Spatio-temporal variation of Chinook salmon in AYK rivers

Chinook salmon in the Yukon and Kuskokwim rivers support important subsistence, commercial, and recreational fisheries that are stabilized by complexity in the biological composition of the stocks, and the diversity of habitats fish spawn, rear, and migrate through in these vast watersheds. This biocomplexity remains poorly described and the stabilizing effects of the diversity in population dynamics of fish in different tributaries has not been rigorously quantified. We propose to use a recently developed framework to quantify the inter-annual variability in the spatial patterns of Chinook salmon production across the Yukon and Kuskokwim rivers over much of the last decade. Our approach uses natural markers (i.e., strontium isotopes in otoliths, and DNA) to accurately determine the natal origins of individual fish from which production patterns of Chinook salmon can be constructed across these remote rivers. These data will also be used to develop tributary-specific adult migration patterns, which are a principal determinant of vulnerability to lower river fisheries, and one of only a few manageable attributes of Chinook salmon in these ecosystems. By quantifying how the spatial patterns of Chinook salmon production shift among years, we will provide novel insights into how the biocomplexity of these large rivers influences the stability of the overall production at the scale of the entire Yukon and Kuskokwim watersheds. Patterns of variation and co-variation among stocks will be quantified from returning adults sampled in several years since 2010 in the Yukon River, and continuously since 2017 in the Kuskokwim River. This proposal will analyze new and existing otolith and genetics collections, to complement existing data from previous projects. The products from this synthesis will be directly applicable to developing and assessing management and conservation strategies for sustaining fisheries in the face of increasingly uncertain futures.

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.