Across taxa, it is generally accepted that there are fitness advantages to rapid growth early in life. For stream-dwelling salmonids, however, high temperatures and associated energetic costs during the summer growing season might offset or even prevent the competitive advantage of large body size. Our overall objective was to understand the relative importance of factors that can cause variation in growth rates in an age-0 cohort of wild steelhead (Oncorhynchus mykiss) in Idaho, USA, where temperatures approach, and temporarily exceed, their tolerance level. For individually tagged fish inhabiting the same stream reach, we found that growth rates were negatively related to fish mass (slopes of the two best approximating models were both À0.024). Comparing growth rates from 16 different stream reaches throughout the watershed, we found that temperature-induced metabolic cost was the single best approximating model (AIC w i = 1.0) of the variation in individual growth rates. The bioenergetic model showed that mass-specific metabolic costs decreased with mass, but the absolute energetic demands increased over the same size range. Because temperature had a multiplicative effect on metabolic cost, our results suggest that the effect of food limitation increased with fish size. We conclude that high water temperatures pose energetic bottlenecks and can be a potentially strong mechanism limiting growth in juvenile salmonids in summer, particularly as streams in the region experience warming trends.

Citation: Myrvold, K. M., and B. P. Kennedy. 2015. Density dependence and its impact on individual growth rates in an age-structured stream salmonid population. Ecosphere 6(12):281. http://dx.doi.org/10.1890/ES15-00390.1Abstract. In organisms with flexible and indeterminate growth, demographic density dependence can affect both individual fitness and population dynamics. Recent work in stream salmonids suggests that individual growth rates can be depressed even at low population densities, which warrants examination of populations previously assumed to be below carrying capacity. We investigated the effects of population density on individual growth rates in a threatened population of steelhead (Oncorhynchus mykiss) in a tributary to the Clearwater River in Idaho, USA. We followed a mark-recapture design where we visited 16 study sites on average five times per year from 2010 to 2012. The 95% confidence interval for average growth rates (% change in body mass per day) for subyearlings were [0.82, 1.0]) and [0.16, 0.30] for yearling steelhead. Variance decomposition showed that the variation in growth rates could be attributed equally to individual-and visit-level factors in subyearlings, whereas almost two-thirds of the variance in yearling growth rates could be attributed to individual-level factors. Growth rates in the subyearling age class were negatively related to the densities of yearling steelhead, but not vice versa. Yearling growth rates showed no evidence of density dependence. Finally, density in interaction with water temperature did not affect growth rates of either age class. Our results demonstrate that density dependence can pose constraints on individual growth rates at low population densities (,1 fish m À2 ) in stream salmonids, and underscore the importance of considering age classes separately when studying density dependence in age-structured populations.

Human activities and climate change threaten coldwater organisms in freshwater ecosystems by causing rivers and streams to warm, increasing the intensity and frequency of warm temperature events, and reducing thermal heterogeneity. Cold‐water refuges are discrete patches of relatively cool water that are used by coldwater organisms for thermal relief and short‐term survival. Globally, cohesive management approaches are needed that consider interlinked physical, biological, and social factors of cold‐water refuges. We review current understanding of cold‐water refuges, identify gaps between science and management, and evaluate policies aimed at protecting thermally sensitive species. Existing policies include designating cold‐water habitats, restricting fishing during warm periods, and implementing threshold temperature standards or guidelines. However, these policies are rare and uncoordinated across spatial scales and often do not consider input from Indigenous peoples. We propose that cold‐water refuges be managed as distinct operational landscape units, which provide a social and ecological context that is relevant at the watershed scale. These operational landscape units provide the foundation for an integrated framework that links science and management by (1) mapping and characterizing cold‐water refuges to prioritize management and conservation actions, (2) leveraging existing and new policies, (3) improving coordination across jurisdictions, and (4) implementing adaptive management practices across scales. Our findings show that while there are many opportunities for scientific advancement, the current state of the sciences is sufficient to inform policy and management. Our proposed framework provides a path forward for managing and protecting cold‐water refuges using existing and new policies to protect coldwater organisms in the face of global change.

Physical habitat characteristics can affect the abundance and distribution of organisms, and are frequently used to predict the standing crop of stream fish for purposes of understanding their ecology and better direct management. However, the spatial scale of the investigation and the resolution of the data can affect the outcome of such analyses. In this study we coupled watershed‐level characteristics with instream habitat variables to model the density of two age‐classes of juvenile steelhead Oncorhynchus mykiss in a watershed in the Clearwater River basin, Idaho. Density varied considerably across time and space. Variance partitioning showed that 41–50% of the variances in density were due to unexplained differences between sampling occasions (residual variance), and the rest resulted from variation at the site and site‐and‐year levels, depending on the age‐class. Instream habitat variables better explained the variation in density than did models that included watershed‐level characteristics. The density of subyearling steelhead was best explained by stream discharge, with a negative relationship. The density of yearling steelhead was best explained by a negative relationship with average weekly temperature; however, this relationship was statistically indistinguishable from zero. Finally, total density (subyearlings and yearlings combined) was best explained by discharge and average daily temperature. We believe that our approach is useful for identifying the physical factors associated with the density of stream salmonids, but we stress that findings from correlative studies should be interpreted in concert with detailed knowledge about life history variation in the study system.

Anadromous salmonids can display considerable movement within their natal streams prior to outmigration, reflecting behavioural responses to biotic and abiotic conditions. Here we quantify dispersal and site fidelity in juvenile steelhead (Oncorhynchus mykiss) over four weeks in the fall, their consequences for individual growth rates, and how habitat, density, and individual body size can explain individual movement decisions. Retention rates ranged from 29% to 66% and were greater among yearlings (61%) than subyearlings (31%). Site fidelity in yearlings was best predicted by individual body mass (negative relationship), and site fidelity in both age classes was positively related to pool habitat availability. An experimental increase in density caused depressed growth rates in both age classes, and site fidelity was lower among translocated fish than among individuals originally residing in the manipulated sites. Within control sites, there were no significant consequences of movement for growth rates in either age class. These results show that movement rates can vary in response to local conditions and that growth rates in movers can equal or exceed those of sedentary individuals.