This thesis addresses the issue of hybridization between native westslope cutthroat trout (Oncorhynchus clarkii lewisi) and introduced rainbow trout (O. mykiss), giving strong consideration to their differing glacial refugia during the Wisconsin Glaciation. We hypothesize that having more recently derived from an anadromous form O. mykiss will possess life history characteristics more typical of a highly anadromous species. This hypothesis would also predict hybrids to be intermediate in these characteristics. In a comparison of growth rates and survivorship (Chapter 2) O. clarkii lewisi were found to employ a slower growing, longer lived strategy than O. mykiss, with hybrids typically being intermediate. Additionally, O. mykiss were also found to have aerobic and anaerobic metabolic capacities superior to O. clarkii lewisi in a first time comparison of these species (Chapter 3). These results support the glacial refuge hypothesis, but furthermore provide a potential explanation of the establishment of the elevational gradient commonly observed in hybridization studies. It would seem likely that O. mykiss would require more productive reaches being a faster growing, shorter lived species with higher metabolic costs. This study confirmed the gradient of O. mykiss persisting at lower elevations, trending through a hybrid zone to pure O. clarkii lewisi in headwater reaches and above migratory barriers (Chapter 2). A similar gradient was also reported when considering only the hybrid population, supporting the notion that habitat preference is under some genotypic control. The importance of migratory barriers was found to decrease with elevation suggesting potential additional limiting factors. Hybrid individuals were also found to be intermediate in morphological characteristics (Chapter 4). The confidence in differentiating between pure and non-pure O. clarkii lewisi was found to increase with the number of O. mykiss alleles (degree of hybridization) an individual possessed. Morphological-based identification was found to be an efficient, cost-friendly, preliminary assessment tool that could be useful in limiting the number of sites needing detailed genetic assessment. / 152 leaves : ill. (some col.) ; 29 cm.

In the Pacific Northwest ecoregion of North America, sculpins represent a major constituent of freshwater assemblages in coastal rivers. Based on their prevalence and abundance, sculpins are likely important ecologically, yet little is known of their interactions with co-occurring species, such as widely studied salmon and trout (salmonines). In this study, I evaluated inter- and intraspecific interactions involving cottids (Cottus sp.) and coastal cutthroat trout (Oncorhynchus clarkii clarkii). I used a response surface experimental design to independently evaluate effects of cutthroat trout and sculpin biomass on growth and behavior. There was evidence of both intra- and interspecific interactions between cutthroat trout and sculpins, but the interactions were asymmetrical with biomass of cutthroat trout driving both intra- and interspecific interactions, whereas sculpins had little influence overall. Cutthroat trout biomass was positively related to conspecific aggressive interactions and negatively related to growth. Sculpin exhibited increased use of cover during the day in response to greater biomass of cutthroat trout, but not sculpin biomass. Nocturnal use of cover by sculpins was unaffected by biomass of either species. This experiment provides insights into the species interactions and the mechanisms that may allow sculpins and salmonines to coexist in nature. As cutthroat trout appear to be superior competitors, coexistence between sculpins and cutthroat trout may depend on some form of refuge, either in the form of in-stream cover or crypsis coupled with diel resource segregation. Cutthroat trout are usually active during the day, indicating that nocturnal foraging by sculpins may in part represent a behavior that minimizes interspecific competition with cutthroat trout. / Graduation date: 2012

Rainbow trout lactate dehydrogenase (LDH) B2' B2' , B2' B2" and B2" B2" phenotypes were tested under a variety of conditions for swimming endurance, oxygen consumption rates, and blood pH, p02, and lactate. The B2" B2" phenotype exhibited a lower swimming endurance, under a limited oxygen supply, than the B2' B2' and B2' B2" phenotypes. Blood pH at fatigue did not differ among the LOH phenotypes that swam under low oxygen conditions (2 mg/1). Blood p02 at fatigue was significantly higher for females than for males under low oxygen conditions. Oxygen consumption rates were significantly higher for males than for females at dissolved oxygen concentrations of 3 mg/1 to 8 mg/1. The depression in oxygen consumption rates at low oxygen levels was greater for males than for females. The increase in blood lactate with swimming under low oxygen concentrations and static exposure to low oxygen depended on LOH phenotype. The adaptive significance of the LDH B2' and B2" alleles was discussed. Thirteen populations of Intermountain cutthroat trout were sampled for biochemical genetic variation. Variation was observed in muscle aspartate aminotransferase for all populations which would fit a genetic model of two loci and three alleles. Unique muscle A group lactate dehydrogenase variation was observed in Snake Valley cutthroat trout which suggested the presence of several variant alleles. Allele frequencies for tetrazolium oxidase changed during three consecutive years in Bear Lake cutthroat trout that were trapped and spawned artificially. The possible causes of those changes were discussed. No variation was observed in alphaglycerophosphate dehydrogenase, isocitrate dehydrogenase, eye lactate dehydrogenase or muscle malate dehydrogenase in cutthroat trout.

