Beyond latitude: Thermal tolerance and vulnerability of a broadly distributed salmonid across a habitat temperature gradient

Salmonid fishes are a focal point of conservation physiology due to their
high value to humans and ecosystems, and their susceptibility to decline
from climate change. A significant challenge in conserving these fishes is
that populations of the same species can be locally adapted to vastly
different habitats within their wild ranges and can therefore have unique
tolerance or vulnerability to environmental stressors within those
habitats. Within the state of Oregon, USA, summer steelhead (Oncorhynchus
mykiss) inhabit both cool, coastal waters most typically associated with
Pacific salmonids as well as arid, inland environments where temperatures
are more extreme. Here, we utilized streamside physiological experiments
paired with habitat temperature monitoring to assess the thermal tolerance
and vulnerability of four populations of summer steelhead from distinct
thermal habitats. All populations had unique responses of critical thermal
maximum, aerobic scope, and exercise recovery to temperature. Despite
populations from warm habitats exhibiting higher thermal tolerance than
populations from cooler habitats, summer steelhead from warm habitats
appear to be more vulnerable to the physiological consequences of warming
based on the extreme temperatures they already experience during the
summer. These results demonstrate an example of thermal physiology varying
between populations within the same portion of their latitudinal range and
highlight the need for habitat-specific conservation strategies for this
species.