Report: Climate change to worsen conditions for Big Hole grayling
Laura Lundquist
(Missoula Current) Recent stream research shows that arctic grayling are going to need more protection to survive in the Big Hole River as summers continue to warm and drought dominates.
This fall, Zachary Hoylman, a Missoula researcher, collected and analyzed several years’ worth of stream and climate data from sites along the Big Hole River to learn what’s been happening to the river since 2017.
Hoylman, owner of Hydrosphere Analytical Laboratory, published his study in September and submitted it to the U.S. Fish and Wildlife Service, which is under a court order to publish a new decision by February 2027 on whether the Big Hole arctic grayling needs Endangered Species protection. Holyman also works as a research assistant professor and climatologist at the University of Montana Climate Office.
When the U.S. Fish and Wildlife Service made its decision in 2020 not to list the fluvial - meaning populations that reside in rivers - arctic grayling, 2017 was the last year the agency collected stream data, and it appeared to show that voluntary measures irrigators had taken were sufficient to keep the river cool enough for the resident arctic grayling.
Hoylman’s report shows conditions have changed since then. Many of the years since 2017 have been some of the hottest years on record globally, and the past three years have seen record-low streamflows in several Montana rivers as mountain snowpack has either been low or melted too early. All of that has made a difference for the Big Hole River, where Hoylman’s analysis shows the flows are slowly declining and the number of hours or days when water temperatures exceed healthy levels for arctic grayling are increasing.
Grayling are an ancient native species that needs cold, clean water, similar to bull trout. They become stressed when water temperatures rise to 21 degrees Celsius or 70 Fahrenheit, and they start to die when temperatures hit 25 Celsius or 77 Fahrenheit. Populations of adfluvial grayling, those restrained to lakes, reside in several high mountain lakes, but the Big Hole grayling is the only remaining native river population.
Hoylman found the maximum daily air temperature between May and October in the Big Hole drainage have increased almost a half-a-degree Celsius every decade since 1980, while the median streamflow for the basin, factored across all the sites to compensate for variability, shows a decrease of about 50 cubic feet per second per decade since 1993.
He then looked at water temperatures, specifically the number of hours that temperatures exceeded 21 degrees Celsius and found a range of fewer than 50 hours at the site near Wise River to more than 700 at the site farthest downstream near Twin Bridges. Arctic grayling don't usually migrate any farther downstream than Divide.
Using this data, Hoylman used two different models to predict the conditions when temperatures would 21 Celsius, and both models produced similar results. The models predict that an average increase of 1 degree Celsius in daily air temperature will increase the odds of exceeding 21 degrees by about 25%. So, as air temperatures increase about 0.5 degrees each decade, each decade will add about 37 additional hours in the summer when water temperatures exceed 21 degrees.
The models also predict that each increase in stream flow of 100 cubic-feet-per-second can reduce the odds of exceeding 21 degrees by 16 %to 18%. So, to offset the additional 37 hours per decade caused by increasing air temperature, the basin streamflow would need to increase by 140 cubic feet per second each decade, which isn’t happening.
Hoylman has made all the data and models public for review.
“This analysis makes clear that the Big Hole River is warming and drying faster than current conservation efforts can compensate, leaving Arctic grayling and other cold-water species increasingly vulnerable,” Hoylman said in a statement. “Without decisive action to safeguard instream flows, the Big Hole River will continue to lose the ecological integrity that made it a renowned stronghold for Arctic grayling and other cold-water dependent fish.”
The Big Hole Watershed Committee developed a Drought Plan where participating landowners voluntarily limit their water use when the Big Hole River drops to specific low flows. But not all landowners participate, and anglers have argued that more needs to be done to improve river conditions to help various trout species, including brown trout that can tolerate higher temperatures than arctic grayling.
Hoylman’s report indicates that climate change is leading to warmer air temperatures, which require higher flows to keep water temperatures down in the river. Based on his measurements, worsening climate conditions are overwhelming the benefits produced prior to 2017 by landowner agreements. A Candidate Conservation Agreement with Assurances is a voluntary agreement where the Fish and Wildlife Service provides incentives for landowners to conserve species of concern to avoid a listing.
“While the (Candidate Conservation Agreement with Assurances) has claimed to have provided measurable benefits through flow augmentation and habitat restoration, those claimed benefits have not led to a detectable change in exceedance sensitivity since 2017,” Hoylman concluded in the report.
The nonprofit Western Watersheds Project points to the report as evidence that more must be done to save the grayling. Western Watersheds Project was involved in the 2023 lawsuit challenging the Fish and Wildlife Service decision not to list the grayling. In February 2025, Missoula federal district judge Dana Christensen ordered the Fish and Wildlife Service to complete a new species special assessment and issue a new listing decision by February 2027.
Western Watersheds Project worked with Hoylman to get a research grant from the Foundation for Sustainability and Innovation to provide the agency with more up-to-date science than was used in its 2020 decision, said Patrick Kelly, Western Watersheds Project Montana and Washington director.
“These findings demand stronger, enforceable action, not voluntary or piecemeal measures,” Kelly said. “Protecting instream flows and restoring functional riparian zones must be treated as non-negotiable requirements for Arctic grayling survival.”
Pedro Marques, Big Hole Watershed Committee director, said he agrees that conditions are worsening in the Big Hole but disagrees about what needs to be done.
“What we're doing is working better than any other alternative out there. What the litigants are offering is not a solution. They don't want to discuss the day-after scenario if they are ever successful at litigating, because a victory for them will only increase stressors on ranchers, which adds to pressures for them to sell out and subdivide, which would further put pressure on the resource, not improve the prospects for conservation. Easier to manage a connected landscape than a fragmented one,” Marques wrote in an email. “The solution for the grayling must involve cooperative participation from working ranches, because all their range is on private land. This is what we have with the current programs.”
Contact reporter Laura Lundquist at lundquist@missoulacurrent.com.