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This report describes a modeling framework to reconstruct the historical time series of drainagewide and stock-specific run and escapement of Pacific salmon Oncorhynchus spp. that return to a single large river system. The model combines historical data from various assessment projects that estimate mainstem passage, harvests, tributary escapements, and stock proportions under a single maximum-likelihood estimation framework. As a demonstration, the modeling framework was applied to reconstruct the historical (1981–2019) drainagewide run size and escapement of Yukon River Chinook salmon (O. tshawytscha) and each of the contributing stock components: Canada, Middle Yukon, and Lower Yukon. The model estimated that the average drainagewide (all stocks) run size was 301,000 (range: 109,000–491,000), escapement was 186,000 (88,000–305,000), and harvest rate was 35% (2–60%). The average Canada stock run size was 125,000 (39,000–214,000), escapement was 60,000 (14,000–126,000), and harvest rate was 48% (2–78%). The average Middle Yukon stock run size was 75,000 (22,000–139,000), escapement was 47,000 (14,000–126,000), and harvest rate was 35% (2–85%). The average Lower Yukon stock run was 101,000 (48,000–204,000), escapement was 78,000 (30,000–164,000), and harvest rate was 21% (1–51%). The next steps regarding the application of this model to Yukon River Chinook salmon should include a comprehensive data review, consideration of alternative model structures, and critical evaluation of model assumptions.
Total run of Chinook salmon (Oncorhynchus tshawytscha) to the Kuskokwim River from 1976 through 2011 was estimated using a model developed for data-limited situations. The model simultaneously combined information on subsistence harvest, commercial harvest and effort, sport harvest, test fish harvest and catch per unit of effort at Bethel, mark-recapture estimates of inriver abundance, and counts of salmon at 6 weirs and peak aerial counts from 14 drainages all spread throughout the Kuskokwim River drainage. The estimates of historic run size were then combined with available information on the age structure of the stock to reconstruct the total return by age and ultimately estimate a brood table.
This report looks at escapements and stock status of chinook salmon, Oncorhynchus tshawytscha, and recommends that the Yukon River chinook salmon stock remain classified as a "stock of yield concern."
Counting tower techniques were used to estimate Chinook salmon Oncorhynchus tshawytscha escapement in the Gulkana River during 2019, 2020, and 2021. The counting tower was located on the mainstem Gulkana River about 3.25 river kilometers upstream of the West Fork Gulkana River confluence. The estimated escapement of Chinook salmon was 8,400 (SE = 436; 95% CI = 7,546–9,254) in 2019, 2,504 (SE = 216; 95% CI = 2,080–2,928) in 2020, and 3,402 (SE = 273; 95% CI = 2,867–3,937) in 2021. These numbers do not represent total inriver escapement, just escapement above the counting tower site. The 2019 estimated escapement of Chinook salmon was the highest on record since the inception of the project in 2002, whereas the 2020 and 2021 estimated escapements were below the average for the years 2002–2018. The dates of the 25th, 50th, and 75th percentile of cumulative passage of Chinook salmon past the Gulkana River counting tower have been getting later since the project’s inception in 2002. The estimated escapement of sockeye salmon O. nerka during the counting tower’s operational period was 20,850 (SE = 661; 95% CI = 19,555–22,145) in 2019, 13,500 (SE = 574; 95% CI = 12,375–14,625) in 2020, and 13,924 (SE = 711; 95% CI = 12,531–15,317) in 2021. Counting tower operations were extended during 2019–2021 to enumerate more of the sockeye salmon return.
This report looks at escapements and stock status of chinook salmon, Oncorhynchus tshawytscha, and recommends that the Yukon River chinook salmon stock remain classified as a "stock of yield concern."
A maximum likelihood model was used to estimate the 2019 drainagewide run size and escapement of Kuskokwim River Chinook salmon (Oncorhynchus tshawytscha). Total run and escapement were estimated to be 226,987 (95% CI: 182,811–281,839) and 188,483 (95% CI: 144,307–243,335) fish, respectively. Model estimates were informed by direct observations of the 2019 escapement at 16 locations (4 weirs and 12 aerial surveys) and harvest, combined with historical observations of escapement (up to 6 weirs and 14 aerial surveys), harvest, test fishery, and mark–recapture data dating back to 1976. Model results are adequate to draw broad conclusions about the 2019 run and escapement. The 2019 total run of Chinook salmon was the largest since 2007 and was probably above the 1976–2018 average of 215,529 fish. The drainagewide sustainable escapement goal of 65,000–120,000 was exceeded in 2019. The 2020 Kuskokwim River Chinook salmon forecast is for a range of 193,000–261,000 fish.