Utilizing stream temperature in regional barrier removal planning does not substantially improve cool-water habitat restoration

Abstract

Mathematical optimization strategies which maximum quantity of restored habitat are a common approach to prioritize small fish passage barriers for removal. Despite the importance of cool summer stream habitat for adult migrating salmon, such approaches have not been evaluated for their efficacy at restoring thermally suitable habitat. In this study, I analyze how well a habitat-maximizing optimization strategy performs at increasing cool-water habitat access, based on mean August stream temperatures, for barrier removals in western Washington. I additionally evaluate whether recent historical stream temperatures can serve as an effective proxy for future stream temperatures in cool-water-focused restoration. I find for this region, there is little relationship between temperature and habitat value, and the habitat-maximizing strategy achieves 70% or better of possible cool-water habitat gains for most relevant summer stream temperatures, making it moderately effective at cool-water restoration. Furthermore, when considering climate change, current stream temperature models indicate spatially homogeneous stream warming, making current temperatures an effective proxy for future temperatures. This study will help inform the utility of incorporating stream temperature information into fish passage planning strategy on regional scales.