Warming waters spell trouble for native trout fisheries

By CHRIS PETERSON

Hungry Horse News

Warmer summers due to climate change don’t bode well for native species like bull trout and westslope cutthroat trout, a recent study by Donovan Bell, a University of Montana fish and wildlife biology doctoral candidate found.

Those species are already threatened by a warming world and things could get much worse in the next 60 years, the study found.

Neither species can tolerate warm water for very long.

Bull trout’s ideal stream temperature range is between 55 and 65 degrees, according to studies and westlope cutthroat trout like it cold, too. Their optimal stream temperature is about 65 degrees, according to a study by Montana State University biologist Elizabeth Ann Bear in 2005.

Bell’s study modeled what the future might hold.

“This study had three main questions: How have the distributions of native and invasive trout shifted in Montana over the last 30 years, how will they change in the future, and what factors are causing those changes?” Bell, the study’s lead author and a doctoral candidate in UM’s Wildlife Biology Program said.

The findings were bleak.

“Using 21,917 surveys collected over 30 years, we quantified the impacts of climate change on the past and future distributions of five interacting native and invasive trout species throughout the northern Rocky Mountains,” the study said. “We found that the occupancy of native bull trout and cutthroat trout declined by 18 and 6%, respectively (1993–2018), and was predicted to decrease by an additional 39 and 16% by 2080.”

The culprit for declines of both native trout species is likely climate change, researchers found, but the specific mechanisms of the declines varied by species.

Bull trout, a threatened species under the Endangered Species Act, require cold streams with adequate flow. But warmer water temperatures and lower summer water levels – both driven by climate change – have degraded stream habitat and likely caused declines of bull trout. Meanwhile, westslope cutthroat trout were strongly limited by the presence of invasive trout species, including brook trout that can outcompete native trout, and rainbow trout that readily hybridize with westslope cutthroat trout. The threat from invasive rainbow trout is particularly concerning as their range is expanding due to climatic warming.

Rainbow trout can also tolerate warmer temperatures — Bear’s study found they could handle stream temperatures as warm as 76 degrees. Westslope could also tolerate those temperatures, but for shorter periods of time.

Her westslope trout, which were raised in lab conditions, lasted about a week at temperatures just under 70 degrees, but then declined sharply, while most the rainbow trout survived — even at warmer temperatures.

Streams, of course, have temperature fluctuations between day and night and fish can often find refuge at springs and seeps, where the water is colder,

But overall, they really need that ideal range — 55 to 65, to thrive, studies have found. When it’s 100 degrees in June and July, like it was last summer, fish can get stressed quickly.

The warmer temperatures also allow more hybridization to occur among westslopes and rainbows, Bell found, as the rainbows can tolerate warmer water. Rainbow and westslope cutthroat trout will interbreed, producing a fish known as a “cuttbow.” The resulting crossbreed is a fish that’s not fit and doesn’t do well.

But there’s still time to act, Bell found.

“Unexpectedly, in the absence of invasive species, our projections suggest that cutthroat trout could occupy more habitat at the end of the century than at present despite rapid climate change, consistent with a recent physiological study that found that cutthroat trout have a higher thermal tolerance than previously documented,” he noted.

There’s appears to be more hope for cutthroats than for bull trout.

“For example, conservation efforts for cutthroat trout may be better aimed at reducing invasive species through intensive suppression and eradication efforts and intentional isolation of at-risk populations,“ Bell noted. “Conversely, conservation efforts for bull trout could focus on protecting, reconnecting, and restoring critical cold-water habitats across entire riverscapes. However, the scope for mitigating climate impacts on bull trout may be more limited because an increasing amount of stream habitat—much of which is in protected areas with minimal human impact —is predicted to exceed their narrow thermal niche as the climate continues to warm.”

In other words, even the best habitat may not be enough to keep streams cool.

Still, over the past 20 years, agencies like the Park Service and Montana Fish, Wildlife and Parks have made great strides in native fish conservation.

Glacier in recent years has begin moving bull trout and native westslope cutthroat trout upstream, where waters are cooler and barriers, like waterfalls, stop migration of nonnative species that live in the mainstems of the Flathead River.

FWP did a multi-year project in the Bob Marshall Wilderness and the South Fork of the Flathead River drainage where it removed non native rainbows and hybrids and replaced them with native westslope cutthroat trout. Since the Hungry Horse Dam is a barrier, the rainbows and cuttbows in the mainstem can’t get into the South Fork.

There’s another method that also looks to genetically alter the westslope cutthroat trout population back to its pure state called “swamping.”

That’s when thousands of fish of a pure strain are stocked into a water. Over the course of time, the pure fish eventually breed out the nonnative genes.

It’s typically done on smaller bodies of water, however.

Another tactic is to put in dams to keep nonnative fish from traveling upstream.

But that can have a downside, as it can lead to inbreeding and isolated populations.

Could swamping be done on a watershed scale?

Bell said that would take another study beyond the realm of what he’s worked on.

Still, more needs to be done, Bell suggests.

“Although native species distributions increased in some watersheds over the past 25 years, our future projections show region-wide declines through 2080. As status quo management is implicit in our model, this suggests that climate change impacts may soon overwhelm current conservation strategies unless more proactive and innovative measures are implemented,” the study noted.