Inter-annual variability of Chinook salmon in AYK rivers

Strontium isotopes in otoliths, and genetics, were used to quantify the inter-annual variability in the spatial patterns of Chinook salmon production across the Yukon and Kuskokwim rivers for fish caught in test fisheries in 2018 (combined with Yukon River data from 2015 & 2017, and data from 2017 in the Kuskokwim River). We partitioned total escapement to each of these rivers into discrete reporting groups with distinct spatial distributions throughout each watershed. We provide novel insights into how the biocomplexity of these large river influences the stability of the overall production at the scale of the entire basin. The spatial distributions of the natal origins of Chinook salmon underscored the importance of the entire watersheds, and their complexity, for maintaining fisheries and informing management approaches. It was possible to make natal habitat assignments based on genetics and Sr isotopes in otoliths for 250 Chinook salmon from the Yukon River in 2018, and based on Sr isotopes alone, from 250 fish from the Kuskokwim River in 2018. In the Yukon, genetic data determined the broad geographic region of origin (i.e., Lower, Middle, and Upper Yukon), while the isotopes further refined these to produce 13 total reporting groups. Strontium isotopes alone produced 5 distinct reporting groups in the Kuskokwim River. In the Kuskokwim River, the spatial distributions of natal origins of adult Chinook salmon were similar in 2017 and 2018, as reflected in the comparable distribution of Sr isotope values in the natal regions of otoliths from fish caught in these years. In both years, the natal origins of most fish were from the George, Holitna, Stony and Takotna Rivers. While the Kwethluk River contributed about 8% of of the total fish in each year. The Sr isotope data showed distinct stock structures associated with seasonal timing of when fish were caught in the Bethel Test Fishery. In both years, fish caught in the first third of the season were mostly from tributaries located in the upper- and mid-watershed; fish caught in the latter part of the season tended to return to tributaries in the lower watershed. In the Yukon River, combining genetic and Sr isotopic data, also indicated that Chinook salmon production for each return year was heterogeneously distributed across the basin. Furthermore, the spatial pattern of production shifted substantially among years, illustrating that some parts of the basin are disproportionately important for any given return year. Like in the Kuskokwim River, a distinct stock structure was detected associated with seasonal timing of when fish were caught in the Lower Yukon Test Fishery, though the spatial patterns were different among years. In 2015, most fish caught in the early third of the run were returning to the Canadian components of the upper watershed, while in 2017 the distribution of the natal origins of early-migrating fish included more of the middle Yukon watershed. Surprisingly, fish caught in the early season of 2018 had natal origins distributed throughout the entire Yukon watershed, though the tendency for higher contributions of upper watershed populations was still evident. These results highlight that the entirety of the Yukon and Kuskokwim rivers is involved in producing Chinook salmon, and that the habitat complexity across the basin and its associated biological diversity are likely important for stabilizing the total production of fish from year-to-year. Results also show that the temporal organization of different stocks migrating through the lower river may present challenges for fishery management with an objective to protect biodiversity within each of the rivers. Although the reasons why patterns of production show complementary dynamics overtime is unclear, the analytical framework presented here provide critical insights as Yukon River Chinook salmon populations respond to the rapid environmental change occurring across the Arctic and Subarctic.

This project is essential for designing effective conservation and management plans for Arctic-Yukon-Kuskokwim (AYK) Chinook salmon that allow both conservation of the biocomplexity within these ecosystems while enabling sustainable fisheries, in the face of increasingly uncertain futures.

Critical insights into what drives freshwater mortality (Theme 1) will come from building a time-series of production patterns of Chinook salmon across a range of environmental conditions. With enough years of data, this would eventually be possible with the analytical framework used herein because it is able to quantify spatial patterns in salmon production across these basins each year. By linking production estimates in space and time with other geospatial data that characterize biologically important environmental conditions, such as patterns in snow cover, precipitation, and air temperature, we will be able to evaluate to what degree freshwater environmental conditions contribute to shifts in production patterns. These environmental conditions throughout the AYK region can be constrained using satellite data (e.g., from MODIS [Moderate Resolution Imaging Spectroradiometer]) or climate modeling efforts such as SNAPS (Scenario Network for Alaska + Arctic Planning).

Managing for sustainability in the face of uncertainty (Theme 4) is directly dependent on accurate and effective monitoring of key biological, chemical, and physical parameters of the ecosystems supporting the production of key ecosystem services such as Chinook salmon fisheries. Building and maintain a time-series of how production patterns are distributed across river basins is an important component of monitoring the status of the stocks because it represents the integration of all ecological processes that affect the production of salmon from large free-flowing river basins. Such analyses will be able to identify how multiple dimensions of biological diversity (locally adapted population structure, life history diversity, and habitat complexity) interact to influence the reliability of salmon production at the scale of the entire river. Furthermore, such baseline monitoring provides critical information for evaluating the tradeoffs among industrial development (e.g., of mines) and habitat conservation within these basins.

In order to assess risk to vulnerable salmon stocks (Theme 5) it is necessary to have the ability to quantify levels of production of the sub-stocks in question. The ultimate goal of the analytical framework we have developed is to do just that: to quantify the relative contribution of individual populations, habitats, and life histories to the overall production of the Yukon and Kuskokwim Rivers basins. Thus, our proposal responds directly to the need to be able to identify which stocks within the Yukon and Kuskokwim Rivers are at risk. This information will provide much-needed input to questions about how system-wide escapement goals and harvest strategies might compromise the population diversity throughout each river because current management makes the implicit assumption that each river can be characterized by a demographically homogeneous stock. Our results will enable a thorough assessment of the biological and management implications of this assumption.