No abstract

Migratory bull trout (Salvelinus confluentus) historically spawned in tributaries of the Clark Fork River, Montana and inhabited Lake Pend Oreille as subadult and adult fish. However, in 1952 Cabinet Gorge Dam was constructed without fish passage facilities disrupting the connectivity of this system. Since the construction of this dam, bull trout populations in upstream tributaries have been in decline. Each year adult bull trout return to the base of Cabinet Gorge Dam when most migratory bull trout begin their spawning migration. However, the origin of these fish is uncertain. We used eight microsatellite loci to compare bull trout collected at the base of Cabinet Gorge Dam to fish sampled from both above and further downstream from the dam. Our data indicate that Cabinet Gorge bull trout are most likely individuals that hatched in above-dam tributaries, reared in Lake Pend Oreille, and could not return to their natal tributaries to spawn. This suggests that the risk of outbreeding depression associated with passing adults over dams in the Clark Fork system is minimal compared to the potential genetic and demographic benefits to populations located above the dams.

There are currently no policy guidelines for treating hybrids under the U.S. Endangered Species Act (ESA). We considered the scientific basis for determining whether hybridized populations should be included as part of the westslope cutthroat trout (Oncorhynchus clarki lewisi) unit considered for listing under the ESA. Westslope cutthroat trout are threatened by genomic extinction because of widespread introgressive hybridization with introduced rainbow trout (O. mykiss) and Yellowstone cutthroat trout (O. c. bouvieri). Experimental results suggest that first-generation hybrids between westslope cutthroat trout and rainbow trout have reduced fitness. However, hybridization may spread even when hybrids have severely reduced fitness because the production of hybrids is unidirectional-that is, all the progeny of a hybrid will be hybrids. In addition, heterosis resulting from the sheltering of deleterious recessive alleles in early-generation hybrids may increase the effective rate of introgression. However, such short-term increases in fitness may disrupt important long-term adaptations of native populations. The loss of these adaptations will be difficult to detect because some local adaptations might only be apparent during periodic episodes of extreme environmental conditions, such as winter storms, drought, or fire. Thus, rapid spread of hybridization could result in the loss of local adaptations in native populations of westslope cutthroat trout and decrease their probability of long-term persistence. Protection of populations with some admixture would protect sources of spreading hybridization. Treatment of hybrids in conservation planning depends primarily on the amount of evolutionary divergence between the hybridizing taxa and the geographical extent of introgression. We recommend that only nonhybridized populations be included as westslope cutthroat trout in the unit to be considered for listing. Populations of unknown status should be protected until more information about these populations becomes available. Entrecruzas y el Acta de Especies en Peligro de E. U. A.: Inclusión de Poblaciones Híbridas de Oncorhynchus clarki lewisi Resumen: Actualmente no hay lineamientos políticos para el tratamiento de híbridos bajo el Acta de Especies en Peligro. Consideramos las bases científicas para determinar si las poblaciones híbridas deben ser incluidas como parte de la unidad de Oncorhynchus clarki lewisi considerada para ser enlistada en el Acta de Especies en Peligro. Oncorhynchus clarki lewisi está amenazada de extinction genómica debido a hibridación introgresiva generalizada con la trucha arco iris introducida (O. mykiss) y O. c. bouvieri. Resultados experimentales sugieren que los híbridos de primera generación entre O. clarki lewisi y O. mykiss tienen adaptabilidad reducida. Sin embargo, la hibridación puede extenderse aun cuando los híbridos tienen adaptabilidad severamente reducida, porque la producción de híbridos es unidireccional (esto es, toda la progenie de un híbrido serán híbridos). Adicionalmente,...

Ecological and life history characteristics such as population size, dispersal pattern, and mating system mediate the influence of genetic drift and gene flow on population subdivision. Bull trout (Salvelinus confluentus) and mountain whitefish (Prosopium williamsoni) differ markedly in spawning location, population size and mating system. Based on these differences, we predicted that bull trout would have reduced genetic variation within and greater differentiation among populations compared with mountain whitefish. To test this hypothesis, we used microsatellite markers to determine patterns of genetic divergence for each species in the Clark Fork River, Montana, USA. As predicted, bull trout had a much greater proportion of genetic variation partitioned among populations than mountain whitefish. Among all sites, FST was seven times greater for bull trout (FST = 0.304 for bull trout, 0.042 for mountain whitefish. After removing genetically differentiated high mountain lake sites for each species FST, was 10 times greater for bull trout (FST = 0.176 for bull trout; FST = 0.018 for mountain whitefish). The same characteristics that affect dispersal patterns in these species also lead to predictions about the amount and scale of adaptive divergence among populations. We provide a theoretical framework that incorporates variation in ecological and life history factors, neutral divergence, and adaptive divergence to interpret how neutral and adaptive divergence might be correlates of ecological and life history factors.

Introgressive hybridization threatens the persistence of several species of native salmonids in the western United States, but little is known about the factors influencing the establishment and maintenance of introgressed populations. We examined the occurrence of introgressive hybridization in westslope cutthroat (Oncorhynchus clarki lewisi) and rainbow trout (O. mykiss) populations in relation to physical characteristics of streams, trout density estimates, and the distance from stocking source. Trout were sampled from 80 stream sites in the Clearwater River Basin, Idaho, USA, and tissues from individual trout were analyzed to detect hybridization using noncoding sequences of nuclear DNA. We found a broad zone of hybridization detected at 64% of the sampled sites. The presence and degree of introgression was negatively related to elevation and positively related to stream width in our logistic regression model. Stream elevation and size likely influence hydrologic and thermal regimes. An interaction between the life history characteristics of the native and nonnative trout with these hydrologic and thermal stream gradients could explain the invasion success of rainbow trout and hence, the extent of the hybrid zone. Alternatively, the influence of elevation and stream width could be the result of habitat selection by the parental species, thereby reducing the opportunity for hybridization. Understanding the relationship between abiotic factors and introgressive hybridization will assist fisheries managers when evaluating the potential threat of introgression in different stream habitats and applying the necessary management actions to conserve the native cutthroat trout genotypes across broad landscapes. Corresponding Editor: K. D. Fausch