Fire and Fish (Aquatic Ecosystems)

One of the many excuses used to justify “thinning” and logging today is to preclude massive wildfires. Notwithstanding, there is considerable evidence that such actions do not impede large fires, which only occur during extreme fire weather; people still use this as an excuse. To generate even more public support, one often hears dire warnings that a large fire will decimate the river ecosystem. In particular, if the river is a popular sport fishery, then “obviously” we need to stop large fires or suffer the consequences of ruined fishing opportunities.

The summer of 2000 will be remembered as one of the driest on record. I will use it here because there was a significant amount of research after that summer relevant to the discussion of aquatic ecosystems. During the summer of 2000, wildfires burned across millions of acres of the West.

The two factors are not unrelated. All historical and paleo-botanical evidence suggests that most large blazes occur in times of drought. In 1910, after one of the driest winters and spring on record, more than 3.5 million acres burned across northern Idaho and western Montana. Although some may suggest that the fires of 2000 were a “disaster,” such characterizations demonstrate a lack of ecological understanding.

All large blazes are inevitable with the right ingredients. Mix fuels, drought, wind, and an ignition source—usually lightning—and you have a recipe for widespread and unstoppable fires. Nevertheless, you need to put together all those ingredients in the same place at the same time, or you may not even get a fire, much less a big fire. Simply piling up fuels won’t create large fires. Nor can all the lightning strikes in the world cause wood or grass to burn if it’s wet and saturated.

It’s somewhat analogous to driving a car with a stick shift. You need to turn the key, push on the accelerator, and let out the clutch all at the same time for the car to move forward. Fail to do any one of these, and the vehicle will remain stationary. Of the four ingredients, drought and wind are the two most critical elements that drive large blazes. And both parts helped to create the conditions that fueled the large flames of 2000.

Not surprisingly, the alignment of all the proper ingredients for large fires doesn’t happen every summer, sometimes not even once or twice a century in many parts of the West. Like a hundred-year flood, we tend to view big blazes as unnatural merely because they are infrequent. Yet large blazes are well within the “normal” disturbance regime that influences most western landscapes, and western ecosystems, including aquatic ecosystems, adapt to the changes that large blazes often create.

Nevertheless, making any generalizations about fire effects on any particular stream system is made difficult by the variation of the landscape across the West. Fires in chaparral tend to be relatively frequent, but high-intensity blazes, while fires in ponderosa pine forests may be more frequent but of lower intensity. Variables that affect a fire’s influence upon fisheries include the intensity of the fire, the percentage of the watershed that is burned, the size of the stream, the kind of soil and vegetative cover, and subsequent events the time before a significant rainfall event. Thus how a low-intensity fire that burn through a relatively flat landscape dominated by a ponderosa pine forest in the volcanic soils of northern Arizona would be different than say a crown blaze through old-growth Douglas fir growing on steep slopes in Oregon.

Despite all these qualifiers, we can still say some generalizations about fish, aquatic ecosystems, and fires. Much of our current knowledge about fire effects on fisheries are the result of intensive research done in Yellowstone National Park in the aftermath of the large fires of 1988 that burned much of the park’s watersheds.

Fire influences can generally be broken down into three categories: short-term, delayed response, and long-term effects. In general, the short-term impacts would generally be considered neutral or negative, while the long-term impact would be regarded as a positive influence.

When watersheds burn, the loss of plant cover and subsequent changes in sediment flows, changes in water temperature, changes in debris flow, and the release of nutrients into the system often alter streams. In general, as the size of the watershed increases, the more of the area that typically is unburned. This geographic area effect provides a higher amount of buffering effect upon the watershed. Thus, first-order headwater streams tend to suffer the greatest alterations and changes due to fires compared to larger, 3rd, and 4th order segments downstream. By the time you get to a river, the size of the Yellowstone, or Clark Fork of the Yellowstone, alterations due explicitly to fires is minimal. In other words, the short term adverse effects that a fire may have on aquatic ecosystems in most of the rivers that are considered essential fisheries are almost non-existent.

