Body size and early marine conditions drive changes in Chinook salmon productivity across northern latitude ecosystems

Disentangling the influences of climate change from other stressors
affecting the population dynamics of aquatic species is particularly
pressing for northern latitude ecosystems, where climate driven warming is
occurring faster than the global average. Chinook salmon (Oncorhynchus
tshawytscha) in the Yukon-Kuskokwim (YK) region occupy the northern extent
of their species’ range and are experiencing prolonged declines in
abundance resulting in fisheries closures and impacts to the wellbeing of
Indigenous people and local communities. These declines have been
associated with physical (e.g., temperature, streamflow) and biological
(e.g., body size, competition) conditions, but uncertainty remains about
the relative influence of these drivers on productivity across populations
and how salmon-environment relationships vary across watersheds. To fill
these knowledge gaps we estimated the effects of marine and freshwater
environmental indicators, body size, and indices of competition, on the
productivity (adult returns-per-spawner) of 26 Chinook salmon populations
in the YK region using a Bayesian hierarchical stock-recruitment model.
Across most populations, productivity declined with smaller spawner body
size and sea surface temperatures that were colder in the winter and
warmer in the summer during the first year at sea. Decreased productivity
was also associated with above average fall maximum daily streamflow,
increased sea ice cover prior to juvenile outmigration, and abundance of
marine competitors, but the strength of these effects varied among
populations. Maximum daily stream temperature during spawning migration
had a nonlinear relationship with productivity with reduced productivity
in years when temperatures exceeded thresholds in mainstem rivers. These
results demonstrate for the first time that well-documented declines in
body size of YK Chinook salmon were associated with declining population
productivity, while taking climate into account.