Density‐dependent processes have repeatedly been shown to have a central role in salmonid population dynamics, but are often assumed to be negligible for populations at low abundances relative to historical records. Density dependence has been observed in overall spring/summer Snake River Chinook salmon Oncorhynchus tshawytscha production, but it is not clear how patterns observed at the aggregate level relate to individual populations within the basin. We used a Bayesian hierarchical modelling approach to explore the degree of density dependence in juvenile production for nine Idaho populations. Our results indicate that density dependence is ubiquitous, although its strength varies between populations. We also investigated the processes driving the population‐level pattern and found density‐dependent growth and mortality present for both common life‐history strategies, but no evidence of density‐dependent movement. Overwinter mortality, spatial clustering of redds and limited resource availability were identified as potentially important limiting factors contributing to density dependence. The ubiquity of density dependence for these threatened populations is alarming as stability at present low abundance levels suggests recovery may be difficult without major changes. We conclude that density dependence at the population level is common and must be considered in demographic analysis and management.

Summary1. Spatial, phenotypic and genetic diversity at relatively small scales can buffer species against large-scale processes such as climate change that tend to synchronize populations and increase temporal variability in overall abundance or production. This portfolio effect generally results in improved biological and economic outcomes for managed species. Previous evidence for the portfolio effect in salmonids has arisen from examinations of time series of adult abundance, but we lack evidence of spatial buffering of temporal variability in demographic rates such as survival of juveniles during their first year of life. 2. We therefore use density-dependent population models with multiple random effects to represent synchronous (similar among populations) and asynchronous (different among populations) temporal variability as well as spatial variability in survival. These are fitted to 25 years of survey data for breeding adults and surviving juveniles from 15 demographically distinct populations of Chinook salmon (Oncorhynchus tshawytscha) within a single metapopulation in the Snake River in Idaho, USA. 3. Model selection identifies the most support for the model that included both synchronous and asynchronous temporal variability, in addition to spatial variability. Asynchronous variability (log-SD = 0Á55) is approximately equal in magnitude to synchronous temporal variability (log-SD = 0Á67), but much lower than spatial variability (log-SD = 1Á11). We also show that the pairwise correlation coefficient, a common measure of population synchrony, is approximated by the estimated ratio of shared and total variance, where both approaches yield a synchrony estimate of 0Á59. We therefore find evidence for spatial buffering of temporal variability in early juvenile survival, although between-population variability that persists over time is also large. 4. We conclude that spatial variation decreases interannual changes in overall juvenile production, which suggests that conservation and restoration of spatial diversity will improve population persistence for this metapopulation. However, the exact magnitude of spatial buffering depends upon demographic parameters such as adult survival that may vary among populations and is proposed as an area of future research using hierarchical life cycle models. We recommend that future sampling of this metapopulation employ a repeated-measure sampling design to improve estimation of early juvenile carrying capacity.

Faulkner et al. (2019) presented data and analyses to support a hypothesis that bypass encounters by juvenile Chinook Salmon Oncorhynchus tshawytscha and steelhead O. mykiss at hydropower dams are not harmful (e.g.,

Grouping populations for management may overlook the fine‐scale diversity underpinning the stability and resilience of meta‐populations and fisheries. A bimodal timing distribution of summer‐run steelhead Oncorhynchus mykiss (anadromous Rainbow Trout) historically was observed at Bonneville Dam (BON), the first barrier to upstream migration in the Columbia River basin. Early mode fish (A‐run) tended to be younger and smaller (<78 cm) than later fish (B‐run). While A‐run fish spawn throughout the Columbia River basin, B‐run fish spawn primarily in the Snake River basin. Managers used indices of these modes to make fishery decisions, and later these criteria were adopted for conservation. It is still unclear how life history and body size differences among wild Snake River populations are related to the categories at BON. We examined population parameters characterizing the two categories (date of passage at BON, length) and parameters directly affecting population dynamics (age composition, sex ratio). The life history portfolio of Snake River steelhead is quite diverse. There was broad overlap among populations in several respects, forming a gradient in life history characteristics rather than a dichotomous break. All populations produced adults <78 cm and adults returning after August 25. Median lengths of putative B‐run populations were close to the criterion that was supposed to be a defining characteristic. In contrast, few A‐run populations produced many adults ≥78 cm. Mean percentage of two‐ocean fish was 52.1% for A‐run populations and 82.0% for B‐run populations. Mean age at spawn was greater in populations producing older smolts. Sex ratio was female biased, and older populations had greater percentages of females. Although the run‐type dichotomy was useful for management of fisheries in the past, it is not useful for conservation. A combination of genetic stock identification at main‐stem dams and population‐specific monitoring in natal streams provides a unified framework for the assessment of fisheries management and conservation objectives. Received March 2, 2016; accepted November 9, 2016 Published online March 10, 2017

The most productive juvenile life history in the Pahsimeroi River Chinook salmon ( Oncorhynchus tshawytscha ; Idaho, USA) population (in terms of smolt production) is being eliminated. Length at emigration and survival from spawning areas to Lower Granite Dam within each of three juvenile phenotypes (age-0 smolts, fall parr, age-1 smolts) were influenced by initial cohort abundance. The proportion of age-1 emigrants reaching Lower Granite Dam was dome-shaped with respect to initial cohort abundance. As initial abundance increased, higher proportions of juveniles adopted the age-1 smolt phenotype or emigrated as fall parr. The age-0 smolt phenotype had the highest relative survival, and the fall parr phenotype, the lowest. The contributions of each emigrant type to cohort smolt production varied with circumstances; hence, the full expression of phenotypic diversity is important to the study population. However, there were no records of tagged age-0 smolts surviving to return from the Pacific Ocean. Given the potential productivity of this life history, management and recovery efforts should be directed at the age-0 smolt phenotype.