Pete Bisson, Aquatic and Land Interactions Program, Pacific Northwest Research Station.
One of the most important long-term threats to fish habitat resilience is climate change. A recent review of the effects of climate change on salmon (ISAB 2007) identified the following probable consequences of global warming along the Pacific coast of North America: 1) warmer temperatures will result in more precipitation falling as rain rather than snow, 2) snowpack will diminish and streamflow timing will be altered, 3) peak river flows will likely increase, and 4) water temperatures will continue to rise. Not all of these anticipated trends are necessarily harmful to aquatic habitat, and many pale in comparison to other anthropogenic factors, but they do have implications for salmon and trout populations.
Climate change scenarios predict an increase in large flood events, wildfires, and forest pathogen outbreaks, all of which have some potential to improve fish habitat complexity as a result of flood plain reconnection and large wood recruitment. Many effects of climate warming, however, will have negative habitat consequences for salmon. A higher frequency of severe floods will result in increased egg and alevin mortality owing to gravel scour, especially for fall- and winter-spawning species. Retreating winter snowpacks will run off earlier in the spring (Mote et al. 2003), potentially altering the life cycles of salmon whose seaward migration is timed to coincide with nearshore plankton blooms (Pearcy 1997). Summer base flows will be lower, and the network of perennially flowing streams in a drainage system will shrink during the summer dry period, forcing fish into smaller wetted channels and less diverse habitats (Battin et al. 2006). Warmer water temperatures will increase physiological rearing costs and lower growth rates if warmer streams do not produce sufficient food resources to offset heightened metabolic demands. Additionally, summer temperatures may approach or exceed incipient lethal levels for salmon and trout (Crozier and Zabel 2006, Crozier et al. 2008), and higher temperatures will likely favor non-salmonid species that are better adapted to warmer water, including potential predators and competitors (Reeves et al. 1987).
As noted by Battin et al. (2006), climate change will force shifts in the distribution of salmon populations that will affect their ability to cope with natural disturbances, particularly drought. Streams located high in watersheds that historically provided some of the best habitat may no longer be accessible to salmon if snowpack is reduced, thus limiting available rearing areas and access to thermal refugia in summer. Crozier et al. (2008) modeled Chinook salmon (Oncorhynchus tshawytscha) population response to alternative climate scenarios in Idaho's Salmon River and found that even moderate changes significantly increased the risk of local population extirpation. Crozier and Zabel (2006) suggested that two climate-related factors (temperature and streamflow) could affect habitat in different ways depending on local site characteristics; narrow, confined streams were more sensitive to flow changes, and wide streams were more sensitive to temperature changes. They concluded that different aspects of climate change were important at different spatial scales, and that a diversity of conditions was needed for metapopulation stability.
Trout and salmon within the interior Columbia River Basin may be especially sensitive to climate change, according to a recent report by a scientific panel (ISAB 2007). Although the intensity of the effects will vary spatially, climate change will alter virtually all streams and rivers in the basin. Current predictions suggest that temperature increases alone will render 2 to 7 percent of headwater trout habitat in the Pacific Northwest unsuitable by 2030, 5 to 20 percent by 2060, and 8 to 33 percent by 2090. Salmon habitat may be more severely affected, in part because these fish are usually restricted to lower, hence warmer, elevations within the region. Salmon habitat loss would be most severe in Oregon and Idaho with potential losses exceeding 40 percent by 2090. Loss of salmon habitat in Washington would be less severe, with the worst-case scenario indicating about 22 percent loss by 2090.
Temperature records show that the Pacific Northwest has warmed 1.8 °F since 1900, approximately 50 percent more than the average global warming during the same period. The warming rate for the region in the 21st century is projected to range from 0.2 to 1.1 °F per decade. Until late in the 21st century, precipitation changes for the region are projected to be relatively modest and likely to be indistinguishable from natural variability; however, some models suggest an increase in winter storm severity. Most climate models project long-term increases in winter precipitation and decreases in summer precipitation. These changes in temperature and precipitation will alter the snowpack, streamflow, and water quality, particularly in the Columbia River Basin. Warmer temperatures will result in more precipitation falling as rain rather than snow. Snowpack will diminish, winter snow lines will retreat to higher elevations, and snowmelt timing will be altered. With earlier runoff, peak river flow will occur earlier in the year, and summer water temperatures will continue to rise as water levels drop.
Climate change has the potential to affect most freshwater life-history stages of trout and salmon. Increased frequency and severity of flood flows during winter will affect over-wintering juvenile fish and incubating eggs in the streambed. Eggs of fall- and winter- spawning fish, including Chinook, coho (Oncorhynchus Kisutch), chum (O. keta) , sockeye salmon (O. nerka), and bull trout (Salvelinus confluentus), may suffer higher levels of mortality when exposed to increased flood flows. Warmer winter water temperatures will accelerate embryo development and may cause premature emergence of fry. Bull trout require very cold headwater streams for spawning; therefore, a warming climate will disproportionately affect this species.
Options for Management
From a habitat resilience standpoint, maintaining as much water as possible in streams and lakes during periods of low flow will likely be the most effective way to combat the harmful effects of climate change, but other management actions could also produce long-term benefits. Zabel et al. (2006) used population viability analyses to predict that "increasing the freshwater carrying capacity for juveniles is most likely important for recovery. This may include improving the quality of existing habitats and making areas currently unoccupied accessible or suitable." Increased flooding associated with higher peak discharge in winter may result in greater societal pressure to prevent damage to homes and infrastructure by isolating rivers from their flood plains; therefore, habitat managers would be well served to ask where flooding can be allowed in a watershed and in particular where flooding will reconnect the river with flood-plain habitats of direct importance to overwintering salmon. Maintaining key flood-plain connections will also act as a hydrologic safety valve that helps reduce the scouring effect of high flows on redds.
Another management response to climate change involves restoring longitudinal connections throughout a drainage network, i.e., removing anthropogenic blockages to fish migrations up and down the watershed. With a constricted system of perennial stream channels in summer it will be important for all potentially usable habitats to be available.
A fourth management safeguard involves protecting and restoring riparian forests on valley floors and on alluvial terraces adjacent to stream channels. Riparian forests play an important role in the dynamics of the water table beneath and adjacent to streams, in moderating discharge during flow extremes, in controlling the concentration of soluble nutrients, in mediating the seasonal input of organic matter and terrestrial food items to aquatic ecosystems, and in regulating microclimate (Naiman et al. 2005).
Policies that explicitly maintain instream flows by limiting water withdrawals, enhancing flood-plain connectivity by opening historically flooded areas where possible, removing anthropogenic barriers to fish movement, and protecting riparian forests will be needed to conserve habitat resilience in the face of climate change. Without such policies in place, aquatic habitats are likely to become increasingly isolated, simplified, and less likely to recover after significant disturbance events.
Although options for forest managers to minimize the harm to aquatic resources from climate change are limited, there are several management actions that can help protect salmon and trout:
- Minimize anthropogenic increases in water temperature by maintaining well-shaded riparian areas.
- Maintain a forest stand structure that retains snow, reduces the "rain on snow" effect associated with forest openings, and promotes fog drip.
- Disconnect road drainage from the stream network to soften discharge peaks during heavy rainstorms.
- Ensure that fish have access to seasonal habitats, e.g., off-channel wintering areas or summer thermal refugia.
- Protect springs and large groundwater seeps from development and water removal, as these subterranean water sources will become increasingly important when surface flows are altered by climate change.