Environmental conditions can influence the way in which density dependence is manifested in populations. These interactions can be particularly important in ectotherms such as fishes, whose metabolism scale with water temperature. With predicted climate change, summer temperatures in freshwater systems are largely expected to increase. In order to forecast the effects of climate change on fish populations there is a need to identify the mechanisms that link the changes in the physical environment with population-level processes.
Here we study the potential effects of predicted climate change on the energetic demands of juvenile steelhead (Oncorhynchus mykiss) and their consequences for local population size and structure in a small watershed in the Snake River Basin. Previous work in the system has shown that temperatures in the summer can approach the thermal tolerance of steelhead, and hence pose an important constraint on individual performance and population dynamics. We combine locally calibrated temperature projections with a bioenergetic model to quantify the added energetic cost associated with warming temperatures, and quantify the shape of the size distributions across the naturally occurring gradient of population densities and temperature. We discuss how local size distributions might change in the face of predicted climate change.
Stream temperatures in the watershed were estimated to increase on average by 1.81°C by 2040, and by 3.11°C by 2080 following the A1B (balanced) scenario. The predicted temperature increases in turn incurred on average a 10% higher energetic cost by 2040 (range 7.0%-12.5% among study reaches in the watershed), and a 16% increase (range 8.5%-21.3%) by 2080. The predicted increase in energetic cost was largest in the currently coolest stream reaches, where the proportional increases in energetic cost exceed that of temperature due to the nonlinear relationship between temperature and metabolism.
In absence of increases in food supply, local densities were consequently expected to decline. We examined which factors best described the shape of current size distributions to explore future size distributions as temperatures increase. A positive skew refers to many small and few large individuals, whereas a negative skew refers to the opposite distribution. Mass distribution skewness was best explained by local biomass (positive relationship) and water temperature (negative relationship). The results suggest that local steelhead cohorts will approach a platykurtic, slightly negatively skewed distribution (i.e. relatively more large individuals and fewer small individuals) with warming temperatures, and demonstrate that temperature can exacerbate demographic density dependence in fish populations.