Comprehensive Monitoring of an ESA-Listed Steelhead (Oncorhynchus mykiss) Population in the Upper Imnaha River PART I & PART II

Steelhead (Oncorhynchus mykiss) serve as a powerful cultural and social symbol for tribal and non-tribal people of the Pacific Northwest. Yet despite the significance of this icon, there have been widespread and dramatic declines in steelhead populations over the last century.  These declines were also witnessed in the steelhead populations of northeast Oregon including in the Imnaha River. In response, managers and policy makers have called for increased and comprehensive monitoring of abundance, population growth, spatial distribution and life history diversity on a tributary specific spatial scale. Since 2011, we have monitored the little known upper Imnaha natural spawning aggregate using data collected via Passive Integrated Transponder (PIT) tag detections, weir interrogations, spawning ground surveys, scale analysis and known age fish. 

We found steelhead return to the upper Imnaha River in large numbers and are significant contributors to the total Snake River DPS. There was diversity in the life history characteristics we observed, the natural spawning population was well distributed in upper Imnaha tributaries as well as the mainstem and not heavily influenced by hatchery fish.  We argue these fish remain an important ecological resource that merits continuing evaluation and protective vigilance. Our findings are presented in two interrelated posters.