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Boise, ID  83702

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Does Wildfire Threaten Extinction for Salmonids?

Responses of Redband Trout and Bull Trout Following

Recent Large Fires on the Boise National Forest


 Bruce Rieman, Danny Lee, Gwynne Chandler, and Deborah Myers

USDA Forest Service, Rocky Mountain Research Station

316 E. Myrtle

Boise, Id 83702 

Boise, ID 83702 Proceedings- Fire Effects on Rare and Endangered Species and Habitats Conference, Nov 13-16, 1995

Coeur dí Alene, Idaho

IAWF, 1997


The potential for wildfire to impact aquatic ecosystems and their associated threatened, endangered, or sensitive species is of increasing concern.  Recent (since 1988) large-scale fires followed by dramatic hydrologic disturbances spark much of this interest.  Broad swaths of western forest lands, where fire suppression and past silvicultural activities have radically altered vegetation structure and fuel loads, are ripe for high-intensity fires.  The potential seems greatest in warm/dry habitat types that historically were dominated by frequent, but low intensity burns.  Interconnected, fuel-laden stands may now link areas that historically burned less frequently or uniformly into large, homogeneous areas that are vulnerable to high-intensity, stand replacing events (Agee 1988; Henjum et al. 1994).  Recent fires in the Pacific Northwest seem to confirm these expectations. 

Wildfires influence aquatic ecosystems both directly and indirectly.  Direct effects include heating or abrupt changes in water chemistry (Minshall et al. 1989; McMahon and de Calesta 1990).  Indirect effects include changes in hydrologic regime, erosion, debris flows, woody debris loading and riparian cover (Swanson and Lienkaemper 1978; Brown 1989; Megahan 1991; Bozek and Young 1994).  Intense fires and related events have killed fish (Bozek and Young 1994) and even caused local extinctions (Propst et al. 1992; Rinne 1996).  Conceivably, large and intense fires could threaten populations of sensitive salmonids such as bull trout, chinook salmon, steelhead, and others that are depressed from other causes.  Historical fires, however, were a natural and potentially important part of the disturbance regime for terrestrial and aquatic systems (Reeves et al. 1995).  Large fires supplied woody debris and triggered hydrologic events and debris flows that transported coarse substrates to stream channels.  These processes may well have provided the materials that maintained productive habitats for fish and other organisms (Swanson et al. 1990; Reeves et al. 1995).


 The magnitude and intensity of recent fires heighten concerns regarding forest/ecosystem health, the potential loss of valuable wood fiber and private property, and the apparent threat to sensitive species.  Such concerns have galvanized new efforts to reduce fuel loads and stand densities through mechanical treatment and the use of prescribed fire.  These efforts create a quandary for biologists and managers working with aquatic systems.  The long-term negative effects of timber harvest activities on aquatic ecosystems are well documented (see papers in Meehan 1991 and Salo and Cundy 1987; Henjum et al. 1994) The effects of fire on fish are more equivocal.  Do large fires really threaten extinction for many existing salmonid populations?  What influences the risk? 


Large fires in the Boise River basin on the Boise National Forest in 1992 and 1994 provided an opportunity to examine these questions relative to populations of two sensitive salmonids.  Bull Trout (Salvelinus confluentus) is a category-one species under the Endangered Species Act (ESA), and redband or interior rainbow trout (Oncorhynchus mykiss) is recognized as a species of special concern by the Idaho Department Fish and Game.  Some isolated redband populations have been petitioned for formal listing under ESA.  Both species inhabit streams caught within fires described as among the most destructive ever observed on the Forest.  We initiated work on the responses of these fishes to wildfire and related effects in 1992.  The work was planned as long term and much is incomplete.  Our preliminary results and the body of literature regarding the disturbance and recovery of aquatic communities provide a base, however, to initiate the discussion. 


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