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Spawning habitat characteristics and egg mortality in relation to river geomorphology and run-times of chum salmon (Oncorhynchus keta) in a metropolitan river system, northern Japan

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Abstract

We investigated the geomorphology and environmental variables in which early- and late-run chum salmon groups spawn in an urban section of the Toyohira River, northern Japan, in relation to egg mortality, where a braided riverbed had been developing before river improvement occurred. Geomorphic units in the river channel having the highest proportions were riffles > the upwelling zone of gravel bars > pools > secondary channels. Most redds (> 60%) in the early-run group were built in the upwelling zone of gravel bars in the primary stream, indicating that salmon chose this geomorphic unit for spawning. A greater proportion of spawning redds in the late-run group occurred in secondary channels (i.e., smaller subsidiary channels that branch from the main, active channel). The buried-egg experiment showed that egg mortality was lower in the early-run group and higher in spawning redds that were shallower and had a higher maximum water temperature in winter. Late-run salmon need to select habitat with relatively higher water temperatures for spawning to compromise between egg mortality risk and the later timing of offspring hatching. Annual variation in the number of out-migrating fry was most associated with the number of spawning redds of the early-run group. A metropolitan river system may be highly regulated by humans and represents a monotonous river morphology, which nonetheless provides favorable spawning habitat for certain run-time populations because wild salmon may facultatively utilize limited diverse environments for natural reproduction.

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Raw data are available by request from the corresponding author.

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Acknowledgements

We thank the members of the Sapporo Wild Salmon Project and staff of the Sapporo Salmon Museum, Fisheries Resources Institute, Japan Fisheries Research and Education Agency, for their cooperation in the field study. We are grateful to JN Negishi for advice on depicting the groundwater levels in the schematic figures; we also thank H. Tanaka for the figure illustrations. We appreciate the anonymous reviewers for their valuable comments on earlier versions of the manuscript.

Funding

This study was partly supported by the research fund for the Ishikari and Tokachi Rivers provided by the Ministry of Land, Infrastructure, Transport, and Tourism of Japan, and by the JSPS KAKENHI grant numbers 21H03647 and 20H01441.

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NA and KM conceived the research idea. NA, KM, MA, KU, KF, KO, and KW designed the methods and performed the collections. NA and KM analyzed the data. NA, KM, and FN led the writing of the manuscript. All authors contributed critically to the drafts and gave final approval for publication.

Corresponding author

Correspondence to Nozomi Aruga.

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The survey was conducted with the permission of the Governor of Hokkaido, Japan. The buried salmon eggs were handled appropriately.

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The authors declare no conflict of interests.

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Aruga, N., Morita, K., Aruga, M. et al. Spawning habitat characteristics and egg mortality in relation to river geomorphology and run-times of chum salmon (Oncorhynchus keta) in a metropolitan river system, northern Japan. Environ Biol Fish 106, 1277–1293 (2023). https://doi.org/10.1007/s10641-023-01415-z

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  • DOI: https://doi.org/10.1007/s10641-023-01415-z

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