Fire and Fish: How a Wildfire Alters River Ecosystems

An uncontrolled fire is normally viewed as destructive, harmful, and dangerous. However, wildfires have critical regenerative properties that certain ecosystems count on. But their increasing frequency, scope, and size threaten unadapted systems and the species that rely on them.

The effects of a wildfire on a watershed are both numerous and complex. A wildfire may compromise the watershed during active burning until years after the fire is contained and smolders out. The immediate five-year period following a wildfire is one of immense transition and rebalancing.

Short-Term Effects

Increased Sedimentation

Without vegetation, trees, or a solid soil structure, massive amounts of debris and ash can wash through a watershed after a wildfire. Excess fine sediment, from the increased erosion, can clog or wear down a fish’s gills and suffocate eggs. It may also occupy small spaces between cobbles where eggs would be laid.

John Moody, USGS Researcher, points out that not all erosion has a negative effect. Course grain sediment, critical to fish habitats, is replenished back into the system.

Nutrient & Temperature Variations

In the first year after a burn, in-stream nutrient levels may increase dramatically. Nutrient variations are due to the heightened amounts of debris being welcomed into the system through run-off or ash. Increased amounts of nitrogen, phosphorous, and potassium are often found. These nutrients are needed for algal and plankton growth- critical as fish food sources. An excess amount of the nutrients, however, can cause algae blooms. When alive, the blooms decrease light penetration and, when dead, decrease amounts of dissolved oxygen.

The ability of water to hold oxygen depends on its temperature. An increase in water temperature, even slightly, decreases the capacity of oxygen that may be held. Many aquatic species, especially trout, can tolerate only a small variation in temperature change. Any shift within the minimum and maximum water warmth can dramatically change the species composition in a wildfire-affected region.

Long-Term Effects

River Reorganization

One of the longest-lasting threats to fish in a watershed post-wildfire is the fragmentation of habitat. Erosion and tree fall may cause reorganization and loss of connectivity in river channels. This means the fish are unable to move freely throughout the watershed. They are unable to migrate to more habitable areas of a river or stream to find food or escape warming waters. Rebecca Flitcroft, U.S. Forest Service fish biologist, says that the survival of a fish species affected by wildfire largely depends on the availability of safe refugees up or downstream.

Ashley Rush, a researcher and fisheries biologist who specializes in the long-term impacts of wildfire, can’t highlight enough the importance of eliminating nuisance blockages in the water system in her interview with Trout Unlimited.

“The best thing we can do is keep our waters connected [by getting rid of unnecessary dams, diversions and other impediments],” she said. “There’s no mitigation that needs to happen after [fires]—we don’t need to restock or be more aggressive on that. We need to acknowledge that we need to clear fish passages, give it time, and recognize that some spots are going to be muddy for a couple years after the fire when it rains.”

Population Density

A study by Rosenberger et al. (2015)  suggests that moderate warming associated with wildfire and channel disturbance history leads to faster individual trout growth. This exacerbates competition for food resulting in an overall decrease in trout populations due to the fish’s quick maturity.

It’s important to note that many species in fire-prone regions are adapted to withstand these natural phenomenons. Rust says that it’s crucial to make sure that these systems become and stay resilient. This includes not intervening in too much habitat recovery to allow the landscape to adapt as it needs.

“In the long term, it’s good to recognize that [fire is] part of the landscape, that we’ve all evolved with it, and at some point it won’t be such a scary-looking burnt-out landscape,” says Rust. “So far I’m seeing that the insects and fish actually bounce back faster than the vegetation or the land does. So, you look around a burnt landscape that might look devoid of life, but the water might be just fine.”

Wildfire plays a critical role in the general balance of an ecosystem. However, their increased occurrence and growing vastness pose both short and long-term effects that may threaten the species that rely on these landscapes. The good news? Research is showing that fish populations are able to bounce back after a decade (or sooner), and even flourish, after the short stint of bad luck.

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