To anglers, trout streams are characterized by free-flowing water that’s clear and, most importantly, cold.
But in the years ahead, the chilly streams that sustain Northwest Montana’s prized native fish could grow increasingly tepid as a result of rising global temperatures. This summer, which has been marked by historic high temperatures and record-low flows due to a scant snowpack and an early spring runoff, researchers say trout will endure stresses that could adversely affect habitat, genetic biodiversity and migratory patterns, while promoting hybridization with nonnative species.
For anglers, the specter of state-imposed fishing restrictions could shut down the cherished Montana pastime on rivers and streams throughout the Crown of the Continent Ecosystem. So-called “hoot-owl” restrictions went into effect July 3 on the Bitterroot, Blackfoot and Clark Fork rivers.
Water temperature strongly regulates the distribution, abundance and physiology of stream-dwelling fish. This is particularly true for cold-water species like bull trout and westslope cutthroat, which are ectothermic (cold-blooded) and constrained to stream and lake environments, said Clint Muhlfeld, a Flathead-based aquatic ecologist with the U.S. Geological Survey in Glacier National Park.
“Native bull trout and westslope cutthroat trout have among the coldest temperature requirements of North American trout, so increases in stream temperatures and reductions in flow like the region is already experiencing this summer can be very stressful to these prized fish,” Muhlfeld said.
And while stream flow is the master variable in assessing the quality of native trout habitat because summer flow declines reduce the amount of available stream habitat, record-high stream temperatures is an increasing concern to biologists.
To determine the long-term effects of elevated stream temperatures, USGS ecologist Leslie Jones is developing a high-resolution stream temperature model for the Crown of the Continent Ecosystem, which stretches from Banff, Alberta, Canada, to Missoula’s Blackfoot River.
Using sensors to glean some 12 million data points from more than 900 temperature-monitoring sites in streams throughout the Crown, Jones is building temperature models in an effort to predict past, current and future climate scenarios as they play out on western Montana streams.
She recently pulled together data from the North Fork of the Flathead River going back to 2000, which show that temperatures this June were 7 degrees Fahrenheit warmer than the 14-year historic average of 55 degrees Fahrenheit, with temperatures peaking at 66 degrees.
According to Muhlfeld and Jones, that means trout could be in hot water this summer as high temperatures and dry conditions persist.
“The take home is that we had below-normal snowfall this winter, which occurred along with above average temperatures, which is a trend that we may begin to see more consistently,” Jones said. “We are going to see extremely low base flows in late summer, and those low flows plus extremely hot temperatures that we are already seeing are going to be extremely stressful to native trout that thrive in these cold-water environments.”
As a result of the warm air temperatures and low flows, stream temperatures are already approaching stressful conditions for trout growth and survival in the Flathead system. Daily water temperatures in the main stem and Middle Fork are also already peaking at 66 degrees Fahrenheit and temperatures could soon approach 70 degrees Fahrenheit if the trends continue this summer.
“At these temperatures, trout experience reduced food consumption, growth, energy reserves, and begin to show signs of outward stress,” Muhlfeld said. “Temperatures of this magnitude for extended periods of time can reduce trout abundance, force trout to retreat upstream to colder waters, may cause direct mortality, and are unlikely to be suitable for long-term survival of these species.”
With snowpack runoff depleted a month ahead of schedule, Montana Fish, Wildlife and Parks is gearing up to protect fish from the life-threatening stress of low flows and elevated water temperatures.
“In many areas we’re seeing stream flows fall below average for this time of year, and some that are the lowest ever recorded,” said Stephen Begley, a water conservation specialist for FWP in Helena.
The situation is particularly grim in Northwest Montana following the driest May and warmest June on record in areas near Kalispell.
Low flows, high water temperatures and competition for space and food are especially stressful to trout. Fish are often physically compromised and can die from the higher water temperatures, lower oxygen levels and disease – all health threats that can adversely affect trout numbers in future years.
To mitigate drought conditions, FWP sometimes employs certain fishing restrictions and maintains state-owned instream water rights, some granted to the citizens of Montana nearly 50 years ago. These water rights are aimed at keeping enough water in a stream to keep fish healthy.
For instance, FWP recently informed junior water-right holders on Young Creek, a Lake Koocanusa tributary three miles south of the Canadian border, that they may be asked to reduce or stop their water diversions. The request could come if stream flows fall below five cubic feet per second – FWP’s instream flow water right from July through December. Young Creek was flowing at 8 cubic feet per second on June 23.
When the need arises to reduce impacts on drought-stressed fish, FWP also can limit fishing hours to midnight to 2 p.m. – commonly called a “hoot owl” regulation.
“Anglers could see hoot owl fishing restrictions on some streams this summer if drought conditions persist,” Begley said. “A full fishing closure is also an option if conditions warrant such restrictions.”
FWP’s drought policy provides for the use of angling closures when flows drop below critical levels for fish, when water quality is diminished, or when maximum daily water temperatures in a stream reach at least 73 degrees for three consecutive days.
The preferred water temperature for rainbow and brown trout is about 55 to 57 degrees. Water temperatures of 77 degrees or more can be lethal to trout.
“Fishing in the cool morning hours helps reduce catch-and-release mortality,” said Bruce Rich, chief of FWP Fisheries Division in Helena. “Because Montana generally doesn’t stock rivers and streams with hatchery fish, we work hard to protect our wild fish, and each stream’s wild brood stocks. Those are the fish that will spawn this fall and next spring.”
Should they become necessary, information about angling restrictions and closures will be posted on FWP’s website at fwp.mt.gov.
Muhlfeld said there are management options to mitigate the stress to trout in the short-term, such as using Hungry Horse Dam to artificially cool the Flathead River downstream, drawing cold water from the deep reservoir to effectively manipulate the stream temperatures.
And while trout have adapted to survive warming periods, wildfire, drought, and flooding through the millennia, often by retreating to colder water refuges, the effects of climate change exacerbate the stressors.
“The maximum temperatures are already at stressful levels. And these trends are just going to continue. We’re not even in the bottleneck yet. We haven’t even seen how warm these rivers can get,” Muhlfeld said. “It’s just not looking good for trout.”