Drastic decline in brown trout numbers
By Benjamin Alva Polley EBS COLUMNIST
Montana Fish, Wildlife & Parks is seeing a significant decline and loss in recruitment in brown trout fish populations along the Upper Clark Fork between Warm Springs and Deer Lodge, a trend that began in 2015.
From the 1970s through 2014, it was common to have estimates of 1,000 to 2,000 and even more brown trout per mile in this section of the river. But, starting in 2015-2017, it started declining to historic lows of 100-200 per mile. The numbers stabilized for a few years. This year, it was down to 25 brown trout per mile.
“We don’t know the cause of the decline,” Region 2 FWP Fisheries Biologist Caleb Uerling said. “But there are lots of variables impacting brown trout. The floodplain has outstanding contamination that interacts with the river at high flows and rain events.”
The Clark Fork is Montana’s largest river by length and volume. It is considered the headwaters of the Columbia River Basin and gets its namesake from the Lewis and Clark expedition. In the 1880s, Montana had a copper mining boom lasting over a century, with most mining and smelting operations occurring upstream in Butte and Anaconda. The processes involved in mining and smelting created significant amounts of toxic waste. In 1908, a massive flood washed millions of tons of contaminated sediments into the Upper Clark Fork and its surrounding floodplain for a 120-mile stretch of river. Nearly every year, the snowmelt and spring rains wash more toxic deposits downstream. This sediment is high in heavy metals like arsenic, copper, lead, and zinc, which, in high doses, kills vegetation, and fish die. Acute and chronic effects like long-term exposure to heavy metals can cause chronic conditions killing trout.
“An acute event causes contaminated sediments to flow into the river from a rain event, in high enough concentrations that kills fish within a couple 100 meters of the point source run-off.”
In 1992, the U.S. Environmental Protection Agency designated the Upper Clark Fork from Warm Springs to Milltown Dam, east of Missoula, a Superfund Site. In the spring of 2013, remediation work began eliminating toxic sediments from the soils in the floodplain, restoring and stabilizing the eroded riverbanks. Teams of workers also making efforts to revegetate riparian areas throughout the floodplain, trying to return to its pristine nature. According to Montana Natural Resource Damage Program, the remediation work is scheduled to be completed in 2038.
“Remediation and restoration can impact things in the short-term, but, in the long-term, those actions are intended to improve the fishery and conditions for the fish,” said Uerling.
It was hot in the summer of 2019, and river flows were low; then, a significant rain event caused contaminants to run into the river, and many dead fish were found nearby.
“We don’t know 100% sure what killed them, but it seemed to be a direct correlation,” said Uerling.
Each April, FWP biologists like Uerling electro-shock stretches of river to count trout within those sections. The Upper Clark Fork is a cold-water brown trout fishery, although many tributaries have other trout species. To effectively sample a fish in the river, the fish must be 7 inches or longer, which is a 2 to 3-year-old in the Clark Fork. They need to get fish to the 7 inches mark, which is also when they become catchable to anglers.
“The issue is somewhere between the fish being spawned and making it to 7 inches,” said Uerling.
There are many life stages for brown trout within that period, including months in the gravel as eggs, spending a month as alevins, and years as juvenile fry and parr until they hit that 7-inch period and then continue maturing in length and girth.
Other variables besides toxic sediments that can affect brown trout recruitment include warmer temperatures, poor habitat quality, high erosion rates, fungus outbreaks, algal blooms, parasites, diseases, proliferative kidney disease, and fire retardants. A recent article in High Country News shows fire retardant is 85% water, 10% ammonium phosphate, and a small amount of thickeners which suffocates fish if dropped in rivers. Retardant doesn’t seem to be the case here in the Clark Fork, but it does have detrimental effects on fisheries.
“There may be a disease that we don’t know how to look for or don’t have the tools to identify,” said Uerling.
FWP is working hard to narrow in on the exact life stage that fish must graduate from to continue their life cycle and increase recruitment throughout this once blue-ribbon brown trout stream.
“FWP is narrowing their focus on the individual issue affecting particular life stages for those brown trout, the geographical location, and narrow it down to a manageable project of what can be done,” said Uerling.
FWP remains hopeful.
Benjamin Alva Polley is a place-based storyteller with stories published in Outside, Adventure Journal, Popular Science, Field & Stream, Esquire, Sierra, Audubon, Earth Island Journal, Modern Huntsman, and other publications at his website www.benjaminpolley.com/stories. He holds a master’s in Environmental Science and Natural Resource Journalism from the University of Montana.