Adult fall Chinook Salmon Oncorhynchus tshawytscha carcasses and redds were surveyed on the mainstem Klamath River, from Iron Gate Dam to Wingate Bar during the 2017 spawning season to estimate annual escapement and characterize the age and sex composition and spawning success of the run. Surveys were conducted over 9 weeks, from October 11 to December 6. Using postmortem mark–recapture methods and a hierarchical latent variables model between Iron Gate Dam and the confluence with the Shasta River, the estimated spawning escapement for this 21.6-km section of the mainstem Klamath River was 4,740 fish. Based on this estimate and age composition data from scale samples, spawning escapement by year class was 1,749 (36.9%) age-2 (jacks and jills), 2,376 (50.1%) age-3, 550 (11.6%) age-4, and 65 (1.4%) age-5 spawners. The presence of jills (age-2 females) was unusually high in 2017 and they accounted for 8.2% of all female carcasses. Jacks (age-2 males) accounted for 53.4% of all male carcasses. An estimated 19.8%
of the fish that spawned in the study area were of hatchery origin. The adult female– male ratio was 1.9:1 and pre-spawn mortality rate of females was 5.5%. Estimated egg deposition by females in the carcass study area was 4.9 million. The redd count in the 125.7-km section of the mainstem river between the Shasta River confluence and Wingate Bar was 478 in 2017. Redd counts over the previous 24-year history of this survey ranged from 243 (in 1993) to 3,456 (in 2014), although the downstream end of these past surveys was the Indian Creek confluence and was thus 11.2 km shorter. Estimated egg deposition in the redd study area was 1.2 million.