Freshwater pressures on Pacific salmon in the coastal watersheds of Alaska

Abstract

This dissertation explores the implications of a warmer and wetter climate on the freshwater life stages of Pacific salmon (Oncorhynchus spp.) in the focal region of southern coastal Alaska. Recent trends and long-term climate change predictions support the notion that arctic and subarctic watersheds will be subject to warming air temperatures, increased rainfall in the autumn and winter, diminishing snowpack, and continued glacial recession. Such atmospheric and terrestrial changes will shift patterns of streamflow, water temperature, and habitat diversity that comprise the basic building blocks of freshwater ecosystems that support Pacific salmon. Alaska's Pacific salmon populations sustain a multi-billion-dollar economy and have supported Indigenous Peoples' way of life for millennia. The character of Alaska is defined, in part, by being one of the last places in North America to support a harvestable bounty of fishes and wildlife. Therefore, studies that describe and inventory current freshwater habitat diversity, predict future habitat change, and model the responses of Alaska Pacific salmon populations to a range of future habitat scenarios will not only advance general ecological understanding, but also provide valuable insights into the trajectory and range of Pacific salmon futures for the remainder of the 21st century. In Chapter 1, A classification of streamflow patterns across the coastal Gulf of Alaska, I classify and map 4,140 coastal Alaska watersheds according to 13 unique patterns of rain, snow, and glacier ice runoff. In Chapter 2, Hypoxia vulnerability in the salmon watersheds of Southeast Alaska, I demonstrate the utility of a mechanistic model of dissolved oxygen dynamics in streams based on low-flow channel hydraulics, water temperature, and spawning salmon density. In Chapter 3, Pacific salmon population responses to a warmer, wetter climate at northern latitudes, I apply a newly established life cycle model to quantify the population responses of chum (O. keta), pink (O. gorbuscha), and coho salmon (O. kisutch) to daily variation in discharge and water temperature patterns, including extreme floods and droughts. Taken together, these findings contribute to the growing body of knowledge on the impacts of a warmer and wetter atmosphere on high-latitude freshwater ecosystems and demonstrate the value of land and water management actions that conserve ecological functioning and promote Pacific salmon resilience.