The life-histories of exploited fish species, such as Pacific salmon, are vulnerable to a wide variety of anthropogenic stressors including climate change, selective exploitation and competition with hatchery releases for finite foraging resources. However, these stressors may generate unexpected changes in life-histories due to developmental linkages when species complete their migratory life cycle in different habitats. We used multivariate time-series models to quantify changes in the prevalence of different life-history strategies of sockeye salmon from Bristol Bay, Alaska, over the past half-century—specifically, how they partition their lives between freshwater habitats and the ocean. Climate warming has decreased the time spent by salmon in their natal freshwater habitat, as climate-enhanced growth opportunities have enabled earlier migration to the ocean. Migration from freshwater at a younger age, and increasing competition from wild and hatchery-released salmon, have tended to delay maturation toward the salmon spending an additional year feeding in the ocean. Models evaluating the effects of size-selective fishing on these patterns had only small support. These stressors combine to reduce the size-at-age of fish vulnerable to commercial fisheries and have increasingly favoured a single-age class, potentially affecting the age class complexity that stabilizes this highly reliable resource.
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All Bristol Bay sockeye age composition, Bristol Bay escapement data, Lake Aleknagik ice break-up dates and Lake Aleknagik temperature data used to support the findings of this study can be found in Supplementary Information. Climate indices PDO and NPGO are available online from http://research.jisao.washington.edu/pdo/ and http://www.o3d.org/npgo/, respectively. North Pacific salmon biomass is available from the online supplementary material for ref. 37.
Code for the multivariate autoregressive state-space models used in this study has been deposited at Github (https://github.com/tjcline/MARSStmb).
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We thank the many graduate students, postdocs, faculty and research scientists who have worked for the University of Washington’s Alaska Salmon Program for help in collecting and organizing these data, particularly J. Carter. Thank you to the Alaska Department of Fish and Game for the brood table data for sockeye salmon returning to Bristol Bay. We thank T. Walsworth for helpful comments throughout the project. Funding was provided by the National Science Foundation (NSF) through a graduate research fellowship to T.J.C. Ongoing monitoring of Bristol Bay lakes has been supported by NSF and the Gordon and Betty Moore Foundation.
The authors declare no competing interests.
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Supplementary Fig. 1, Supplementary Tables 1–4, Supplementary References
Returns of sockeye salmon to Bristol Bay since brood year 1963 aggregated by river system, freshwater age, and ocean age. Returns are in thousands of fish and ages are in years
The escapement of sockeye salmon to the 9 river systems in Bristol Bay since 1963. Escapement numbers are in thousands. Escapement is counted near the mouth of each river by the Alaska Department of Fish and Game
Average summer surface (0–20 m) watertemperature (ºC) and day of spring ice off for Lake Aleknagik, AK
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Cline, T.J., Ohlberger, J. & Schindler, D.E. Effects of warming climate and competition in the ocean for life-histories of Pacific salmon. Nat Ecol Evol 3, 935–942 (2019). https://doi.org/10.1038/s41559-019-0901-7
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