Assessing Thermal Habitat Variability to Identify Refugia in SE Alaska Subsistence Salmon Watersheds

Temperature data from 69 streams representing a wide range of watershed types and sizes across southeast Alaska were analyzed to understand how climate change may alter the suitability of stream temperatures for salmon. Modeled relationships between stream temperature, seasonal weather metrics, and watershed characteristics were applied to 2,203 unmonitored streams across southeast Alaska to predict stream temperatures under current and projected future climate conditions. Results suggest increasing sensitivity to warming as snow and ice are lost in the future, with a dramatic increase in the fraction of streams (>40%) periodically experiencing temperatures that exceed lethal thresholds for salmon by 2080. However, many watersheds are likely to remain suitable for salmon, and some streams that are currently very cold may become more productive in the future. Although the most downstream reaches of many streams are expected to warm dramatically, there is often important spatial heterogeneity in upstream water temperatures. More spatially intense temperature monitoring was conducted within eight watersheds, four of which support important subsistence sockeye runs. These efforts identified multiple groundwater-fed reaches, tributaries, and side channels that can provide summer refugia, while other areas, such as beaver ponds and lake outlets can provide warmer, more productive habitat during colder seasons, highlighting the importance of identifying and protecting critical areas for salmon.

Understanding current and future thermal conditions in Southeast Alaska’s salmon streams is critical for making informed decisions about which watersheds are likely to continue to be (or may become increasingly) suitable for salmon productivity and should therefore be the focus of future restoration and protection actions. This project will produce a regionally-relevant assessment that provides insights into the types of watersheds likely to be resilient (or resistant) to climate change with respect to water temperatures, and will identify specific locations within eight salmon watersheds (833 km2), including four important subsistence watersheds (582 km2), that may provide thermal refugia and should be considered priorities for restoration and/or protection actions.