Climate Change and the Rocky Mountains - Impact of Global Changes on Mountains: Responses and Adaptation - page 443

spring runoff decreases summer base fl ows and available habitat, which
will further accelerate increasing water temperatures during a thermally
stressful time for many aquatic organisms.
Climate-induced changes in fl ow and temperature regimes may also
have direct effects on salmonid reproduction and recruitment across Rocky
Mountain streams. Native salmonids are either spring spawners, adapted
to deposit eggs in the streambed on the descending limb of the snowmelt
hydrograph (e.g., cutthroat trout) or fall spawners, which deposit their
eggs during the fall and emerge the following spring (e.g., bull trout)
(Northcote 1997). In either case, salmonids may be especially sensitive to
increased fl ood events because high fl ows can scour incubating eggs from
gravel substrates or wash away newly emerged fry. In some regions, winter
fl oods are projected to continue increasing due to warming temperatures
causing precipitation to shift from snow to a snow-rain mix. Likewise,
more rapid spring snowmelt runoff may reduce recruitment of recently
emerged juveniles, creating an additional environmental stressor that these
fi sh must respond to.
Native fi shes have persisted for millennia in these cold mountainous
landscapes that have undergone tremendous geologic and hydrologic
change shaped by continental glaciation, fl ood, wildfi re, and periods of
extreme temperature warming. These differences in climatic regimes and
environmental conditions have allowed salmonids to adapt to a diversity
of aquatic habitats (e.g., streams, rivers, lakes, ponds) through highly
plastic life histories, which are primarily linked to temperature and fl ow.
Shifts in the timing and magnitude of ecologically important fl ows and
temperatures are clearly evident across the Rocky Mountain region. For
example, migratory cutthroat trout and bull trout spawn and rear in natal
headwater streams, but move downstream to grow and mature in rivers
(i.e., fl uvial) or lakes (i.e., adfl uvial) (Rieman and McIntyre 1995, Muhlfeld
and Marotz 2005, Muhlfeld et al. 2009). These unique evolutionary
behaviors sustain the genetic diversity of populations and are critical
for the persistence of salmonid metapopulations in complex riverscapes.
If climate warming trends continue, however, headwater streams used
for spawning and early rearing will be at greater risk of extirpation by
stochastic perturbations, whereas downstream habitats used for foraging,
migrating and overwintering may become thermally and hydrologically
unsuitable, resulting in the loss of migratory life history forms, genetic
diversity and headwater source populations. Thus, the migratory nature of
salmonid fi shes may make them particularly susceptible to extirpation or
decline in a rapidly warming world. However, the diversity of life history
strategies found in these fi sh may enable some populations to adapt through
phenotypic or genetic means or move to track suitable habitats to complete
their life cycle. Generally, the adaptive capacity of aquatic organisms is still