Kuskokwim Strontium Isotope Baseline

Kuskokwim River Chinook salmon have experienced low returns during the last decade, critically challenging commercial fishing and subsistence-based human communities of this watershed. Fundamental knowledge gaps concerning the ecology of its Chinook salmon populations remain unknown but are critical to their effective management. In particular, it is unknown how Chinook salmon production is distributed across the Kuskokwim’s diverse tributaries and habitats, and how these production patterns change over time in response to climate forcing and fishery interceptions lower in the river. Spatial variability in salmon production patterns across large river basins and how these patterns shift through time are integral to the resilience of populations and fisheries to environmental change. This largely stems from intact watersheds being able to distribute the risk of low production across a variety of
distinct populations, habitats, and life history strategies. We currently lack the tools to easily delineate how production patterns change over time across the Kuskokwim River basin. Here, we built a strontium isoscape (the spatial variation in 87Sr/86Sr ratios) of the Kuskokwim River that is able to determine Chinook salmon production patterns and life histories at small spatial scales. Using the isoscape and a geographically continuous Bayesian assignment framework, we determined the natal origins of 262 fish caught in the Bethel Test Fishery based on a match between the strontium isotopes in their otoliths and the spatial variation across the watershed. By aggregating the natal origin maps of these fish, we estimated the spatial pattern in production for the 2017 return year. Production of Chinook salmon returning in 2017 to the Kuskokwim was heterogenous across the river network, with
most fish being produced from tributaries such as the Aniak River and Salmon Fork of the Pitka River. This information can be used to inform stock assessments that are currently hampered by a distinct lack of data describing the distribution of returning fish among tributary populations. In addition to analyzing 87Sr/86Sr ratios across the Kuskokwim River, we also analyzed concentrations in mercury (Hg) in slimy sculpin tissues, plus a suite of other dissolved constituents in river waters (e.g., dissolved organic carbon) known to covary with bioavailable Hg concentrations. We constructed a spatial hydrology model which quantified the patterns of mercury contamination in slimy sculpins throughout the ecosystem. Hg concentrations varied across the basin at multiple spatial scales reflecting the influence of multiple biological, chemical, and physical landscape processes. The baseline map of the spatial variation in fish tissue Hg can be used to better assess the potential risk of consuming resident fish to human communities and the influence of proposed industrial development in the basin.

Kuskokwim River Chinook salmon have experienced low returns during the last decade, critically challenging commercial fishing and subsistence-based human communities of this watershed. Fundamental knowledge gaps concerning the ecology of its Chinook salmon populations remain unknown but are critical to their effective management. In particular, it is unknown how Chinook salmon production is distributed across the Kuskokwim’s diverse tributaries and habitats, and how these production patterns change over time in response to climate forcing and fishery interceptions lower in the river. Spatial variability in salmon production patterns across large river basins and how these patterns shift through time are integral to the resilience of populations and fisheries to environmental change. This largely stems from intact watersheds being able to distribute the risk of low production across a variety of distinct populations, habitats, and life history strategies.
We currently lack the tools to easily delineate how production patterns change over time across the Kuskokwim River basin. Here, we built a strontium isoscape (the spatial variation in 87Sr/86Sr ratios) of the Kuskokwim River that is able to determine Chinook salmon production patterns and life histories at small spatial scales. Using the isoscape and a geographically continuous Bayesian assignment framework, we determined the natal origins of 262 fish caught in the Bethel Test Fishery based on a match between the strontium isotopes in their otoliths and the spatial variation across the watershed. By aggregating the natal origin maps of these fish, we estimated the spatial pattern in production for the 2017 return year. Production of Chinook salmon returning in 2017 to the Kuskokwim was heterogenous across the river network, with most fish being produced from tributaries such as the Aniak River and Salmon Fork of the Pitka River. This information can be used to inform stock assessments that are currently hampered by a distinct lack of data describing the distribution of returning fish among tributary populations.
In addition to analyzing 87Sr/86Sr ratios across the Kuskokwim River, we also analyzed concentrations in mercury (Hg) in slimy sculpin tissues, plus a suite of other dissolved constituents in river waters (e.g., dissolved organic carbon) known to covary with bioavailable Hg concentrations. We constructed a spatial hydrology model which quantified the patterns of mercury contamination in slimy sculpins throughout the ecosystem. Hg concentrations varied across the basin at multiple spatial scales reflecting the influence of multiple biological, chemical, and physical landscape processes. The baseline map of the spatial variation in fish tissue Hg can be used to better assess the potential risk of consuming resident fish to human communities and the influence of proposed industrial development in the basin.