Rainbow and brown trout populations on the Bighorn River below Yellowtail Dam are growing and healthy.
That’s good news for anglers and outfitters who saw populations on the stream drop after three years of high flows, from 2017 to 2019. High springtime flows are especially hard on rainbow trout, which are spring spawners.
Shannon Blackburn, Region 5 fisheries manager for the Montana Department of Fish, Wildlife & Parks, provided an update on the fishery during a Montana/Wyoming Bureau of Reclamation meeting on Thursday. Other presentations included runoff forecasts, reservoir levels and a report on the fisheries in Bighorn Reservoir and the upper Bighorn River in Wyoming.
According to a graph Blackburn showed, total trout numbers in the Bighorn River below Yellowtail Dam climbed steadily after 2019 to a peak of around 2,500 trout per mile in 2021 before dipping slightly last year. The 30-year average for the stream, from 1992 to 2022, is about 2,200 fish per mile.
“2019 was pretty much the bottom of our population estimates, and that can be related to those sustained, high-flushing flows that happened for three years in a row,” Blackburn said.
Brown trout numbers mirrored total trout populations, bottoming out at 449 per mile in 2019, while rainbows dropped from 2019 to 2020 before climbing back the last two years. In 2022 the rainbow numbers were estimated at 1,463 per mile compared to 1,185 brown trout per mile. So both species have seen considerable gainis.
“It is pretty awesome to see the river recover,” Blackburn said. “These are wild trout so they’re pretty resilient.”
The fish are also healthy compared to some years.
“When we have a lot of fish in the system, like in 2015, we saw really what we called eel-pike brown trout (because they were long and skinny) … as of last year we’re seeing some big, healthy fish. So we’re trying to find that balance between big healthy fish and big populations,” she said.
By sampling 200 brown trout in 2020 by taking scales and otoliths, the fish’s ear bone, FWP got an idea of the fishes’ growth and age.
“And what we saw was some pretty incredible growth,” Blackburn said. “So at age 2 our brown trout had an average mean length of 15.3 inches,” which she dubbed “pretty incredible rates.”
The survey also revealed a handful of older fish that were age 8-plus.
“I really wasn’t expecting to see older fish. The older fish that survived those floods, they seemed to have a pretty high survival rate.”
With a new fisheries biologist dedicated to the Bighorn, Blackburn said FWP will be looking at what the river’s carrying capacity is for trout as well as what environmental factors are driving the population. In addition, a “wetted perimeter analysis” will examine whether the optimum 2,500 cubic feet per second flows that FWP recommended almost 25 years ago are still applicable.
A roving creel count to interview anglers, which hasn’t been done since the early 1990s, could be undertaken next spring. FWP is also planning more habitat improvement projects and opening more side channels with the help of the Bighorn River Alliance conservation group. Side channels are important rearing and spawning habitat for young fish. Some were cut off by debris and gravel during high water years.
The Bighorn Reservoir, which straddles the Montana-Wyoming border, contains two distinct fisheries. On the Wyoming side the lake is more of a warmwater fishery containing yellow perch, shorthead redhorse and native sauger, Blackburn explained, while the end closest to the dam has become a stronghold for smallmouth bass and also contains walleye, rainbow trout and the occasional shovelnose sturgeon.
In 2009, FWP stocked the reservoir with sterile walleye to protect the existing native sauger. Sauger are often mistaken for walleye and will interbreed with the fish. Sauger are a species of special concern in Montana.
Walleye stocking has varied widely, from no fish to more than 200,000 in 2015. When sampling for walleye, biologists are seeing fast growth until about age 6, then the fish disappear.
“It’s a little difficult to say what the older walleye are doing in Bighorn Reservoir,” Blackburn said.
More sampling will be done this year to evaluate the stocking strategy and where the fish should be placed because demand for the species by anglers is “really high,” she said
Like the reservoir it empties into, there are 97 miles of the upper Bighorn River that create a warmwater fishery containing species like channel catfish, sauger and shovelnose sturgeon, all native sportfish.
The lake plays an important role in forage fish production for many of the river species that overwinter in the lake before migrating upstream as far as Worland, Wyoming, to spawn in the spring. Wyoming Game and Fish hasn’t stocked walleye in the reservoir since 2000.
“From 2008 to 2022 it’s been relatively stable and above objectives,” said Joe Skorupski, Cody-based fisheries biologist for Wyoming Game and Fish. “We’re happy with where the population is. Things are looking pretty good.”
Unlike below Yellowtail Dam, a lot of water is good for sauger recruitment, Skorupski said. Unfortunately, the last two years were fairly dry resulting in poorer recruitment which means fish in the system are growing larger. The average size of the sauger is around 18 inches.
“That’s pretty remarkable for a sauger fishery,” he said. “And a really healthy population overall.”
The department is hoping this year’s heavy mountain snowpack may signal better spawning success.
New this year is the beginning of a two-year effort to look at small-bodied fish in the river to see where they live.
“The Bighorn River is a really intact native fishery and fish community, so understanding that information as much as we can and how that functions is … a great question to be asking,” Skorupski said. “Today it’s becoming more and more rare to see these unique individuals that have adapted to these really large river systems. They’re highly migratory. These flathead chubs and sturgeon chubs move really long distances. This will be the first time we’ve looked at this in this much depth.”
The project will divide the river into two sections, from Worland to where the Nowood River enters the Bighorn near Manderson will be one section. After leaving Boysen Reservoir, the river slowly warms as it flows downstream until the Nowood adds cool, clear water from the Bighorn Mountains. Consequently, water temperature and discharge and how that relates to the populations of the smaller fish will be key to the study and analyzing the difference between what goes on between the lower 65 miles and the upper 32 miles of the Bighorn River.
“These fish are highly migratory so we want to know how they are using the river based on different time periods, so spring, summer and fall,” Skorupski said. “It will give us a good snapshot of what’s going on with some of our unique native fishes, one of those being sturgeon chub that was petitioned for (Endangered Species Act) listing back in 2017.”