Increasing water temperatures exacerbate the potential for density dependence in juvenile steelhead

We studied 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 Idaho, USA. Projected increases in water temperature 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 following the A1B scenario. The predicted increase in energetic cost was largest in the coolest stream reaches, where the proportional increases in energetic cost exceed that of temperature. Energetically, and 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. 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 with increasing temperatures and demonstrate that temperature can exacerbate demographic density dependence in fish populations.