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A maximum likelihood model was developed to estimate the 1968–2020 drainagewide run size and escapement of Nushagak River Chinook salmon (Oncorhynchus tshawytscha). The model simultaneously combined information by direct observations of escapement at 8 locations (1 tower and 7 aerial surveys); harvest of fish from commercial, subsistence, and sport fisheries; inriver abundance indices from the Nushagak River sonar project; and inriver abundance estimates from acoustic tag and mark–recapture studies. Results showed that reconstructed total run size ranged from 74,000 to 629,000 Chinook salmon with an average run size of 282,000 fish, and escapement ranged from 49,000 to 476,000 fish with an average of 210,000 fish. The model estimated total run and escapement appeared to be reasonable and tracked well with previous estimates. The major deficiency of this model is the absence of overlapping, long-term escapement and run monitoring data.
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 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.
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.
A maximum likelihood model was used to estimate the 2020 drainagewide run size and escapement of Kuskokwim River Chinook salmon (Oncorhynchus tshawytscha). The total run was estimated to be 124,486 fish (95% CI: 102,661–150,952) and escapement was estimated to be 88,285 fish (95% CI: 66,460–114,751). Model estimates were informed by direct observations of the 2020 escapement at 15 locations (3 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 2020 run and escapement. The 2020 total run of Chinook salmon was below the 1976–2019 average of 215,870 fish. The drainagewide sustainable escapement goal of 65,000–120,000 was met in 2020. The 2021 Kuskokwim River Chinook salmon forecast is for a range of 94,000–155,000 fish.
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.
A maximum likelihood model was used to estimate the 2021 drainagewide run size and escapement of Kuskokwim River Chinook salmon (Oncorhynchus tshawytscha). The total run was estimated to be 129,751 (95% CI: 94,489–178,171) fish and escapement was estimated to be 101,000 (95% CI: 65,738–149,420) fish. Model estimates were informed by direct observations of the 2021 escapement at 3 weirs and 2021 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 2021 run and escapement. The 2021 total run of Chinook salmon was below the 1976–2020 average of 214,475 fish. The drainagewide sustainable escapement goal of 65,000–120,000 was met in 2021. The 2022 Kuskokwim River Chinook salmon forecast is for a range of 99,000–161,000 fish.
The Susitna River drains approximately 52,000 square kilometers of the southern slopes of the Alaska Range and the Talkeetna mountains. This watershed supports wild populations of all 5 species of Pacific salmon and vibrant sport fisheries when production allows. Chinook salmon spawning escapements have been monitored since the late 1970s by aerial survey and a weir has been used to count returning adults on the Deshka River (a tributary) since 1995. The Deshka River is currently managed by an escapement goal based on weir passage whereas several other spawning populations are managed using escapement goals based only on once-per-year aerial surveys. Other fishery data, such as inriver and marine harvest estimates, age estimates, recent mark–recapture abundance estimates, and spawner distribution data are also available. We present a state-space model that incorporates all available datasets to generate annual inriver and spawning escapement abundance estimates of 4 stocks of Susitna River Chinook salmon. These stocks were created by dividing the drainage into geographical units similar to existing management units used in Alaska Department of Fish and Game sport fishing regulations: Deshka River, Eastside Susitna, Talkeetna River, and Yentna River. The state-space model estimates a spawner-recruitment (S-R) relationship for each stock that is used in developing escapement goal recommendations based on the number of spawners that provide maximum sustained yield (SMSY). SMSY was estimated for each stock: 12,564 for Deshka River; 12,971 for Eastside Susitna; 10,570 for Talkeetna River; and 13,614 for Yentna River. We used a decision matrix to choose escapement goals based on the probability of achieving maximum sustained yield for the 4 stocks. We recommend discontinuing escapement goals for individual spawning populations within these stocks and replacing them with stock-based escapement goals of 9,000–18,000 for Deshka River, 13,000–25,000 for Eastside Susitna, 9,000–17,500 for Talkeetna River, and 13,000–22,000 for Yentna River stocks.
Objectives of this study are to estimate the spawning escapement and run timing of Chinook salmon Oncorhynchus tshawytscha in an index area of the Gulkana River using counting tower methodology. In addition, sockeye salmon O. nerka escapement at the tower site will be estimated during the period of tower operation. The number of Chinook salmon and sockeye salmon passing the tower site will be estimated by visually counting fish as they pass 2 counting towers located approximately 2.5 km upstream of the confluence of the West Fork. Ten-minute visual counts will be conducted for each river channel every hour, 24 hours per day, 7 days per week. The abundance estimates will be stratified by day. Because counts are planned for all hours, daily estimates of abundance will be a single-stage direct expansion from the 10 min counting periods. The count schedule will start prior to the beginning of the Chinook salmon run, approximately 3 June, and continue until after 10 August when the run is complete. Tower operations may be extended to a later date to enumerate more of the sockeye salmon run if funding is provided by Prince William Sound Aquaculture Corporation.
A maximum likelihood model was used to estimate the 2022 drainagewide run size and escapement of Kuskokwim River Chinook salmon (Oncorhynchus tshawytscha). The total run was estimated to be 142,495 (95% CI: 107,579–188,743) fish, and escapement was estimated to be 107,980 (95% CI: 73,064–154,228) fish. Model estimates were informed by direct observations of the 2022 escapement at 3 weirs and 2022 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 2022 run and escapement. The 2022 total run of Chinook salmon was below the 1976–2021 average of 211,081 fish. The drainagewide sustainable escapement goal of 65,000–120,000 was met in 2022. The 2022 Kuskokwim River Chinook salmon forecast is for a range of 115,000–170,000 fish.