By Michael Howell

Ever wonder how a raging wildfire affects the fish population in streams that are burned over? Well, things look pretty good for the native westslope cutthroat trout, according to Chris Clancy, FWP Fisheries Biologist for the Bitterroot.

We are in a unique position to learn a few things about it here in the Bitterroot, thanks to some studies that were done over the years and thanks to a very significant summer fire event – the Fires of 2000 – and the studies done since. In 2000, almost one quarter of the Bitterroot National Forest land base (22%) burned, more than 307,000 acres. Many streams were burned across, some lightly, some moderately and some severely. Some streams suffered significant debris events the following year due to mudslides and avalanches. One hundred and twenty-six streams were affected along some stretch by wildfire. About 150 miles of streams were burned either moderately or severely. There were major fish kills in seven streams and widespread population declines in a number of streams.

Data on fish populations had been collected over the years on the upper reaches of many tributaries to the Bitterroot River by FWP and Bitterroot National Forest biologists. But following the Fires of 2000, a student from Montana State University did his master’s thesis, a three-year study of fish populations following the burns, here in the Bitterroot. The question he asked in his thesis was, “Did severe fire events provide an opportunity for the replacement of native species by non-native species in the streams?”

To answer his question, fish population studies were done on several streams from 2001 to 2003 hoping to show recovery trends. Populations in 13 unburned streams were studied as well as populations in three streams in low intensity burn areas, five in moderately burned, and five in high intensity burns, as well as in four streams that experienced major debris flows.

Using historical data and the three year study data as well as data collected on a number of streams up to 2013, we can answer a few of the questions, according to Clancy. The westslope cutthroat trout is not being replaced following burn events by a non-native species, in fact they show a remarkably good recovery rate, even exceeding pre-fire levels in some streams. Bull trout, another native species, does not fare as well as the cutthroat, but shows some decline in some streams while showing population increase in others. They are not being replaced by non-native species.

Clancy asked if anyone had heard of the phrase “walking on fish.” He said that’s just what it was like up Meadow Creek, a tributary of the East Fork of the Bitterroot, in 2010.

“It’s like you were walking on fish,” he said. “The cutthroat population had exploded.”

There is an exception to every rule, he said, and had an example, but on the whole, cutthroat populations recovered very well after fire. Bull trout were so-so, increasing here, decreasing there, remaining stable elsewhere and disappearing in at least one case. Brook trout did not recover well, losing their pre-fire dominance in some streams and suffering diminished population in other streams, too.

Another odd thing to turn up in the tally is that Brown trout, another imported, non-native species, showed signs of enlarging its traditional range and moving upstream in major fashion on certain streams, like Sleeping Child Creek. Clancy said this increase in the range of Brown trout had been recognized in other drainages around the state as well.

One reason Bull trout are negatively impacted by burns is that they thrive in cold waters and have less tolerance of warmer temperatures than other fish. Water temperatures spike following a wildfire when cover and shading is reduced or eliminated.

What helps the fish populations recover is the immediate rebound of the insect and invertebrate populations which are their food. More sunlight and rising water temperatures creates an opportunity for algae to form on the rocks and a certain kind of insect will begin to flourish, the “scrapers”. They live by scraping algae off the rocks. Also the woody debris that is added to the streams, including fallen trees, creates habitat beneficial to both the “shredders”, the insects that live off shredded foliage and other biomass, and the fish themselves, providing refuge and some deeper water.

Aside from the effects of wildfire on our streams, Clancy said that temperature measurements across the valley show an overall warming trend in all our creeks over the last twenty years.