Hypoxia vulnerability in the salmon watersheds of Southeast Alaska

The frequency of dissolved oxygen depletion events (hypoxia) in coastal aquatic ecosystems has risen dramatically since the late 20th century, yet the causes and consequences of hypoxia for some culturally and economically impor-tant species remain poorly understood. In rivers, oxygen depletion can be caused by high densities of spawning Pacific salmon (Oncorhynchus spp.) consuming oxygen faster than can be replaced by reaeration. This process may be exacer-bated when salmon densities are artificially inflated, such as when hatchery-origin salmon stray into rivers instead of returning to hatcheries. In Southeast Alaska, hatchery salmon production has increased rapidly since the 1970s, with over 553 million chum salmon (O. keta) and 64 million pink salmon (O. gorbuscha) released in 2021 alone. Straying is pervasive in streams with outlets <25 km from nearshore marine hatchery release sites. Using a previously ground-truthed mechanistic model of dissolved oxygen dynamics, we examined how water temperature and low-flow channel hydraulics contribute to hypoxia vulnerability. We then applied the model to predict hypoxia vulnerability for watersheds within 25 km of hatchery salmon release points, where straying salmon spawner densities are expected to be higher and promote dissolved oxygen depletion. Our model predicted that low-gradient stream reaches, regardless of water temper-ature, are the most prone to hypoxia due to low reaeration rates. Our spatial analysis determined that nearly 17,000 km of anadromous-accessible stream reaches are vulnerable to high densities of hatchery-origin salmon based on 2021 release sites. To our knowledge, this study is the first to map the spatial variation of hypoxia vulnerability in anadromous water-sheds, identify habitat conditions most likely to promote hypoxia, and provide a repeatable analytical approach to identify hypoxia-prone stream reaches that can be updated as empirical data sets improve.

Automated summary

The frequency of hypoxia events in coastal aquatic ecosystems has increased since the late 20th century. The causes and consequences of hypoxia for important species are poorly understood. In rivers, oxygen depletion can be caused by high densities of spawning salmon consuming oxygen faster than reaeration. In Southeast Alaska, hatchery salmon production has rapidly increased, and straying salmon densities are high near hatchery release sites. Low-gradient stream reaches are the most prone to hypoxia. About 17,000 km of streams are vulnerable to high densities of hatchery-origin salmon.