Quantifying thermal exposure for migratory riverine species: phenology of Chinook salmon populations predicts thermal stress

Migratory species are particularly vulnerable to climate change because
habitat throughout their entire migration cycle must be suitable for the
species to persist. For migratory species in rivers, predicting climate
change impacts is especially difficult because there is a lack of
spatially-continuous and seasonally-varying stream temperature data,
habitat conditions can vary for an individual throughout its life cycle,
and vulnerability can vary by life stage and season. To predict thermal
impacts on migratory riverine populations, we first expanded a spatial
stream network model to predict mean monthly temperature for 465,775 river
km in the western U.S., and then applied simple yet plausible future
stream-temperature change scenarios. We then joined stream temperature
predictions to 44,396 spatial observations and life stage-specific
phenology (timing) for 26 ecotypes (i.e. geographically distinct
population groups expressing one of four distinct seasonal migration
patterns) of Chinook salmon (Oncorhynchus tshawytscha), a phenotypically
diverse anadromous salmonid that is ecologically and economically
important but declining throughout its range. Thermal stress, assessed for
each life stage and ecotype based on federal criteria, was influenced by
migration timing rather than latitude, elevation, or migration distance,
such that sympatric ecotypes often showed differential thermal exposure.
Early-migration phenotypes were especially vulnerable due to prolonged
residency in inland streams during the summer. We evaluated the thermal
suitability of 31,699 stream km which are currently blocked by dams to
explore reintroduction above dams as an option to mitigate the negative
effects of our warmer stream temperature scenarios. Our results showed
that negative impacts of stream temperature warming can be offset for
almost all ecotypes if formerly occupied habitat above dams is made
available. Our approach of combining spatial distribution and phenology
data with spatially- and temporally-explicit temperature predictions
enables researchers to examine thermal exposure of migrating populations
that use seasonally-varying habitats.