How long were young salmon at sea? Tracing coho salmon origins with otoliths.

Jenn Hoskins
16th January, 2026

How long were young salmon at sea? Tracing coho salmon origins with otoliths.

This study distinguished between the diverse juvenile life histories of Coho salmon (Oncorhynchus kisutch) by comparing the visual stages of winter annulus formation in the otolith (a) with the timing of saltwater entry, which is identified by a sharp increase in the strontium-to-calcium ratio (b).

Image adapted from: Anderson et al. / CC BY (Source)

Key Findings

  • Coho salmon in Washington’s Chehalis River basin exhibit diverse life histories, with seven unique strategies identified
  • Most coho salmon migrate to the ocean as year-old smolts, but 20-39% emigrate earlier while still growing
  • Identifying these previously unrecognized migration strategies improves understanding of population stability and more accurate run size forecasts
Coho salmon populations face increasing challenges from environmental changes, making effective management and conservation crucial. A key aspect of this is understanding the diverse ways these fish live and grow – their ‘life history strategies’ – and how each contributes to the overall population. Traditionally, assessments of salmon populations have focused on fish that spend a year in freshwater before migrating to the ocean as ‘smolts’[1]. However, recent research from the Washington Department of Fish and Wildlife demonstrates a far more complex picture. The study focused on coho salmon returning to tributaries of the Chehalis River in Washington state. Researchers aimed to identify the different pathways juvenile salmon take from their birth streams to the ocean and, crucially, determine how successful each pathway is. To do this, they analyzed ‘otoliths’ – small ear bones within the fish. Otoliths grow throughout a salmon’s life, creating annual rings, much like trees, that record their environment. By examining these rings and the chemical composition of the otoliths, scientists can reconstruct a fish’s life history, including when it moved between freshwater and saltwater. The research team examined otoliths from nearly 200 adult coho salmon returning in 2021 and 2022. They discovered seven distinct life history strategies in total – three in the East Fork Satsop River and six in the Newaukum River. The timing of when fish left freshwater varied considerably, ranging from August to June. While the majority (52-80%) of returning fish followed the traditional pattern of migrating as age-1 smolts in the spring, a significant proportion (20-39%) emigrated earlier, while still actively growing their first otolith annulus. In the Newaukum River, a small percentage (2-3%) even left before the annulus had begun to form, representing an even earlier migration. This finding is significant because it reveals previously unrecognized successful migration strategies. Prior research has highlighted the importance of population diversity for stabilizing ecosystem services[2][3]. The Bristol Bay sockeye salmon fishery, for example, thrives due to the hundreds of discrete spawning populations with diverse life histories, allowing the overall stock to withstand environmental fluctuations[2]. This ‘portfolio effect’ – similar to diversifying financial investments – means that if one population struggles, others can compensate, ensuring the fishery remains productive. The study expands on this concept by demonstrating life history diversity within a single species. It shows that coho salmon aren’t a single homogenous group, but rather a collection of individuals employing different strategies. The identification of these strategies is important because conventional estimates of juvenile production, which typically focus only on spring-migrating smolts, may be underestimating the total number of fish successfully reaching adulthood. Furthermore, understanding these different life histories is vital for accurately forecasting future salmon returns. The variability in habitat use, as shown in other studies of coho salmon[4], can influence which strategies are most successful in a given year. By incorporating this knowledge into population models, scientists can improve their ability to predict how salmon populations will respond to changing environmental conditions. The study[5] on strontium isotope records reinforces this, demonstrating that the reliability of salmon production is linked to shifting habitat mosaics, and understanding these dynamics is key to accurate forecasting. The Washington Department of Fish and Wildlife’s research underscores the importance of considering the full range of life history diversity when managing salmon populations. Ignoring these previously unrecognized strategies can lead to inaccurate assessments and potentially hinder conservation efforts.

WildlifeEcologyMarine Biology

References

Main Study

1) When did you leave home? Reconstructing juvenile life histories of coastal Washington adult coho salmon (Oncorhynchus kisutch) using otolith structure and chemistry

Published 13th January, 2026

https://doi.org/10.1371/journal.pone.0340362

Related Studies

2) Biocomplexity and fisheries sustainability.

Journal: Proceedings of the National Academy of Sciences of the United States of America, Issue: Vol 100, Issue 11, May 2003

3) Population diversity and the portfolio effect in an exploited species.

https://doi.org/10.1038/nature09060

4) The contribution of estuary-resident life histories to the return of adult Oncorhynchus kisutch.

https://doi.org/10.1111/jfb.12380

5) Shifting habitat mosaics and fish production across river basins.

https://doi.org/10.1126/science.aav4313


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