Stock Assessment of Yukon River Chinook in 2024

Status The anticipated Yukon River Chinook salmon major fish stock is comprised of two Stock Management Units (SMU) – Yukon Chinook SMU and Porcupine Chinook SMU. The major stock is below its Conservation Unit (CU)-status based Limit Reference Point, based on Wild Salmon Policy (WSP) rapid status approach, because at least one CU is in the Red status zone. Five of nine CUs in the Yukon Chinook SMU, and two of three CUs in the Porcupine SMU, are assessed as in the Red status zone. Spawner abundance over the most recent generation (2019–2024) in the Yukon Chinook SMU was 25,852 which is below a proposed Fishery Reference Point – Lower of 31,000 and an Upper Stock Reference Point of 86,000. The SMU harvest rate over this same time period averaged 16% which is well below a proposed Maximum Removal Reference Point of 36%. Recent (2019–2024) estimates of spawner abundance to the Porcupine SMU averaged 695, but with data limited to two CUs. Harvest of Porcupine Chinook is not well characterized but Chinook salmon from the SMU are assumed to experience similar harvest rates to those experienced by the Yukon SMU in in-river fisheries in Alaska. Trends Total returns to the Yukon Chinook SMU averaged ~175,000 from the 1980s to the mid-1990s, and have declined to the lowest observed returns on record 2021–2024 (an 87% decline from 1981–2024). In contrast, spawning abundance has varied from year to year but has been relatively stable over time with the exception of very low spawning abundances in recent years (2021–2024). Model based harvest rates in the predominantly mixed-stock fishery in mainstem of the Yukon River in Alaska and Yukon territory ranged from an average of 68% in the late 1980s and early 1990s to 18% over the most recent decade. In the last four return years, harvest rates were dramatically reduced and ranged from 3%–8%. Female Yukon River Chinook salmon are returning at younger ages and the proportion of the total return that is females has declined, leading to a reduction in eggs per spawner (by approximately 25% from the late 1980s to the early 2020s). Intrinsic productivity has declined by an average of approximately 60% across CUs from the late 1980s to the early 2020s. Ecosystem and Climate Change Considerations Rapid and ongoing climate and ecosystem change occurring throughout the freshwater and marine habitats Yukon River Chinook salmon use is expected to result in conditions that continue to keep population productivity relatively depressed. Factors affecting adult Yukon River Chinook salmon during their upriver migration and spawning include the Whitehorse dam, Ichthyophoniasis disease, low water levels, and warm stream temperatures which may all contribute to increased pre-spawn mortality. Changes in the Bering Sea ecosystem that Yukon River Chinook salmon use through their marine life include increased competition with other salmon, shifts in prey availability, and increased predator abundance which may contribute to decreased fitness and survival. The consequences of ecosystem and climate change were implicitly taken into account in the evaluation of harvest control rules (HCRs) by using population models with intrinsic productivity estimated from recent brood years (2012–2017). Stock Advice Harvest Advice Closed loop simulations to 2050 were used to evaluate alternative HCRs assuming productivity remains the same as it was over the most recent generation. A subset of these HCRs included: recently implemented seven-year agreement1,which limits harvest unless border passage is expected to exceed 71,000; the Interim Management Escapement Goal that was in place prior to the seven year agreement (lower bound of 42,500 aggregate spawners); and a Precautionary Approach compliant HCR, which allowed harvest at lower abundances than other HCRs considered, and limited harvest rates at large run sizes. Extending the seven-year agreement until 2050 is expected to allow for limited rebuilding of some CUs and modest harvest opportunities. Under the agreement, spawner abundances are projected to exceed their CU-specific lower biological benchmarks for six CUs, and exceed upper biological benchmarks for four of those six. Using the lower bound of the Interim Management Escapement Goal after the seven-year agreement is expected to allow for higher average harvests than if the agreement remains in place, but at the cost of one additional CU in the Yukon Chinook SMU falling below its lower biological benchmark. Implementing an alternative Precautionary Approach-compliant HCR after the seven-year agreement expires, which allows harvest at lower abundances than other HCRs considered, has similar biological and fishery performance to the seven year agreement, but reduces the frequency of fishery closures from four in every five years to one in every two. Under simulations that assume productivity remains very depressed (35% below most recent generation average), all HCRs resulted in very limited opportunities for harvest and at least four of nine CUs within the Yukon Chinook SMU falling below their lower biological benchmarks. Enhancement Advice An analysis to determine feasibility and a risk-benefit assessment would inform appropriate enhancement for rebuilding activities following enhanced contribution guidelines. If enhancement is undertaken, the development and implementation of an enhancement plan that summarizes recovery goals, recovery phases, benchmarks, triggers, and intended outcomes in collaboration with Yukon First Nations and stakeholders is strongly recommended. 1Agreement of April 1, 2024 regarding Canadian-origin Yukon River Chinook Salmon for 2024 through 2030