Written by Dr. Robert Lusardi, the California Trout-UC Davis Wild and Coldwater Fish Scientist – republished from The Current Winter 2019 issue
California has been ravaged by an increase in the frequency and intensity of wildfires. During 2018, there were more than 8,400 wildfires that burned nearly two million acres across the state, the most in recorded history. The increase in fire frequency is the result of an interaction between climate change and increased fuel loads and fire suppression over the last century, among other factors. Despite the recent increase in wildfires throughout the state, we know relatively little about how they affect salmonids. Understanding the effects of wildfire on salmonids, however, is important. Wildfire was found to be one of the primary anthropogenic threats affecting inland salmonids in the recent State of the Salmonids Report. Further, recent fires such as the Carr Fire near Redding, California and its proximity to the remaining population of winter-run Chinook salmon suggest we need to better understand the various ways wildfire can affect vulnerable species or populations.
Similar to many large-scale ecological processes, the magnitude of effects of wildfire on salmonids are context specific and depend on numerous factors. Such factors include the location of the fire within the watershed, the amount of area burned, the intensity of the fire, the species and/or populations affected and their status, and the condition of the watershed prior to fire. Similar to droughts and floods, wildfires also represent important disturbances on the landscape that salmonids have evolved to over millions of years. While the short-term effects of wildfire on salmonids can negatively affect populations in numerous ways, similar to floods and drought, some of the long-term effects can be beneficial to salmonids.
The most notable short-term effects of wildfire on salmonids include a loss of riparian habitat (and subsequent shading) and an influx of fine sediment. Riparian habitat loss affects both stream water temperature and increases sediment delivery to rivers. In turn, significant increases in fine sediment to watersheds affected by fire have been shown to reduce salmonid spawning and rearing habitat principally through a loss of viable egg habitat. Local hydrology and water chemistry can also be greatly impacted by fire as runoff and overland flow increase with an abrupt loss of riparian vegetation. For instance, Oliver et al. (2012) found that runoff occurred earlier in recently burned areas in California and that the delivery of nutrients, such as nitrate and phosphorous, increased. In general, intense wildfires adjacent to streams can alter local hydrology and change sediment delivery patterns and in some cases even lead to the direct mortality of fish. Bozek and Young (1994) found that a combination of intense rainfall immediately after a wildfire and increased sedimentation lead to the direct mortality of salmonids in the greater Yellowstone ecosystem. Yet, even in these instances, recolonization of recently impacted habitats can be relatively quick and largely depends on the location of viable source populations. For instance, Rieman et al. (1995) found that most stream reaches affected by wildfire were repopulated within the first year and approached densities of unimpaired stream reaches in one to three years.
Wildfires can also positively affect salmonid habitat. Flitcroft et al. (2015) recently found that large wood recruitment increased in recently burned watersheds and improved overwintering habitat for juvenile salmonids through the creation of complex pool habitats. Increased delivery of nutrients, as documented by Oliver et al. (2012) and others, may also improve ecosystem productivity by stimulating stream food webs and improving the quantity and quality of prey resources. Changes in hydrology and increases in overland flow may also lead to large debris flows in upstream tributaries which, as a result, may deliver larger, cobble-size sediments to spawning areas to help improve spawning habitat. Similar to the effects of flooding, wildfires can also promote and even enhance instream processes known to benefit salmonids, yet this often occurs over longer time periods.
However, as with floods, there’s an important caveat with wildfires and the evolution of salmonids. Much like dams have harnessed rivers throughout California and changed the frequency and magnitude of ecologically important flood events with dire consequences for salmonids, climate change combined with increased fuel loads and fire suppression have likely accomplished the same for wildfires with similar consequences. Historically, both floods and wildfires negatively affected salmonids in the short-term, but abundant and diverse populations enabled these fishes to recolonize disturbed habitats quickly and allow them to persist in the long-term. Changes in the frequency and intensity of wildfire combined with a general loss of population diversity across the landscape may, inevitably, create a phenomenon where salmonids lack the tools to recover and ultimately adapt to the new wildfire regime we are currently witnessing in California. Our best bet to confront this new threat is to enhance population diversity and adaptive capacity in order to improve species resilience and provide salmonids with the tools necessary to adapt to a rapidly changing landscape.
Bozek, M.A., and M.K. Young. 1994. Fish mortality resulting from delayed effects of fire in the Greater Yellowstone Ecosystem. Great Basin Naturalist 54: 91-95.
Flitcroft, R.L., Falke, J.A., Reeves, G.H., Hessburg, P.F., McNyset, K.M., and L. Benda. 2016. Wildfire may increase habitat quality for spring Chinook salmon in the Wenatchee River subbasin, WA, USA. Forest Ecology and Management 359: 126-140.
Oliver, A.A., Bogan, M.T., Herbst, D.B., and R.A. Dahlgren. 2012. Short-term changes in-stream macroinvertebrate communities following a sever fire in the Lake Tahoe basin, California. Hydrobiologia 694: 117-130.
Rieman, B., Lee, D., Chandler, G., and D. Meyers. 1995. Does wildfire threaten extinction for salmonids? Responses of redband trout and bull trout following recent large fires on the Boise National Forest. Conference Proceedings – Fire Effects on Rare and Endangered Species and Habitats. Coeur d’ Alene, Idaho. Pages 47-57.