We assessed infection prevalence and intensity by the ectoparasitic copepod Salmincola californiensis among salmonid species rearing in reservoirs and streams upstream of reservoirs in the Willamette River basin, Oregon, during 2012 and 2013. Infection levels of juvenile Chinook Salmon Oncorhynchus tshawytscha, Rainbow Trout O. mykiss, and Cutthroat Trout O. clarkii were greater in reservoirs than in streams and increased with the age and size of fish. Copepods were more likely to be attached within the brachial cavity of reservoir fish (79%), whereas fins were the most common attachment site on stream fish (71%). Chinook Salmon in reservoirs were more vulnerable to infection than other species. Age-0 Chinook Salmon in reservoirs showed increasing infection prevalence throughout the year, reaching 84% by fall (compared with 11% in streams). Infection intensity was greater for age-0 Chinook Salmon in reservoirs than for those in streams. Infection prevalence for reservoirrearing Rainbow Trout was < 1% at age 0, 22% at age 1, 36% at age 2, and 38% at age 3. Intensity was low for age-1 Rainbow Trout and increased for age-2 and age-3 fish. Infection prevalence for reservoir-rearing Cutthroat Trout collected in spring (39%) was greater than for those rearing in streams (4.5%). Juvenile kokanee O. nerka were only present in reservoirs and were rarely infected with copepods. The lack of water current in reservoirs may increase the likelihood of infection in the brachial cavity. Greater infection levels observed for juvenile Chinook Salmon compared with the other species in reservoirs may be a function of behavioral, physiological, and habitat differences. We concluded that copepod infection in reservoirs reached levels that could decrease the fitness and survival of Chinook Salmon smolts, potentially hampering conservation and recovery efforts.

Complex predator-prey interactions over time have the potential to limit survival of threatened native species. Reservoirs created by large dams in temperate ecosystems are sites where both coldwater and warmwater fish species overlap in distributions, forming assemblages that would not occur under natural settings. For example, in many western North American reservoirs, juvenile native salmonids now overlap with native and non-native predators such as Northern Pikeminnow Ptychocheilus oregonensis and bass Micropterus spp. Currently, native Northern Pikeminnow are considered by many to be the most formidable predator of salmon smolts in freshwater systems of the Pacific Northwest. However, their consumption of salmon fry and the role of non-native warmwater predators remain unclear. Predation on fry has proved more difficult to identify than on smolts, due to smaller sizes and high digestibility, but is important for prioritizing management strategies. Here, we use multiple lines of evidence, including large datasets of stable isotopes and fish stomach contents, to identify which fish consume Chinook Salmon Oncorhynchus tshawytscha fry (<55 mm) as they enter reservoirs below their spawning grounds in the Middle Fork Willamette River, Oregon. Unexpectedly, we found that non-native warmwater game fishes, including bass and crappie Pomoxis spp., preyed more heavily on Chinook Salmon fry in the spring than native fish predators including Northern Pikeminnow. Of the native species, only Rainbow Trout Oncorhynchus mykiss showed evidence for significant predation on Chinook Salmon fry. Fish management in reservoirs that attempts to promote both warmwater and coldwater recreational fisheries simultaneously may thus be in conflict, necessitating future prioritization of species composition in individual reservoirs. This prioritization is especially important as these species include popular game fishes as well as salmonids threatened under the U.S. Endangered Species Act. Understanding predation pressure by popular invasive game fishes on sensitive life stages will allow managers and policymakers to evaluate trade-offs in the management of these novel assemblages.

Smolts of spring Chinook salmon Oncorhynchus tshawytscha experience substantial mortality while migrating through free-flowing reaches of the Snake River basin before reaching Lower Granite Dam, the first dam encountered in the Columbia-Snake river hydrosystem. We investigated the patterns of travel time and survival of hatchery and natural smolts fitted with passive integrated transponder (PIT) tags through specific reaches of the migration corridor during the 2000-2006 migration years for two populations originating in the Grande Ronde River basin (Lostine River and Catherine Creek). For both populations, median travel times for natural smolts were significantly longer in the upper reaches of the migration corridor but shorter in the lower reaches than for their hatchery counterparts. Also, among both hatchery and natural smolts, smaller individuals spent more time in the upper reaches, presumably feeding to attain a larger size before continuing their migration. Within populations, both hatchery and natural smolts showed similar patterns of survival through the reaches of the migration corridor above Lower Granite Dam. Size-selective mortality was evident for hatchery and natural smolts from both populations, especially in the upper reaches, larger individuals experiencing higher survival. The Catherine Creek population experienced the majority of natural and hatchery smolt mortality (32.8-65.8%) in a relatively short (91-km), low-gradient reach immediately below the summer rearing habitat. In contrast, the Lostine River natural and hatchery smolts experienced lower mortality (3.6-46.1%) in a 174-km reach below the summer rearing habitat. The results of this study demonstrate the dynamic nature of survival and migration rate among spring Chinook salmon smolts during their initial seaward migration from tributaries.

Through spawning ground and snorkel surveys, we confirmed the presence of adfluvial spring Chinook Salmon Oncorhynchus tshawytscha in a tributary upstream from a high‐head dam in the upper Willamette River in northwest Oregon. Spring Chinook Salmon previously had been extirpated above the dam but juvenile hatchery fish were released in the reservoir in subsequent years. In 2012, we recovered six carcasses of adfluvial Chinook Salmon adults, identified nine live adults, and recorded nine redds. Analyses of scales from carcasses revealed those fish were ages 5–6. Otolith microchemistry from an unmarked adult female Chinook Salmon did not indicate ocean residence, and no hatchery thermal marks were observed, suggesting this fish was the progeny of adfluvial adults. In 2013, we observed one live, unclipped adult and three juvenile Chinook Salmon. We conclude that adfluvial spring Chinook Salmon exist in Green Peter Reservoir and successfully reproduce. This is the first documentation of adfluvial Chinook Salmon in Oregon, and this unusual life history should be considered in the context of research, monitoring, and recovery actions pertaining to ongoing reintroduction programs for threatened Willamette River spring Chinook Salmon above dams. Received February 21, 2014; accepted May 7, 2014