diel water temperature. Given the strong influence and potential limitations exerted by temperature on fish physiology, we were interested in how acute temperature change and thermal history influenced the physiological capabilities and biochemical characteristics of these trout. To this end, we studied wild redband trout inhabiting two streams with different thermal profiles by measuring (1) critical swimming speed (U(crit)) and oxygen consumption in the field at 12 degrees and 24 degrees C; (2) biochemical indices of energy metabolism in the heart, axial white skeletal muscle, and blood; and (3) temperature preference in a laboratory thermal gradient. Further, we also examined genetic and morphological characteristics of fish from these two streams. At 12 degrees C, maximum metabolic rate (Mo2max) and metabolic power were greater in Little Blitzen redband trout as compared with those from Bridge Creek (by 37% and 32%, respectively). Conversely, Bridge Creek and Little Blitzen trout had similar values for Mo2max and metabolic power at 24 degrees C. The U(crit) of Little Blitzen trout was similar at the two temperatures (61+/-3 vs. 57+/-4 cm s(-1)). However, the U(crit) for Bridge Creek trout increased from 62+/-3 cm s(-1) to 75+/-3 cm s(-1) when water temperature was raised from 12 degrees to 24 degrees C, and the U(crit) value at 24 degrees C was significantly greater than for Little Blitzen fish. Cost of transport was lower for Bridge Creek trout at both 12 degrees and 24 degrees C, indicating that these trout swim more efficiently than those from the Little Blitzen. Possible explanations for the greater metabolic power of Little Blitzen redband trout at 12 degrees C include increased relative ventricular mass (27%) and an elevation in epaxial white muscle citrate synthase activity (by 72%). Bridge Creek trout had 50% higher lactate dehydrogenase activity in white muscle and presumably a greater potential for anaerobic metabolism. Both populations exhibited a preferred temperature of approximately 13 degrees C and identical mitochondrial haplotypes and p53 gene allele frequencies. However, Bridge Creek trout had a more robust body form, with a relatively larger head and a deeper body and caudal peduncle. In summary, despite the short distance ( approximately 10 km) and genotypic similarity between study streams, our results indicate that phenotypic reorganization of anatomical characteristics, swimming ability at environmentally pertinent temperatures and white axial muscle ATP-producing pathways occurs in redband trout.