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Developing High-Resolution Strontium Isotope Maps of Alaska Rivers to Track Pacific Salmon Migrations: The Nushagak River as a Case Study to Evaluate Spatial and Seasonal Variability


Matthew WoollerWater and Environmental Research Center
Institute of Northern Engineering



Alaska's fisheries managers need cost effective, accurate, and reliable tools to accurately identify the birthplace, called natal tributaries, of adult salmon harvested in mixed-stock commercial fisheries. In this case study of the Nushagak River in southwest Alaska's Bristol Bay, researchers will examine the feasibility of using strontium (87Sr/86Sr) isotope signatures found in Nushagak River tributaries and those found in salmon otoliths to identify the fine-scale tributary-level origins of adult salmon. If successful, this study could result in an important new tool to effectively manage the region's commercial salmon fisheries, which generate more than $2.9 billion in local harvesting and processing-related economic activity. This study will be the first thorough attempt to resolve salmon mixed-stock fisheries in Alaska rivers using isotopic data integrated with genetic data.

For more information, see the 2014 news story “Student’s strontium isotope project offers a new map of Alaska”.


The issue

Alaska's fishery managers have a critical need for tools that can accurately and cost-effectively identify natal sources of salmon in mixed-stock fisheries. Researchers hope to develop new, accurate, and economically sound techniques to describe the fine-scale salmon population structure important to the health and sustainability of both wild salmon populations and human coastal fishing-dependent communities. As reported in recent literature, there is great potential for genetic and isotopic techniques to work in concert to identify local breeding populations in complex mixed-stock fisheries. Genetic differentiation has the ability to identify broad regional and subregional population structure, while isotopic techniques (87Sr/86Sr in otoliths) are likely to be able to differentiate population structure to a much finer scale, perhaps down to the tributary level. This case study to evaluate 87Sr/86Sr spatial and temporal variability with respect to both fish populations and river water, is necessary to determine the feasibility of using isotopic techniques as a management tool.

Why is this an Alaska Sea Grant project?

One of Alaska Sea Grant's six key goals outlined in the 2009–2013 Strategic Plan is sustained, well-managed, and healthy marine, coastal, and watershed ecosystems in Alaska. The program pursues this goal through support of research that provides decision-makers with science-based information that can be used to craft well-informed policies governing the use and conservation of Alaska's marine and coastal resources.

How will researchers conduct their study?

The primary objective of this project is to develop and apply strontium isotope (87Sr/86Sr) ratio analyses of otoliths and tributary river water chemistry to trace adult Nushagak River salmon (chinook, coho, and sockeye) caught in mixed-stock fisheries conducted at the river's mouth and lower river back to respective natal sources.

To effectively evaluate the feasibility of using strontium 87Sr/86Sr ratios to identify salmon natal origins, researchers will investigate the following four sub-objectives:

1. Evaluate 87Sr/86Sr spatial variation within Nushagak River watershed and quantify isotopic compositions of its geologically distinct tributaries to link to salmon populations.

2. Evaluate seasonal variation of the Nushagak River and its tributaries and investigate how potential seasonal Sr-isotopic variation is recorded in otoliths.

3. Use otolith chemistry to evaluate seasonal movement of fish between tributaries and mainstem river.

4. Evaluate a model of intra-watershed (~50 square km and larger) 87Sr/86Sr variation using high resolution digital geologic maps and available rock geochemistry data (Sr ppm and 87Sr/86Sr) in ArcInfo Geographic Information Systems (GIS) to characterize potential salmon natal sources.

During the summers of 2012 and 2013 baseline fish and water samples will be collected on the Nushagak River. Seasonality in tributary and mainstem 87Sr/86Sr water values will be evaluated using nonmigratory slimy sculpin (Cottus cognatus) otoliths (sub-objective 2). Seasonal movements of fish, as related to 87Sr/86Sr otolith time-series, will be evaluated using seasonally migratory arctic grayling (Thymallus arcticus) otoliths (sub-objective 3). Otolith 87Sr/86Sr values of juvenile and adult chinook, coho, and sockeye salmon (Oncorhynchus tshawytscha, O. kisutch, and O. nerka, respectively) captured in tributaries will document variation among local breeding populations (sub-objective 1) and potential seasonal juvenile salmon migration (sub-objective 3). Natal origins and the proportions of different breeding populations will be discerned using both 87Sr/86Sr values of otoliths and Genetic Stock Identification (conducted by Alaska Department of Fish and Game) from salmon sampled during commercial harvest in the Nushagak fishing district and subsistence fish camps in the lower river. The intra-watershed 87Sr/86Sr variation of otoliths and waters researchers plan to describe will be compared with the current evaluation of chinook salmon genetic variation by ADFG and the most recent report on sockeye salmon genetic variation. The isotopic variation researchers observe within the population of coho salmon in the Nushagak River will be the first baseline data characterizing coho salmon population structure, as no genetic data exist. Additionally, using the proposed water collections and analyses they will evaluate the accuracy of a priori modeled 87Sr/86Sr variation within the Nushagak River (sub-objective 4). This model uses high-resolution geologic maps and rock geochemistry data to predict spatial variation of 87Sr/86Sr values of river waters.

