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

Abstract

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 the 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 explicit and temporally explicit temperature predictions enables researchers to examine thermal exposure of migrating populations that use seasonally varying habitats.

Capacity of Two Sierra Nevada Rivers for Reintroduction of Anadromous Salmonids: Insights from a High‐Resolution View

“…2008; see also the discussion by FitzGerald et al. 2020). Overall, the scope for local adaptation to the prevailing thermal regime seems greater for the spawning stage than for the holding stage.…”

“…However, FitzGerald et al. (2020) recently reviewed accounts of spawn timing throughout California and found that spring‐run Chinook Salmon may spawn as early as August or as late as December. Thompson et al.…”

Does global change increase the risk of maladaptation of Atlantic salmon migration through joint modifications of river temperature and discharge?

“…Diadromous fish populations such as Atlantic salmon are in drastic decline worldwide due to the effects of global change on both freshwater and marine environments [ 78 , 79 ]. Changes in one life stage can propagate throughout the life cycle and affect subsequent life stages or accumulate over several generations [ 80 ].…”

Quantification of thermal impacts across freshwater life stages to improve temperature management for anadromous salmonids

“…For aquatic ectotherms, temperature is the primary abiotic factor that controls biophysical, biochemical and bioenergetic processes ( Fry, 1971 ), thus influencing spatial distributions ( Fourcade et al, 2018 ), physiological rates, daily and long-term survival and evolutionary trajectory ( Brannon et al, 2004 ). Seasonal patterns in temperature have resulted in corresponding patterns in life history, phenology and spatial distribution for aquatic populations, such that a geographically widespread species may consist of multiple populations with unique thermal environments and thermal adaptations ( Chen et al, 2013 ; Eliason et al, 2011 ; FitzGerald et al, 2021 ; Flitcroft et al, 2016 ; Nadeau et al, 2017 ; Quinn, 2018 ; Stitt et al, 2014 ; Zillig et al, 2021 ). Sudden shifts in the long-term thermal average can induce conditions for which a population has not experienced.…”

“…The temperature of the water, timing of release, duration of release, amount of water and discharge rate are constantly debated because the thermal requirements for one life stage or species may be harmful to another (e.g. Zarri et al, 2019 ), interspecific populations may exhibit different adaptations ( Eliason et al, 2011 ; Zillig et al, 2021 ) and thermally suitable habitat may vary spatially and temporally ( Armstrong et al, 2021 ; FitzGerald et al, 2021 ). These conditional effects are not well-studied due to their complexity, indicating a need for a spatially and temporally explicit comparative model that can estimate population-specific thermal impacts on sympatric and successive life stages ( Crozier et al, 2017 ; Crozier et al, 2021 ; Snyder et al, 2019 ).…”