Salmon Recovery, Science on the Street|

Ecosphere – 2023 – Sorel – Juvenile life history diversity is associated with lifetime individual heterogeneity

Differences in the life history pathways (LHPs) of juvenile animals are often
associated with differences in demographic rates in later life stages. For migratory
animals, different LHPs often result in animals from the same population
occupying distinct habitats subjected to different environmental drivers.
Understanding how demographic rates differ among animals expressing different
LHPs may reveal fitness trade-offs that drive the expression of alternative
LHPs and enable better prediction of population dynamics in a changing environment.
To understand how demographic outcomes and their relationships
with environmental variables differ among animals with different LHPs, we
analyzed a long-term (2006–2021) mark–recapture dataset for Chinook salmon
(Oncorhynchus tshawytscha) from the Wenatchee River, Washington, USA.
Distinct LHPs represented in this population include either remaining in the
natal stream until emigrating to the ocean as a 1-year-old (natal-reach rearing)
or emigrating from the natal stream and rearing in downstream habitats for
several months before completing the emigration to the ocean as a 1-year-old
(downstream rearing). We found that downstream-rearing fish emigrated to
the ocean 19 days earlier on average and returned as adults from the ocean at
higher rates. We detected a positive correlation between rate of return from
the ocean by downstream-rearing fish and coastal upwelling in their spring
of outmigration, whereas for natal-reach-rearing fish we detected a positive
correlation with sea surface temperature during their first marine
summer. Different responses to environmental variability should lead to
asynchrony in adult abundance among juvenile LHPs. A higher proportion of
downstream-rearing fish returned at younger ages compared with
natal-reach-rearing fish, which contributed to variability in age at reproduction
and greater mixing across generations. Our results demonstrate how
diversity in juvenile LHPs is associated with heterogeneity in demographic
rates during subsequent life stages, which can in turn affect variance in
aggregate population abundance and response to environmental change.

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