2023 Run Timing and Escapement of Wild Summer Coho Salmon in the Upper Sol Duc River Based on ARIS Multi-Beam SONAR

The Sol Duc River on the Olympic Peninsula of Washington State has a unique population of wild summer-run Coho Salmon (Oncorhynchus kisutch) that return to spawn during low summer flow conditions. Most of these fish spawn upstream of a partial natural barrier known as the Sol Duc Salmon Cascades. Above this barrier during the summer months, adult Coho Salmon are generally the only large-bodied salmonid present. Furthermore, essentially no hatchery summer coho have been documented upstream of this partial barrier. Traditionally, abundance estimation for these wild coho is accomplished by spawning ground surveys, using a redd-based methodology. However, recent discussions among co-managers determined that sound navigation and ranging, or SONAR, could provide an alternative method to enumerate Sol Duc summer coho above the Salmon Cascades. In 2023, a pilot study was initiated to compare SONAR counts with redd-based estimates at location above the Salmon Cascades. In addition to enabling abundance estimates, monitoring coho with SONAR also facilitated greater understanding of timing and environmental conditions associated with summer coho passage during their upstream spawning migration. We deployed an ARIS Explorer 1800 SONAR unit at an ideal location 415 m upstream of the Salmon Cascades throughout the entire spawning migration period and estimated an escapement of 161 fish, nearly 2.5 times higher than the redd-based estimate of 65 fish. Most coho passed the SONAR site upstream of the Salmon Cascades when water temperatures were between 8.5° to 11°C and when stage height at the Department of Ecology gauge was between 15.1 and 15.24 ft. The first adult coho observed using SONAR imagery occurred on July 29, 2023, and the last on November 10, 2023, with a median migration date of October 6, 2023. Stream flow during the late summer of 2023 was at or near record lows, likely delaying migration and passage of the Salmon Cascades, leaving many fish downstream of the partial natural barrier. Future years of SONAR operation will provide valuable insight into run-timing and environmental conditions for this unique population of Coho Salmon whose return timing makes them susceptible to changing environmental conditions. Continued operation will also provide managers with an alternative estimate to compare with redd-based abundance, and a way to potentially calibrate or correct estimates from other coarser methods. The skills and experience learned operating a SONAR unit at this location in 2023 will be transferable to other river systems and advancements in SONAR technology and data analysis will make SONAR a more viable tool for fisheries management and science.