To describe the life histories and demography of a fluvial population of Bull Trout Salvelinus confluentus, we PIT-tagged and radio-tagged Bull Trout captured in Mill Creek, a tributary of the Walla Walla River (Washington-Oregon), during 1998-2009. Adult abundance declined 63% during 2006-2010, driven primarily by a 10-fold reduction in subadult-to-adult returns. Larger subadults and fall-winter emigrants survived at higher rates, but they were a small proportion of the subadult migrants. The survival rates of larger, generally older adults were also more than 40% greater than those of smaller adults. Changes in abundance influenced other characteristics of the population. For example, adult upstream movement into spawning areas during 1999-2005 peaked in late July, whereas the smaller runs observed during 2006-2010 peaked in early September, and the relationship between fish size and migration timing shifted. Unlike many adfluvial populations, more than 90% of the adults in Mill Creek spawned annually. Bull Trout that spawned in main-stem Mill Creek were primarily larger migratory adults; however, about 20% of the large adults were strictly or intermittently resident, remaining in the spawning area year-round. The downstream extent of individuals' migratory distributions varied greatly-from just downstream of the spawning area to the mouth of the Walla Walla River and potentially hundreds of kilometers into the Columbia River. Despite a large sample size of radio-tagged fish, radiotelemetry substantially underestimated the distribution and range that were evident from PIT tag detections. Life history terms such as "migratory," "resident," and "fluvial" and their associations with body size, movement, and distribution are useful for describing general patterns, but they fail to reflect the diversity and complexity within and among populations. For Bull Trout in Mill Creek, that life history diversity, including small, resident adult forms in the tributaries and a continuum of distribution for large adults, maximizes the use of available habitat and likely contributes to the population's persistence.

Understanding thermal habitat use by migratory fish has been limited by difficulties in matching fish locations with water temperatures. To describe spatial and temporal patterns of thermal habitat use by migratory adult bull trout, Salvelinus confluentus, that spawn in the Lostine River, Oregon, we employed a combination of archival temperature tags, radio tags, and thermographs. We also compared temperatures of the tagged fish to ambient water temperatures to determine if the fish were using thermal refuges. The timing and temperatures at which fish moved upstream from overwintering areas to spawning locations varied considerably among individuals. The annual maximum 7-day average daily maximum (7DADM) temperatures of tagged fish were 16-18°C and potentially as high as 21°C. Maximum 7DADM ambient water temperatures within the range of tagged fish during summer were 18-25°C. However, there was no evidence of the tagged fish using localized cold water refuges. Tagged fish appeared to spawn at 7DADM temperatures of 7-14°C. Maximum 7DADM temperatures of tagged fish and ambient temperatures at the onset of the spawning period in late August were 11-18°C. Water temperatures in most of the upper Lostine River used for spawning and rearing appear to be largely natural since there has been little development, whereas downstream reaches used by migratory bull trout are heavily diverted for irrigation. Although the population effects of these temperatures are unknown, summer temperatures and the higher temperatures observed for spawning fish appear to be at or above the upper range of suitability reported for the species.

From 1997 to 2004, we used radio telemetry to investigate movement and distribution patterns of 206 adult fluvial bull trout (mean, 449 mm FL) from watersheds representing a wide range of habitat conditions in northeastern Oregon and southwestern Washington, a region for which there was little previous information about this species. Migrations between spawning and wintering locations were longest for fish from the Imnaha River (median, 89 km) and three Grande Ronde River tributaries, the Wenaha (56 km) and Lostine (41 km) rivers and Lookingglass Creek (47 km). Shorter migrations were observed in the John Day (8 km), Walla Walla (20 km) and Umatilla river (22 km) systems, where relatively extensive human alterations of the riverscape have been reported. From November through May, fish displayed station-keeping behavior within a narrow range (basin medians, 0.5–6.2 km). Prespawning migrations began after snowmelt-driven peak discharge and coincided with declining flows. Most postspawning migrations began by late September. Migration rates of individuals ranged from 0.1 to 10.7 km/day. Adults migrated to spawning grounds in consecutive years and displayed strong fidelity to previous spawning areas and winter locations. In the Grande Ronde River basin, most fish displayed an unusual fluvial pattern: After exiting the spawning tributary and entering a main stem river, individuals moved upstream to wintering habitat, often a substantial distance (maximum, 49 km). Our work provides additional evidence of a strong migratory capacity in fluvial bull trout, but the short migrations we observed suggest adult fluvial migration may be restricted in basins with substantial anthropogenic habitat alteration. More research into bull trout ecology in large river habitats is needed to improve our understanding of how adults establish migration patterns, what factors influence adult spatial distribution in winter, and how managers can protect and enhance fluvial populations.

The use of redd counts to monitor abundance and trend of bull trout Salvelinus confluentus has been clouded by uncertainties concerning measurement error, life history variation, and correspondence of redd counts to adult population size. We compared census redd counts with population estimates of mature females for a migratory fluvial population of bull trout (primarily ≥ 300 mm fork length) and for a population of small (<200 mm), likely resident, bull trout. We also compared the measurement error of the experienced surveyors who conducted the redd counts to that of a group of inexperienced surveyors. Although the regression of redd counts on adult females for the migratory population was statistically significant, a large proportion of the variation in the relationship was unexplained (r 2 = 0.47). Despite that variation, redd counts accurately reflected a greater than 50% decline in the population over 10 years; however, 5-year trends in redd counts could be misleading. Power analysis parameterized by using the variation in the number of females per redd and measurement error of experienced surveyors indicated that minimum declines of 44-56% or increases of 78-118% over 10-15 years would be necessary for detection using traditional statistical criteria. Geometric mean abundance of migratory adults derived from redd counts and adults-per-redd values from the present study and published averages were similar to measured adult numbers in most cases. For both migratory and resident populations, redd counts by experienced surveyors were substantially more accurate and precise than those by inexperienced surveyors. Counts of migratory bull trout redds were more accurate and precise than counts of resident bull trout redds, which were significantly smaller and consistently underestimated. Thus, bull trout redd counts can be used to estimate abundance levels and to detect substantial longer-term changes in abundance, particularly for migratory populations. However, the reliability of the counts depends on the skill of the surveyors.

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