Brown trout reduce native cutthroat numbers, study finds

BILLINGS – One of anglers’ favorite trout species to target is the predatory brown trout, a nonnative first introduced in 1889 to Montana’s Madison River and now found in all of the state’s major drainages.

As fisheries officials have dosed mountain streams with fish-killing poisons to remove nonnatives like rainbow trout and brook trout to make room for native Yellowstone and westslope cutthroat trout, not much has been said about the much-beloved brown trout.

Now a recently completed study on a stream on the south end of the Crazy Mountains shows that where brown trout and Yellowstone cutthroat trout coexist, cutthroat trout are the losers.

“Nonnative brown trout can substantially alter food webs, compete with and reduce recruitment of native fishes, and lead to declines in native fish abundance,” according to the paper published by scientists Robert Al-Chokhachy and Adam Sepulveda of the U.S. Geological Survey in Bozeman.

The effects the study found were based on two populations of Yellowstone cutthroat trout on the Henry’s Fork and East Fork creeks within the Duck Creek watershed near Springdale. The streams drain into the Yellowstone River.

“During the 1980s surveys in Duck Creek indicated that brown trout were well distributed in the watershed, but Yellowstone cutthroat trout were numerically dominant in the headwaters,” the scientists wrote. “However, sampling in 2012 indicated a considerable shift as brown trout became the numerically dominant species.”

Even before that 2012 sampling, in 2009 and 2010 FWP moved some cutthroat trout above a waterfall on the Henry’s Fork where there were no brown trout present. That created two separate populations allowing the scientists to compare and contrast growth rates and survival, as well as diets.

From 2013-16, Sepulveda and Al-Chokhachy sampled fish above and below the waterfall twice a year – usually around mid-July and mid- to late October. In 2015, they also pumped the fishes’ stomachs to see what they were eating.

Out of the 981 cutthroat captured during the study, few juveniles were found in the stretches of creek where brown trout were present. Also, cutthroat trout growth rates “in length and mass were greater for both juveniles and adults” where the fish weren’t competing with brown trout compared to those fish that did compete. Specifically, juvenile cutthroat trout growth rates where there was no competition was “significantly greater,” the study found.

“While there has been much focus on the negative effects of nonnative brook trout … and rainbow trout … in cutthroat trout conservation, our results suggest brown trout represent an additional threat,” the scientists concluded.

Montana’s native fish have long been in decline. In the past decade Montana Fish, Wildlife & Parks as well as Yellowstone National Park have stepped up efforts to remove nonnatives from mountain streams and lakes to ensure cutthroat trout have a few places to themselves.

“Historically across western North America there were 14 subspecies of cutthroat trout,” the study noted.

“However, due to a variety of threats, only nine subspecies currently remain, with each of the remaining subspecies designated with state or federal protection status.”

Although brown trout are a favorite of anglers because they grow large and fight hard, the European fish “have been included in the top 30 worst invasive species on the globe,” the study noted.

Even though favored, “studies indicate brown trout fundamentally alter the aquatic ecosystems they invade,” according to A Handbook of Global Freshwater Invasive Species, published in 2012.

Anglers may be more likely to attach the word “invasive” to zebra mussels or weeds like spotted knapweed, yet brown trout also fit the definition since they can have a “detrimental impact on some aspect of their introduced location (whether ecological, economic or cultural),” according to the handbook.

“(Invasive aquatic species) are ranked among the most significant threats to global biodiversity, ecosystem services and natural resource economics.”

Such a statement seems contrary to the fact that many anglers complain when brown trout numbers on the Missouri or Bighorn rivers drops, a fact noted in the handbook.

“In comparison to most other freshwater invasive species, Salmo trutta (brown trout) pose a paradox. They are regarded as one of the ‘world’s worst invasive alien species’ by international conservation authorities for their impact on native species, but they are simultaneously loved by recreational anglers.

Not only do they threaten cutthroat trout in the West, brown trout also outcompete native brook trout in Eastern waters. In a 2016 USGS fisheries study, scientists concluded “that nonnative brown trout limited the ability of brook trout to utilize warmer stream temperatures. In contrast, removal of brown trout expanded brook trout’s reach into warmer waters.”

“We know streams are warming due to climate change and non-native species are becoming increasingly abundant in many places,” said Nathaniel Hitt, U. S. Geological Survey research fish biologist, in a press release about the 2016 study. “We wanted to understand how these changes may synergistically affect native brook trout, a species of cultural and economic importance in eastern North America.”

Brown trout populations are growing in some Montana streams, according to Al-Chukhachy and Sepulveda.

“Recently, brown trout have been increasing in abundance in headwater streams within the region,” they wrote. “The invasion and proliferation of brown trout in headwater streams in combination with our results suggest potential concerns for (existing) populations of cutthroat trout.”

Hence the study, which demonstrates the effects brown trout are having on native species to provide “critical information in garnering support by the public for confronting these socially and economically valuable fishes,” the study reported.

As FWP and Yellowstone fisheries officials have poisoned streams to remove brook and rainbow trout to set aside habitat for cutthroat trout, some members of the public have decried the loss of the nonnative fishes. Given the popularity of brown trout, directing efforts at removing them from certain streams could be even more vocal and unpopular.

“Efforts directed at the management of nonnative brown trout need to be increased,” A Handbook of Global Freshwater Invasive Species reported. “There are few published accounts of attempted brown trout control, though isolated efforts have been undertaken and/or are ongoing in Australia, New Zealand and the U.S.”