As biologists consider proposals to stem the tide of steep brown trout population declines in southwest Montana, the situation in the upper reaches of the Bitterroot River couldn’t be more different.
Instead of numbers dropping, brown trout are showing up in places they haven’t been before and threatening to displace native species, including endangered bull trout.
Montana Fish, Wildlife and Parks fisheries research coordinator David Schmetterling said brown trout have been expanding their range into tributaries where the waters were once thought to be too cold for the European species first stocked into the state in the 1930s.
Biologists were studying the range expansion of brown trout in tributaries in the western half of the state before the documented drop in populations in popular fisheries in southwest Montana that came to a head this past year.
On the popular Big Hole River, the number of brown trout dipped from 1,800 fish per mile to 400 in the most fished section of the river near Melrose. This week, the state closed 14 miles of the Ruby River due to low flows and warm water temperatures. In the lower end of the Ruby River, biologists estimate that is 31 trout per mile in places where the long-term average neared 800.
Similar declines have been documented in the Beaverhead and Jefferson rivers.
With the large drops in brown trout numbers, the state announced this week that it was considering adopting fishing closures in eight southwest Montana rivers from September to May to protect spawning brown trout as well as catch-and-release and standing “hoot-owl” restrictions for during the hottest part of the summer.
While no one is ruling out the water getting low and warm enough for hoot-owl restrictions later this summer on the Bitterroot River, biologists have not seen a major decline in brown trout numbers on the main stem of the river.
Last year’s sampling on the main stem of the Bitterroot River showed numbers of both brown and rainbow trout were a little below average, but nothing that caused biologists concern.
FWP Bitterroot fisheries biologist Jason Lindstrom said the drop is likely the result of drought conditions the valley experienced four or five years ago. Last year’s good flows could bump up numbers in the future.
“We really have not seen the same thing that’s happening in southwest Montana,” Lindstrom said. “All of our numbers are within the range of variability.”
Before the sudden decline in brown trout numbers in southwest Montana, biologists in western Montana have been more concerned that brown trout are invading areas once considered strongholds for native trout.
Fifteen years ago, Schmetterling said those native fish strongholds in tributaries were considered to be places where brown trout couldn’t survive.
“It was the habitat we thought was unsuitable for brown trout,” he said “Water temperatures were cool enough that we thought it would be inhospitable for brown trout. What we started to see — and it happened pretty rapidly — brown trout were showing up and it didn’t take them long to get established.”
And it wasn’t happening only in the upper reaches of the Bitterroot. Other watersheds in western Montana were experiencing the same brown trout expansion.
Biologists looked to see if brown trout were out-competing bull trout and displacing the native fish. What they found was the habitat had changed enough that it wasn’t suitable for bull trout, which require cold and clear waters to survive.
Brown trout moving into the tributaries in western Montana might be linked to the population declines in southwest Montana.
Biologists and researchers are tapping into decades of data that’s been diligently gathered on Montana’s blue-ribbon fisheries. It shows decades of overall declines in stream flows.
“A lot of people remember the drought years of the 2000s or 1980s and think things haven’t been bad lately,” Schmetterling said. “What we are seeing is that the snowmelt runoff happens in a shorter period of time and occurs earlier. It seems like we go from 100 percent of snowpack to talking about the start of fire season. The big years are not as big as they used to be or as frequent.”
That pattern of decline in stream flows has been happening over the last 20 years.
That change may be one of the reasons behind the movement of brown trout in the tributaries in places like the Bitterroot.
Both brown and bull trout spawn in the fall. Native bull trout have adapted to large spring runoffs by their young staying protected in the stream gravels where brown trout fry may have been flushed downstream.
Brown trout can also tolerate warmer temperatures.
“This displacement has happened over a pretty short period of time,” Schmetterling said. “Up in the tributaries of the Bitterroot or the Blackfoot, the places that were once considered cutthroat or bull trout streams, are now predominately brown trout streams.”
Schmetterling said there are a lot of reasons why that could be happening.
Brown trout haven’t been in western Montana for all that long. The first brown trout in the state were planted in the Madison River in the 1930s. Some streams in western Montana may not have been planted until the 1950s.
“For large part, they haven’t been here all that long,” he said. “They might still be exploring, moving around and trying to get established. Along with that, we have had a change in the climate. Since the mid-1990s, conditions in the tributaries are warmer and more suitable for brown trout.”
There has also been a change in when the runoff happens.
“Before that happened, brown trout redds might have been scoured or young fish blown back into the river,” Schmetterling said. “Now we might have conditions that allowed them to stay in the stream. I think the habitat has just gotten more suitable for them.”
“The Bitterroot River is definitely not alone in what’s happening with brown trout moving into tributaries,” he said. “What’s happening in the Bitterroot’s tribs is being seen everywhere there are brown trout.”