Nevertheless, since drought is almost a pre-requisite for large blazes, the real negative impacts on fish and other aquatic life as a consequence of drought are low water flows. Low flows lead to more significant dewatering of tributary streams for irrigation, causing a decline in spawning success and recruitment. Low flows also reduce the amount of water in streams leading to higher temperatures and greater concentration of pollutants—all negatively affecting fish and aquatic systems.

For instance, the loss of vegetation has several effects. The loss of screening streamside plants, particularly on smaller streams, can lead to higher water temperatures. If temperatures rise too high, they may be lethal to fish and other cold water-dependent species. In most of the West, however, small headwater streams are typically quite cold due to high elevation and snowmelt as a water source. The water temperature in such streams remains well within the tolerance of trout and other aquatic insects even if streamside vegetation is removed.

In some cases, rising temperatures may be a positive benefit, increasing biological activity, growth rates, and food supplies. But again, like any generalization, there are exceptions. Some lower elevation waters may rise above lethal temperatures for fish if enough streamside vegetation is killed or destroyed by fire.

Fire induced vegetation loss can also affect streamflow and timing. Snowmelt may come earlier and proceed more rapidly in burned watersheds. Plus, the loss of trees and shrubs can reduce the amount of moisture transpired by plants, increasing soil moisture, which can lead to higher stream flows. These higher flows, in turn, can mobilize sediments and debris affecting channel morphology.
Nevertheless, how higher flow affects individual streams has much to do with the stream size, steepness, and bedrock characteristics.

For example, the upper headwaters of Cache Creek in Yellowstone National Park is a short steep tributary of the Lamar River. The Lamar, in turn, flows into the Yellowstone River. More than 80% of the Cache Creek drainage burned in 1988. The watershed is composed of loosely consolidated volcanic debris. After the fire, subsequent heavy summer thunderstorms contributed to significant stream channel changes combined with higher sediment flow that led to a subsequent decline in aquatic insects and fish.

Researchers found that the further downstream one moved from the headwaters, the less fire-affected aquatic ecosystems and flow characteristics. For instance, measurements taken on the Yellowstone River outside of the park showed that the fires’ overall effects were relatively minor, with runoff increasing only 4-5% as a consequence of fires. This compares to the natural variation that results from a major flood that may change flows as much as 161% over the long-term average. In other words, when you get to the level of a major river, the effects of a fire are minor compared to other natural events like floods or droughts.

At the Stream Ecology Center at Idaho State University in Pocatello, Wayne Minshall studied the effects of Yellowstone’s fires on stream systems. They found that burned watershed in Cache Creek and other small tributaries of the Lamar River (where more than 50% of the area had been scotched) had more sheet erosion, gully formation, and mass movement compared to unburned control streams. Though these channel alternations may at first be seen as unfavorable, they are, for the most part, temporary. The regrowth of vegetation stimulated by the increase in sunlight, water, nutrients, and fertilization from the fire’s ashes rapidly reduces erosion and sediment flow. Within a few years, the stream systems begin to stabilize.

In a comparison of sediment flow in the Lamar River before the 1988 fires with post-fire conditions, Roy Ewing found that sediment transport initially increased, but diminished by 1992 to less than pre-fire levels. Much of the decrease in sediment transport was due to storage behind fallen logs and other debris that had begun to trap gravels. Plus, after the initial rush of fine sediments is reduced, stream flows start to stabilize the newly deposited stream gravels and even create a crucial new source for spawning habitat.

Another generally positive benefit of fires is a large amount of woody debris—logs, branches, and other burnt materials that are carried or fall into rivers. These logs and other materials reduce water velocity contributing to greater channel stability over time, somewhat countering the effects of higher flows.

The new wood and logs create cover and food resources for aquatic insects and fish. It’s important to note that these inputs of wood are episodic. A large percentage of the deposition of trees and woody debris in a stream may be the result of one large fire event occurring once every hundred or two hundred years.

