Ecologist Christopher Sergeant of the National Park Service set out to investigate if hypoxic events were to blame for salmon die-offs in Alaskan streams and, if so, what caused the hypoxia.
He teamed up with research fish biologist Ryan Bellmore of the USDA Forest Service Pacific Northwest (PNW) Research Station, graduate student Casey McConnell of the University of Alaska Fairbanks, and associate professor of aquatic ecology and conservation Jonathan Moore of Simon Fraser University to investigate these questions.
A central focus of their study was to determine if the dense populations of salmon themselves significantly contributed to the formation of hypoxic conditions.
The researchers used dissolved oxygen probes to measure and record data in the Indian River and Sawmill Creek of southeastern Alaska. This resulted in a valuable, high-frequency, long-term data series on seasonal hypoxic conditions. They also measured water temperature, river flow, and did periodic visual counts of salmon.
The scientists defined the hypoxic threshold in this study as 7 mg oxygen per liter, Alaska's minimum level of dissolved oxygen in streams with salmon. Previous studies found that salmon demonstrated poor swimming performance and delayed migration below this level.
By measuring dissolved oxygen, the scientists recorded four hypoxic events between 2010 and 2015 that lasted for very different lengths of time (1.5 hours to 37 days). Two of these events resulted in documented salmon die-offs.