Linking aerobic scope to fitness in the wild reveals key opportunities to recover imperiled salmon populations

Aquatic ectotherms are hypothesized to be vulnerable to warming and
deoxygenation associated with environmental change because temperature and
oxygen (O2) supply can restrict aerobic scope (AS) in captivity. However,
evidence of a direct association between AS and fitness in the wild is
lacking, inspiring debate about the circumstances under which AS is the
primary driver of population fluctuations. Using the metabolic index (ɸ),
we related AS to two Chinook salmon (Oncorhynchus tshawytscha) population
bottlenecks in the wild, juvenile rearing and migration. We found that AS
governed success probability for these bottlenecks only under a relatively
narrow window of viable environmental conditions, depending on
intraspecific metabolic trait diversity and hydrologic conditions.
Opportunities for high-impact temperature- and O2-specific conservation
and management actions using existing hydraulic engineering infrastructure
therefore exist when AS is between critical (ɸcrit) and stable (ɸstable)
values. Outside of this ecological threshold, increases in AS did not
yield appreciable fitness benefits because successful rearing and
migration were either exceptionally improbable (i.e., AS<ɸcrit), or
seemingly independent of AS (i.e., AS>ɸstable). In addition, AS
impairments likely increased susceptibility to predation, and this may
have been involved in the putative association between AS and fitness in
the wild. We propose that ɸstable may serve as a more conservative
benchmark than ɸcrit to prevent extirpations and recover imperiled
populations under a changing climate.