It should also be noted that these logs, snags, and down wood store carbon on-site for decades to centuries, while little carbon is lost from a fire.

One difference between fires and logging activity, particularly “salvage logging,” is the repeated remobilization of sediments that occurs every time machinery and road construction occurs in a watershed. While a blaze may release an initial flush of deposits, within a few years, sediment flow tends to decline to pre-fire levels or even lower as the post-fire slopes revegetate and fallen woody debris begins to trap sediments both on the slopes and in the streams.

However, logging may repeatedly disturb slopes, releasing sediments for years or decades, depending on how long logging continues in the drainage. Fish and aquatic insects can cope with a few years of limited reproduction due to high sediment flow. Still, they can’t deal with extended periods of repeated flushes of fresh new sediments. This is one of the significant differences between logging and its effects on fish habitat compared to fire-induced habitat changes.

Another short-term effect of fire is a greater loss in organic materials—at least in first-order high gradient streams. Much of the organic matter that supports macro invertebrates is leaf fall, grass, and other organic matter that falls into streams. The higher velocity of streamflow resulting post-fire can transport a greater amount of the organic materials downstream, reducing the organic material needed by aquatic insects. The organic material retained in the streams shifts to high amounts of charcoal that is inedible for most fish species. Although certain species, including some mayflies can feed on charcoal materials and may increase, the majority of stream macroinvertebrates find such source inedible.

These effects, however, are often very short-term. Post-fire revegetation is rapid and even enhanced by the removal of streamside conifers and other evergreen vegetation.

Studies of Cache Creek in Yellowstone showed dramatic changes in post-fire stream invertebrates. The loss of streamside vegetation contributed to a decrease in organic matter, such as leaves, leading to a decline in aquatic insects that shred debris. Countering these effects was an increase in algae production due to greater sunlight penetration that favored aquatic insects like mayflies and riffle beetles that feed on algae. Still, the most significant long-term effect of the fires on Cache Creek aquatic insect populations was not due to changes in food resources, rather a consequence of the alterations in stream channel morphology that occurred post-fire.

The findings in Cache Creek differed significantly from results measured in other parts of Yellowstone. In the majority of streams monitored by the USFWS, macroinvertebrates increased between 1988 and 1991 that may be attributed to higher post-fire primary productivity. Before the 1988 fires, Montana State University entomologist George Roemhild had sampled aquatic insects throughout the park. He resampled many of those sites in 1991 and 1992 some three and four years post-fire and found no large changes in the number or diversity of stoneflies, mayflies, or caddis flies before and after the fires in the park as a whole.

So what was the effect on fish? In a situation like Yellowstone’s 1988, fish in small headwater drainages like Cache Creek suffered some mortality from the blazes. Temperatures were not the blame; instead, ammonia from smoke increased in tiny streams to lethal conditions. But within a year, fish had recolonized all these streams.

Dead trout were documented in several wilderness streams outside of Yellowstone in nearby Forest Service wilderness areas two years later. These fish died from high sediment loads resulting from several severe but very localized thunderstorms. These storms in August of 1990 created a “flash flood” that washed in massive quantities of gravel and dirt into several streams, including Crandell Creek and Jones Creek—both in the North Absaroka Wilderness east of the park.

Despite the severity of blazes that charred many of Yellowstone’s major watersheds, researchers could find no evidence for fire-related effects on fish populations in any of the park’s major rivers, including the Gibbon, Madison, Firehole, Yellowstone, Lamar, and Gardner. Furthermore, post-fire data shows that trout growth rates in all of these rivers were some of the highest ever recorded.
Researchers also conducted an inventory of cutthroat trout spawning runs in Yellowstone Lake. Before the fires, some 58 tributaries of Yellowstone Lake had cutthroat trout spawning runs, and in 2000 at least 60 streams were documented to have trout spawning activity. Again this suggests no direct long-term negative impacts on fisheries.

