Sensitivity to climate change
The survival of steelhead embryos or recently emerged fry may be sensitive to the timing and magnitude of spring runoff rather than the fall and winter aspects of flow regimes. For example, high winter flows that threaten the egg-to-fry survival of fall-spawning salmonids are not predicted to negatively affect steelhead. Steelhead juveniles, which typically live in freshwater for 2 years, would be sensitive to lower summer flows due to threats posed by reduction in amount of rearing habitat, increased exposure to higher temperatures, and/or forced movement into less preferred habitat (e.g., competition with other species).
Steelhead may also exhibit some sensitivity to warming water temperatures. Direct measures of Oncorhynchus mykiss thermal physiology suggest many parameters do not differ significantly from those of other salmonids (except in locally adapted populations of redband rainbow trout in desert streams). In addition, contemporary temperature regimes in the Columbia River cause steelhead and Chinook salmon to use the same thermal refuges during spawning migrations. Similar to Chinook salmon, steelhead are vulnerable to high angling pressure when seeking refuge in cold refugia such as tributary junctions; thus warmer temperatures can have indirect effects on mortality. However, the geographic distribution of steelhead suggests they may be less sensitive to warm temperatures than other anadromous salmonids—steelhead occur in Southern California, farther south than any Pacific salmon. Further, the resident life history form of O. mykiss can persist in desert streams that often exceed 20˚C through what appears to be local adaptation. Whether steelhead populations from warmer streams exhibit higher thermal tolerance is poorly understood, as is the potential rate of evolution in attributes of thermal physiology.
Similar to Chinook salmon, steelhead exhibit alternative life histories in regards to run-timing, which confer different sensitivities to climate. Summer-run steelhead migrate higher in river networks, entering freshwater between late spring and fall, and overwinter before spawning the following spring. In contrast, winter-run steelhead migrate during winter or early spring and spawn immediately. Because they spend more time in freshwater, summer-run populations of steelhead may be more sensitive to changes in flow and temperature regimes across river networks. For example, higher temperatures will increase the metabolic costs accrued by summer-run steelhead during the several months that they hold in streams prior to spawning.
The existence of a resident life history form likely buffers O. mykiss from environmental stochasticity and may make populations less vulnerable to extirpation. For example, anadromous individuals can survive ephemeral periods of unsuitability in their natal streams while they are away at the ocean, whereas residents can survive in years where conditions are poor along migratory routes.
Exposure to climate change
- Altered spring runoff timing and amount/magnitude
- Increased water temperatures
- Lower summer flows