Spatio-temporal variation of Chinook salmon in AYK rivers

The diverse life history strategies of Pacific Salmon in western Alaska pose a challenge for resource allocation and management. However, this genetic and life history diversity expressed across space and time reduces variability in overall returns by distributing risk across a continuum of habitat types and localized environmental conditions. Variability in life history among subpopulations increases population resilience to short term or localized environmental stressors, and maintaining this variability may therefore be key to maximizing climate adaptation and population resilience. To do so, however, requires knowledge as to the spatial ecology of Chinook salmon in Western Alaska watersheds, which is currently poorly understood. The use of otolith-based isotope signatures has emerged as a tool for reconstructing patterns of spatial ecology in Pacific Salmon. By relating the isotopic signature found in the otolith to those in the environment, this method can be used to reconstruct population scale estimates of provenance and habitat use where traditional monitoring is not possible. Here, we use otolith-based methods to estimate the spatial distribution of natal origin locations for Chinook salmon in the Yukon and Kuskokwim River basins over several years. In doing so, we explore how life history strategies of Chinook salmon are structured in these systems and expressed at various spatial and temporal scales. For example, in the Yukon River, most Chinook salmon that returned in 2015 were derived from populations in the lower watershed, while in 2021 most of the production was derived from populations in the upper parts of the watershed. In parallel, analyses that partition the run into the lower river according to catch date within a season, demonstrate considerable stock structure associated with migration timing. Fish that migrate early in the season have a tendency to be headed for the upper river while fish migrating later in the season are more likely to be derived from populations lower in the watershed. These results not only contribute towards our understanding of the spatial ecology of Chinook salmon in Alaska’s largest watersheds but may help to inform targeted management and conservation strategies which may best preserve fishery resilience by maintaining life history diversity in Chinook salmon across the landscape.

Understanding the ecology of Chinook salmon in western Alaska’s large rivers is hampered by the immense size, complexity, and remoteness of these ecosystems, which severely limit appropriately scaled observations of fish and habitat conditions across these vast watersheds. Stock assessments and management of fisheries for Chinook salmon must make a series of simplifying assumptions about how distinct the biology and population dynamics of individual stock components are throughout the stock complexes produced from these ecosystems. Stock assessments of both Yukon and Kuskokwim Chinook salmon have relied heavily on weirs located on a small number of tributaries, with little data beyond highly uncertain aerial surveys on the majority of habitat. This lack of reliable information on the diversity of Chinook salmon populations from the Yukon and Kuskokwim rivers makes stock assessments prone to uncertainty about the stock composition of returns to these rivers, thereby challenging management intending to balance conservation and fisheries outcomes.

The principal research question is to quantify how production patterns of adult Chinook salmon shift in space inter-annually across the Yukon and Kuskokwim river basins. We have produced the data and developed the analytical approach to quantify shifting patterns in production of Yukon River Chinook salmon for adult fish caught in lower river test fisheries in 2010 and, 2015-2018. By extending this time-series to 2019 and 2021, we will be able to provide more robust estimates of inter-annual variability in the spatial patterns of production across the Yukon River. We have also determined the production patterns of Chinook salmon for the 2017 and 2018 return years to the Kuskokwim River. Extending the Kuskokwim analyses to 2019-2021 will be the next step towards quantifying how production patterns shift across the basin year-to-year. These data will then be used to statistically describe the biocomplexity of Chinook stocks that reside throughout each of the Yukon and Kuskokwim rivers that are a critical input to analyses to examine trade-offs in sustaining yield and minimizing risk to biocomplexity under different harvest strategies. Furthermore, continuing such time-series provides the foundation for gaining key insights into the causes of shifting production dynamics in free-flowing river basins by evaluating how production patterns change across a range of environmental conditions.

The second objective of this proposal is to use natal origins based on Sr in isotopes to quantify the relationships between adult migration timing and tributary origins of Chinook salmon returning to different regions of the Yukon and Kuskokwim rivers. Eiler et al. (2014) showed that there was substantial variation in the migration timing of different Chinook salmon stocks within the Yukon River, based on telemetry data from 2002-2004. In general, fish returning to the upper and middle watershed returned early during the migration season and stocks that migrated to the lower river migrated in the latter parts of the migration season. Such general organization of the stock composition during different migration stanzas is common to other Chinook salmon stocks in Alaska including the Kuskokwim River. We will use our natal assignments (based on otoliths and genetics) to reconstruct temporal changes in the stock composition of Chinook salmon migrating through the lower Yukon and Kuskokwim rivers. Our observations will be integrated with earlier telemetry and genetic studies on these rivers to develop Bayesian estimates of average migration timing and variance within and among years over the time period from the early 2000’s to the present. Such information will be extremely useful for assessing different temporal closure strategies for regulating harvest on different stocks within the Yukon and Kuskokwim stock complexes.