The general overall conclusions from Yellowstone research are that small, high gradient streams like Cache Creek, where more than 50% of the drainage was burned, suffered significant changes in stream channel morphology that included down cutting, channel scouring, and loss of pool habitat. Responding to this, the macroinvertebrate populations shifted towards more generalist species. And fish populations declined but did not disappear from these drainages.

On the other hand, larger downstream segments of these watersheds appear to have suffered no negative impacts from fires. Indeed, there is some evidence to suggest that overall the effects were positive, including the deposition of more woody debris that has increased habitat structure and higher fish growth rates due to the influx of nutrients.

Although nearly half of Yellowstone’s acreage was within the perimeter of a major burn, no apparent short term or long term adverse fire effects on fish nor aquatic invertebrates were observed in any of the larger rivers or to fish populations.
What can we say about the long terms impacts of fires on the West’s fisheries? Well, all you need to do is look back in time. Many of the West’s last strongholds for native fish and high-quality fish habitat are areas that burned extensively in the past. For example, the drainages of the Selway River, North Fork of the Clearwater, St. Joe River, Kelly Creek, and the Lochua River in Idaho were all extensively burned in the 1910 blazes that charred more than 3.5 million acres of the Northern Rockies. Today they are among the most famous trout streams in northern Idaho and known as refugia for native species like Westslope cutthroat that are endangered elsewhere.

A similar conclusion could be made about the North Fork and the Middle Fork of the Flathead Rivers in Montana. Both drainages have burned extensively in the past, and today are among the last refuge and stronghold for bull trout and westslope cutthroat trout.
And research on fire ring history documents even more massive fires in Yellowstone in the centuries past. Despite these large blazes, Yellowstone remains a premier fishery.

One of the key differences between the impacts associated with fires and those from other human activities like logging and livestock grazing has to do with the temporal component. While a thunderstorm may send massive amounts of sediments from fire-denuded slopes into a stream, such events only occur for a short time after the blazes. Very shortly after a blaze, new plant growth stimulated by the fire-released nutrients and greater sunlight begin to take hold of slopes, combined with the down woody debris that acts as mini check dams both act to reduce sediment flow.

Fish populations can deal with a short-term impact on habitat quality and quickly recover from population declines. While on-going human activities like livestock grazing or logging disturb the slopes, continuously adding new sediments year after year gradually causes fish populations to dwindle, sometimes to the point of extinction.

Another difference between fires and human activities has to do with the structural component. While logging removes wood from the site and streams, fires add wood to the sites and streams. Over the long term, the input of fallen snags creates more fish habitat.

The same thing can be said about fires and livestock impacts. Year after year, cattle trample streambanks, destroying bank structure, removing vegetation. At the same time, fires may temporarily upset the stream channel stability, over the long term, it has a chance to stabilize, and eventually improve as riparian vegetation regrows. Channel structure is secured by the addition of wood, and riparian vegetation.

All the research suggests is that the adverse effects of fires tend to be localized and short term, while the positive results appear to be long term and more widely distributed in a watershed. Any regional effect of fires is dwarfed by the negative impacts of dewatering, combined with drought. If there are any lessons we take away from the summer of 2000 and subsequent drought years with large blazes is that natural events like fires, even big fires, are well within the natural range of variation for aquatic ecosystems, and fish are well adapted for coping with these occasional blazes.

Not surprisingly, there is evidence that native trout are better adapted to periodic large fires than exotic species. One study in Montana’s Bitterroot River found that native cutthroat and bull trout recovered quicker than exotic brown trout and brook trout.

I want to editorialize a bit here. If people are concerned about fisheries, we would shift money away from fire fighting and “thinning” for forest health or wildfire prevention, which are largely fruitless and wasted effort. This money would be better spent on restoring aquatic ecosystems impacted by livestock and logging, creating on-going and long-term negative impacts on our fisheries.

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