Analyses of 87Sr/86Sr ratios in otoliths and waters will be done using multicollector-inductively coupled plasma mass-spectrometry (MC-ICP-MS) by Sean Brennan (PhD student supported by this proposal) and co-PIs Drs. Cerling and Fernandez at the University of Utah ICP-MS Laboratory using established laser ablation and solution methods. A suite of other geochemical analyses will be conducted on water samples as well including: analyses of elemental concentrations, alkalinity, δ18O and δ2H, pH, temperature and conductivity. Otolith analyses will: (a) target the region accreted during a salmon’s freshwater stage (~250 mm distal of primordia—after yolk absorption, during exogenous feeding, but before outmigration) and (b) produce 87Sr/86Sr time-series of individuals.

Research collaborators

United States Geological Survey
Alaska Department of Fish and Game
University of Utah
Village of New Stuyahok
Peter Pan Seafoods
National Institute of Water Resources
Center for Ocean Sciences Education and Excellence
University of Alaska Fairbanks, Bristol Bay Campus


What researchers learned

In this project we developed a spatially and temporally robust strontium isotopic baseline map of the Nushagak River, which is able to determine the birthplace and freshwater migration patterns of Pacific salmon produced in this large watershed. Using the map, we simultaneously determined natal origins at fine spatial scales (a seven-fold increase compared to the leading genetic methods) and freshwater life history variation of adult Chinook salmon harvested during a mixed stock fishery. This baseline can also be used to determine the same ecological dimensions for coho and sockeye salmon produced within the Nushagak River. In so doing, this study presents a new framework to investigate how the ecological dimensions influence overall stock productivity and respond to perturbation. Alaska is home to some of the world’s last thriving wild salmon and salmon-based cultures in the world. Metrics such as the one developed here will be integral to their effective conservation. Our results also demonstrate how coupling spatially and temporally robust baseline maps of strontium isotopic variation with sequentially growing animal tissues can inform conservation planning of other highly mobile species.

Research impacts

Alaska Sea Grant researchers confirm tributary variability in fish isotope ratios for use in salmon stock identification

Relevance: Managing salmon harvest in mixed stock fisheries is a challenge. Trace metals (for example strontium isotope ratios) can be used to identify natal origins and movement patterns of fishes, yet time variability in these tracers can make the task difficult.

Response: Alaska Sea Grant–funded researchers investigated space and time variability in strontium isotopes in slimy sculpin, a non-migratory fish, to develop a baseline signature across a watershed. This research applies to using the strontium isotope tracer to track individual Chinook salmon to their natal stream. Alaska is an ideal site to create strontium isotope models of river waters because of the low human impact on rivers and a wealth of geochemical and geological data. However, evaluating temporal (time) variability in river water strontium isotopes is imperative for effectively applying this tool to salmon migration ecology. Researchers used the time-keeping properties of the sculpin to evaluate time variability in strontium isotope ratio throughout the Nushagak River in Western Alaska.

Results: Strontium isotope ratios recorded in sculpin otoliths tracked temporal variability of site-specific water chemistry with relatively high sensitivity. These ratios were found to be temporally stable and as a result strontium ratios recorded in the otoliths of sculpins can be used to evaluate temporal variability of the tracer—a prerequisite for research aiming to discern migrations of fish, including salmon, within this watershed. The method has broad applicability, as non-migratory fish can serve as sentinel species for monitoring temporal changes in freshwater ecosystems. The results were published in Geochimica et Cosmochimica Acta.