A key component to conservation is an accurate understanding of genetic subdivision within a species. Despite their ecological and economic importance, relatively little is understood about the genetic structuring of Yellowstone cutthroat trout in Yellowstone National Park. Here, we use traditional (Fst, Rst, Nm, and AMOVA) and modern (Bayesian assignment tests, coalescent theory, and nested clade analysis) analytical approaches to describe the population genetic subdivision of cutthroat trout spawning populations in Yellowstone Lake and to identify genetically distinct population segments throughout Yellowstone National Park. Evidence for restricted gene flow between spawning populations within Yellowstone Lake was detected using nested clade analysis. This is the first molecular evidence for restricted gene flow between spawning populations in Yellowstone Lake. In contrast, traditional methods such as Fst and Rst as well as the Bayesian clustering program STRUCTURE v2.0 failed to detect evidence for restricted gene flow. Across our sampling range within Yellowstone National Park, eleven genetically distinct cutthroat trout population segments were detected. These showed a general pattern of small, isolated populations with low genetic diversity in headwater streams and wide-spread, genetically diverse populations in higher-order rivers. We recommend populations be managed to maintain current levels of genetic diversity and gene flow. Based on the recent decline of and distinct morphological, behavioral, and genetic nature of cutthroat trout in Yellowstone Lake, we recommend the Yellowstone Lake spawning populations collectively be recognized as an evolutionarily significant unit.

Visual estimation techniques were used to quantify habitat characteristics, habitat type (pool, riffle) use and longitudinal distribution of steelhead (Oncorhynchus mykiss), cutthroat trout (0. clarki), and coho salmon (0. kisutch) in spring, summer and fall in 8.8 km of Cummins Creek, a basin in the central coast of Oregon. Fish were distributed significantly different than habitat type availability in most samples. Pool habitats contained a disproportionate percent of the salmonid assemblage and 1+ fish in each sample, and the percentage of fish in pools increased as flow decreased. In spring, coho salmon fry were concentrated in side channels and valley floor tributary habitats. Large woody debris formed 57- 68% of pool habitats and was significantly correlated with pool volume, maximum pool depth, slow surface velocity in pools, and pieces of small woody debris. Longitudinal distribution of the salmonid assemblage did not differ from habitat distribution seasonally or between years, even though certain species differed Coho salmon and cutthroat trout were distributed in proportion to longitudinal habitat availability only when fish abundance was relatively high and streamflow was low. In most samples, both 0+ and 1+ steelhead were distributed in proportion to longitudinal habitat availability. Differences in coho salmon abundance between years appeared to influence longitudinal distribution of each species and age class. Certain reaches had consistent numbers of fish between years while the number of fish in other reaches varied widely. In most samples, reaches with highest abundance for steelhead were in the lower basin, cutthroat trout in the upper basin and coho salmon between the two other species. Timing of reduction in number of fish varied among species. Fifty-five percent of 0+ steelhead and 73% of 1+ steelhead lost between August 1988 and April 1989 were lost between August and October during low flow conditions. However, only 18% of the losses, for 0+ coho salmon, occurred between August and October with the remaining losses occurring after October. This study illustrates that habitat availability is not a good index of fish distribution when fish abundance is low, and it highlights the importance of habitat in the lower portions of basins when fish abundance is high. It also demonstrates that the basin wide distribution of salmonids varies among species, age classes, seasons, and years and suggests that our understanding of salmonid distribution and abundance could be greatly enhanced by adopting a basin-wide, community, and seasonal perspective. In addition, the methods used in this study offer one way to assess the seasonal distribution and abundance of salmonids in a relatively quick, inexpensive, and non-destructive manner. / Graduation date: 1994

Selenium (Se) is an essential element, toxic at concentrations only slightly above those required. It can become detrimental to fish health when available in excess in the aquatic environment. This study examined the effects of Se on adult female rainbow trout, brook trout, and cutthroat trout fed Se-methionine for five months, and on their larvae. Differences between fish species and between tissues were observed in the accumulation of Se in adults, and dose-dependent effects of Se on plasma thyroid hormone concentrations in brook trout, gonad size in all three species, and indicators of oxidative stress in rainbow trout and cutthroat trout, were detected. Survival of larvae and swim-up success decreased with increasing egg Se concentrations in rainbow trout and cutthroat trout, however, the same effects were not observed in brook trout. Data from this study provide new evidence for species-specific Se accumulation and toxicity in salmonid fish. / xi, 123 leaves : ill. ; 29 cm

Swimming performance strongly influences fitness in aquatic organisms and is closely tied to external body morphology. Although this connection has been closely examined at the individual and species level, few studies have focused on this relationship as it pertains to functional group assemblages. Using functional groups based on similarities in habitat use and morphology, I tested the hypothesis that swimming performance can be reliably predicted by functional group composition. I measured swimming performance as burst speed using a simulated predator attack and as prolonged speed using a step-endurance test in a laboratory flume. I measured morphology using geometric morphometric techniques. A difference in swimming behavior in four of the seven species was observed in the step-endurance test. Benthic species exhibited bracing behavior as an alternative to body-caudal fin (BCF) propulsion in the prolonged speed trials. Swimming performance exhibited a weak relationship with functional groups based on habitat or morphology. Rather a species-based model was the best predictor of swimming performance. Although species exhibited variation in swimming performance, body size was the strongest predictor of absolute swimming performance across all models. Relative swimming performance (measured in body lengths/sec) was negatively related to body size. The results of this study suggest that functional groups are not always reliable predictors of performance and they necessitate empirical testing to validate their effectiveness. This study also provides critical swimming performance data for previously unstudied Great Basin fishes which could be valuable for predicting fish passage through culverts, weirs and fish ladders.