Water Temperature Thresholds for Coho Salmon in a Spring-fed river, Siskiyou County, California

Coho salmon (Oncorhynchus kisutch) populations in California have declined at an alarming rate in the last 40 to 50 years. Detrimental water temperatures in the Shasta River have contributed to this decline. At one time, the Shasta River was a cool water stream with flows dominated by springs originating from underground flow from Mt. Shasta and snowmelt from the Eddy Mountains. Agricultural practices and water diversions have eliminated much of the historic high-quality aquatic habitat, and only remnants of the once abundant cool water habitat exist. Cool water temperatures are critical for the freshwater phase of the coho salmon life cycle, and are imperative for population recovery. Based on a literature review of the effects of physiology, behavior, and survival of coho salmon, we break water temperatures into optimal, suboptimal, and detrimental ranges. Identifying water temperature thresholds for coho salmon will support the implementation of monitoring stations and adaptive management practices to assure that suboptimal temperature thresholds are not exceeded. It is well documented that the establishment and use of locally determined thresholds as performance criteria in the monitoring and adaptive management of ecosystems is critical to conducting restoration activities. We conclude that protecting the cool water produced by springs located in the upper Shasta River springs complex will improve the likelihood of coho salmon persistence in this watershed and contribute to coho salmon recovery.