Recap: Alaska Sea Grant–funded researchers developed a method for identifying natal streams of individual Chinook salmon in the Nushagak River in western Alaska, and using non-migratory fish they validated the technique by showing the isotopic tracer varies across tributaries and is stable over time.

Researchers refined and further developed bedrock-water strontium isotope models for Alaska rivers, which were successfully used in three graduate student research projects on provenance of dinosaurs, extinct bison, and Bering cisco.

Relevance: Alaska fisheries managers need cost effective, accurate, and reliable tools to identify the birthplace of adult salmon harvested in mixed-stock commercial fisheries. This project used strontium isotope ratios to track salmon yielded tools that can fill knowledge gaps and provide data on origins of other animals, including extinct species. Differences in rock type, age, and weathering create variations in the strontium concentration and isotope ratio in water, which is reflected in animals that spend time there.

Response: Researchers developed tools to track salmon to natal streams, based on strontium concentrations and isotope ratios in the streams and in fish. Aware that Alaska is an ideal site to create bedrock-water strontium isotope models, because of the low human impact on rivers and a wealth of geochemical and geological data, researchers continued to build and refine the tools. They created an extensive data set that documents ratios of the Sr isotopes in 61 Alaska rivers. New models predicting 87Sr/86Sr ratios in bedrock and water across Alaska were the first baseline and modeling efforts on Alaska strontium isotopic variation. These data products can be used for other regional life form provenance studies.

Results: Two MS students and one PhD student at the University of Alaska Fairbanks have used data products from this research to track the past migration of dinosaurs on Alaska’s North Slope, extinct bison in interior Alaska, and Bering cisco in the Yukon River.

Researchers developed a method for identifying natal streams of individual Chinook salmon in the Nushagak River in western Alaska.

Relevance: Managing commercial, sport, and subsistence salmon harvests in mixed stock fisheries is a challenge. The management goal is to allow as much harvest as possible while enabling enough salmon to return to their natal streams to ensure plentiful fish in future years. Currently sonar is used at river mouths to count returning salmon, but managers have no way to follow salmon movement through the system to hundreds of natal streams.

Response: Over several years graduate student Sean Brennan collected water samples from the main river and tributaries of the Nushagak River in western Alaska. At the same time he collected otoliths from salmon in the bay. Otoliths, the auditory structure of the salmon, contain natural chemicals from the waters where the fish swim. The water samples identified two isotopes of strontium, a natural chemical element. Differences in rock type, age, and weathering create variations in the strontium isotope ratio of the stream, which then becomes incorporated into the otoliths of young salmon.

Results: By matching strontium isotope ratios from otoliths with those in the waterways, scientists may be able to determine the natal streams of salmon caught in the ocean. Such tools are important for the effective conservation of salmon biodiversity and the management of sustainable fisheries.

Research outcomes

Brennan, S.R, C.E. Zimmerman, D.P. Fernandez, T.E. Cerling, M.V. McPhee, and M.J. Wooller. 2015. Strontium isotopes delineate fine-scale natal origins and migration histories of Pacific salmon. Science Advances 1(4). https://doi.org/10.1126/sciadv.1400124

Brennan, S.R., D.P. Fernandez, C.E. Zimmerman, T.E. Cerling, R.J. Brown, and M.J. Wooller. 2015. Strontium isotopes in otoliths of a non-migratory fish (slimy sculpin): Implications for provenance studies. Geochimica et Cosmochimica Acta 149:32–45. https://doi.org/10.1016/j.gca.2014.10.032

Brennan, S.R. 2014. Using strontium isotopes to track Pacific salmon migrations in Alaska. Ph.D. dissertation, University of Alaska Fairbanks, SGT-14-02, 152 pp.

Brennan, S.R., D.P. Fernandez, G. Mackey, T.E. Cerling, C.P. Bataille, G.J. Bowen, and M.J. Wooller. 2014. Strontium isotope variation and carbonate versus silicate weathering in rivers from across Alaska: Implications for provenance studies. Chemical Geology 389:167–181. https://doi.org/10.1016/j.chemgeo.2014.08.018

Bataille, C.P., S.R. Brennan, J. Hartmann, N. Moosdorf, M.J. Wooller, and G.J. Bowen. 2014. A geostatistical framework for predicting variations in strontium concentrations and isotope ratios in Alaskan rivers. Chemical Geology 389:1–15. https://doi.org/10.1016/j.chemgeo.2014.08.030