Oligochaetes, triactinomyxons (TAMs), and age-0 trout were sampled in the upper Cache la Poudre River, Colorado, to determine the distribution of Myxobolus cerebralis during 1997 and 1998. Densities of the intermediate host, the oligochaete Tubifex tubifex, were 3.5 orders of magnitude higher in the M. cerebralis-infected Poudre Rearing Unit (PRU) trout rearing ponds than at any of the river sampling reaches. Oligochaetes, including T. tubifex, were rare in the river (1 oligochaete m -2 ), except in a few stream side alcoves and eddies (50 oligochaete m -2 ). Species composition of oligochaetes in the river reaches was more diverse than in the PRU. Tubifex tubifex constituted 50% or less of the oligochaete community in the river and 98% in the PRU. Infection rates of T. tubifex were 1% in the area above the PRU, 2% in the PRU, and 6% below the PRU. An increased M. cerebralis intensity of infection in age-0 trout below the PRU could not be attributed entirely to the high numbers of TAMs in its effluent (3.7 TAMs l -1 ). Low densities of TAMs ranging from 0 to 0.2 TAMs l -1 were found in the river reaches, yet nearly all of the age-0 trout were infected soon after emergence. This suggests that very few TAMs, as measured by filtration, need be present in the water column to bring about infection in the majority of trout present. This also indicates that the parasite can persist and potentially cause reduced juvenile trout recruitment in cold, oligotrophic, sediment poor, highgradient streams. KEY WORDS: Salmonid whirling disease · Myxobolus cerebralis · Tubifex tubifex · Trout rearing unit Resale or republication not permitted without written consent of the publisherDis Aquat Org 49: [51][52][53][54][55][56][57][58][59][60] 2002 Wildlife, pers. comm.). Rainbow trout populations in at least 370 km of stream have been affected .Myxobolus cerebralis was first described in Germany in 1893 (Hoffman 1990). Though it is believed to be endemic to Europe , it was not detected until rainbow trout, imported from the USA to a German trout rearing facility, displayed classic WD signs (Hoffman 1990). The mechanism of infection remained unknown for almost a century until Markiw & Wolf (1983) found that tubificid worms were a necessary alternate host for the parasite. Antigenic and genetic evidence has since confirmed the relationship between triactinomyxons (TAMs), which are the waterborne, fish-infective stage of the parasite, and the myxospore, which infects Tubifex tubifex (Markiw 1989, Andree et al. 1997. The oligochaete host for M. cerebralis is apparently restricted to T. tubifex; attempts to infect other oligochaete species with M. cerebralis myxospores have been unsuccessful (Wolf et al. 1986, El-Matbouli & Hoffman 1989, Hedrick et al. 1996.Myxobolus cerebralis was first detected in the Cache la Poudre River in 1988 (Nehring 2000). Drastic declines in the number of age-1 wild rainbow trout became evident in the early 1990s at 4 different monitoring stations in a 30 km reach of the river. In 1995, 5 yr after ...

Host-parasite interactions can drive rapid, reciprocal genetic changes (coevolution), provided both hosts and parasites have high heritabilities for resistance/infectivity. Similarly, the host’s mating system should also affect the rate of coevolutionary change in host-parasite interactions. Using experimental coevolution, we determined the effect of obligate outcrossing verses partial self-fertilization (mixed mating) on the rate of evolutionary change in a nematode host (Caenorhabditis elegans) and its bacterial parasite (Serratia marcescens). Bacterial populations were derived from a common ancestor. We measured the effects of host mating system on host adaptation to the parasite. We then determined the extent of parasite adaptation to their local host populations. Obligately outcrossing hosts exhibited more rapid adaptation to parasites than did mixed mating hosts. In addition, most of the parasites became adapted to infecting their “local” hosts; but parasites from obligately outcrossing hosts showed a greater level of local adaptation. These results suggest that host populations evolved along separate trajectories, and that outcrossing host populations diverged further than partially selfing populations. Finally, parasites “tracking” outcrossing host populations diverged further than parasites tracking the partially selfing host populations. These results show that the evolutionary trajectories of both hosts and parasites can be shaped by the host’s mating system.

Condit Dam, at river kilometer 5.3 on the White Salmon River, Washington, was breached in 2011 and completely removed in 2012. This action opened habitat to migratory fish for the first time in 100 years. The White Salmon Working Group was formed to create plans for fish salvage in preparation for fish recolonization and to prescribe the actions necessary to restore anadromous salmonid populations in the White Salmon River after Condit Dam removal. Studies conducted by work group members and others served to inform management decisions. Management options for individual species were considered, including natural recolonization, introduction of a neighboring stock, hatchery supplementation, and monitoring natural recolonization for some time period to assess the need for hatchery supplementation. Monitoring to date indicates that multiple species and stocks of anadromous salmonids are finding and spawning in the now accessible and recovering habitat.

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Restoration of access to lost habitat for threatened and endangered fishes above currently impassable dams represents a major undertaking. Biological monitoring is critical to understand the dynamics and success of anadromous recolonization as, in the case of Oncorhynchus mykiss, anadromous steelhead populations are reconnected with their conspecific resident rainbow trout counterparts. We evaluate three river systems in the Lower Columbia River basin: the White Salmon, Sandy, and Lewis rivers that are in the process of removing and/or providing passage around existing human-made barriers in O. mykiss riverine habitat. In these instances, now isolated resident rainbow trout populations will be exposed to competition and/or genetic introgression with steelhead and vice versa. Our genetic analyses of 2,158 fish using 13 DNA microsatellite (mSAT) loci indicated that within each basin anadromous O. mykiss were genetically distinct from and significantly more diverse than their resident above-dam trout counterparts. Above long-standing natural impassable barriers, each of these watersheds also harbors unique rainbow trout gene pools with reduced levels of genetic diversity. Despite frequent releases of non-native steelhead and rainbow trout in each river, hatchery releases do not appear to have had a significant genetic effect on the population structure of O. mykiss in any of these watersheds. Simulation results suggest there is a high likelihood of identifying anadromous x resident individuals in the Lewis and White Salmon rivers, and slightly less so in the Sandy River. These genetic data are a prerequisite for informed monitoring, managing, and conserving the different life history forms during upstream recolonization when sympatry of life history forms of O. mykiss is restored.