Anatomy of a range contraction: Flow-phenology mismatches threaten salmonid fishes near their trailing edge

Climate change is redistributing life on Earth, with profound impacts for
ecosystems and human well-being. While repeat surveys separated by
multi-decadal intervals can determine whether observed shifts are in the
expected direction (e.g., poleward or upslope due to climate change), they
do not reveal their mechanisms or time scales: whether they were gradual
responses to environmental trends or punctuated responses to disturbance
events. Here we document population reductions and temporary range
contractions at multiple sites resulting from drought for three Pacific
salmonids at their ranges’ trailing edge. During California’s 2012-2016
historic multi-year drought, the 2013-14 winter stood apart because
rainfall was both reduced and delayed. Extremely low river flows
during the breeding season (‘flow-phenology mismatch’) reduced or
precluded access to breeding habitat. While Chinook (Oncorhynchus
tshawytscha) experienced a down-river range shift, entire cohorts failed
in individual tributaries (steelhead trout, O. mykiss) and in entire
watersheds (coho salmon, O. kisutch). Salmonids returned to impacted sites
in subsequent years, rescued by reserves in the ocean, life history
diversity, and, in one case, a conservation broodstock program. Large
population losses can, however, leave trailing-edge populations vulnerable
to extinction due to demographic stochasticity, making permanent range
contraction more likely. When only a few large storms occur during high
flow season, the timing of particular storms plays an outsized role in
determining which migratory fish species are able to access their riverine
breeding grounds and persist.