Estimations des points de référence biologiques pour le complexe de stocks de saumon chinook (Oncorhynchus tshawytscha) du cours principal du fleuve Yukon d’origine canadienne

Abstract

The Yukon River Basin is one of the largest salmon producing river basins in the world and Chinook salmon (Oncorhynchus tshawytscha) from the river have historically supported commercial, subsistence, and First Nations fisheries in both Alaska and Canada. The Yukon River Chapter of the Pacific Salmon Treaty specifies a spawning escapement goal for Canadian-Origin Mainstem Yukon River Chinook salmon which has been revised over time and considered interim in nature until a comprehensive review and analysis of available data could be completed. To inform a biologically based aggregate escapement goal we developed a semi integrated state-space run reconstruction and spawner-recruitment model fit to data (1981-2019) from various assessment projects that estimate mainstem passage, harvests, tributary escapements, stock-proportions, and age-composition, under a single Bayesian estimation framework. We found that the Canadian origin Yukon River Chinook salmon stock aggregate is moderately productive. Equilibrium stock size (SEQ) was estimated to be 110,601 (95% CRI: 81,708-234,252, the spawner abundance expected to maximize long-term sustainable yield (SMSY ) was estimated to be 43,125 (29,874- 93,070) and the spawner abundance expected to maximize recruitment (SMSR) was estimated to be 70,447 (41,094-186,200). Female Chinook salmon age at maturity, and to a lesser extent the proportion of females in the spawning population, has declined over time. We adapted the integrated state-space run reconstruction and spawner-recruitment model to account for these demographic changes and found that the spawner abundance expected to maximize yield or recruitment was estimated to be on average 13% and 19% greater, respectively, in recent years than in our baseline analysis that did not take demographic changes in escapement into consideration. We outline key considerations when developing an escapement goal based on the information we provide and conclude with recommendations for future work. These include a more comprehensive consideration of the consequences of demographic change in the spawning stock and explicit consideration of trade-offs between the harvest rates, and escapement goals, predicted to maximize aggregate yield (or recruitment) and risk to individual weak (less productive) populations within the Canada stock aggregate. We also recommend undertaking a Management Strategy Evaluation that quantifies trade-offs among a broad range of objectives and evaluates the ability of alternative management strategies to meet them as part of a collaborative process with fishery participants, Traditional Knowledge holders, and resource managers.